Revised: January, 2004
Watermelons are produced throughout the state of Florida. The region of greatest production is the north to north-central region (Suwanee, Columbia, Gilchrist, Alachua, Levy, Marion, and Sumter Counties), which comprised 34.4 percent of the harvested acreage and 37.1 percent of the state’s watermelon producing farms in 1997. The west-central region (Manatee, Hardee, DeSoto and Highlands Counties) comprised 20.5 percent of the acreage and 8.4 percent of the watermelon producing farms in that year, while the south-west region (Charlotte, Lee, Hendry and Collier Counties) held 19.9 percent of the watermelon acreage and 6.5 percent of the farms. An additional 7.5 percent of acreage was found in the north-west region (Jackson, Washington, and Holmes Counties), containing 14.6 percent of the state’s watermelon producing farms. Remaining watermelon production is distributed among 24 of the state’s other counties (3).
Both seeded and seedless watermelons are produced in Florida. Varieties of seeded watermelons include Celebration, Fiesta, Mardi Gras, Regency, Royal Flush, Royal Star, Royal Sweet, Sangria, Sentinel, StarBrite, Stars-n-Stripes, Summer Flavor 800 and Summer Flavor 900. Available varieties of seedless watermelons include Freedom, Genesis, Millionaire, Revere, Summer Sweet 5244, Summer Sweet 5544, and Tri-X-313 (9). Production of seedless varieties in Florida has been increasing, particularly in north Florida (10). Increases in national watermelon consumption have paralleled the availability of greater amounts of seedless watermelons in U.S. markets, and the popularity of seedless watermelons is expected to grow. In addition to being more convenient for the consumer, seedless varieties are sweeter and have a longer shelf life (11).
Seedless varieties are sterile hybrids, the seeds of which have been produced by a cross between a normal watermelon and one that has been genetically changed through chemical treatment at the seedling stage. When pollinated with normal watermelon plants, seedless plants produce only the small, white undeveloped seedcoats, which are soft and tasteless and are eaten with the flesh of the watermelon. The parent watermelon of a seedless plant produces only five to ten percent as many seeds as the normal plant, resulting in a seed cost that is five to ten times greater than that of seeded hybrid varieties and ten to 100 times greater than that of standard, open-pollinated varieties. Seedless varieties require soil temperatures above 80EF (26.7EC) for germination, and both germination and seedling emergence are slower for seedless varieties (11,12).
Seedless varieties of watermelons are transplanted because of the high cost of hybrid seed. In addition, seedless varieties cannot be established by direct seeding (4). As seedless watermelon production increases in Florida, the use of transplants is becoming increasingly popular (7). Approximately half of the watermelons in Florida are grown from transplants at the present (8). Generally, while growers in the southern half of the state tend to grow transplanted watermelons on plastic mulch, in the northern and western portions of the state (starting at around the Gainesville area), there is a wide mix of cultural practices used. While approximately 30 percent of growers use direct seeding and bare ground culture, many others (70 percent) use transplants on plastic mulched beds, a practice that is increasingly utilized in north Florida (8,13). When plastic mulch is not used, the crop is grown on an open bed system, particularly when the soils are likely to flood. The height of beds in Florida varies from 3 to 8 inches (7.6 to 20 cm), depending on the slope and drainage of the land (14).
When transplants are used, they are usually field ready in three to five weeks, after being grown in greenhouses. Bare-root transplants cannot be used. Instead, transplants are grown in planter flats to maintain the root and soil ball (14,15). Transplanting watermelons permits earlier harvesting, particularly if used with plastic mulch (6). Yields are also generally higher for transplanted watermelons, and the resulting plant uniformity makes cultivation easier. In a comparison of costs and returns from direct seeded versus transplanted watermelons grown on plastic mulch in north Florida in 1995, production costs for both methods were found to be similar. However, higher yields and a higher market price as a result of early harvest were shown to result in higher profitability of transplanted than of direct seeded watermelons (16). Direct seeding also has advantages, including lower labor requirements, the availability of precision planting, which has improved efficiency, and the possible production of greater vine area, which reduces sunburn (15,16).
In south Florida, watermelons are primarily grown on plastic mulch as a second crop in a double-cropping system, following tomato or pepper. The north Florida growers utilize the mulch to warm the soil, allowing them to plant earlier in the season and get their product into the market early. In addition to the ability to harvest earlier, the use of polyethylene mulch aids in weed control and improves the efficiency of water and fertilizer use. Plastic mulch is also a requirement for soil fumigation. When plastic mulch is used, a bed press shapes a smooth bed to maximize contact between the mulch and the bed surface, and fertilizer and soil-applied pesticides are added to the bed before the mulch is laid down. Plastic mulch measuring approximately 48 inches (122 cm) wide is placed on beds that are approximately 20 to 24 inches (51 to 61 cm) across the top. Mulch used for double-cropping needs to be able to survive two crop seasons (7,10,14).
Worker activities for the season commence with laying mulch, if this system is employed. Some seedless watermelon growers may use methyl bromide, but this is a small percentage. Worker activities during fumigation include mostly tractor-driven related operations, such as cultivation, fertilization, operating the fumigation rig, and laying drip tape. The only field task is shoveling dirt on the mulch to bury it, which generally requires three people per end. The two-row fumigation rig will cover about eight acres a day. With an average size farm of 40 acres, shovel crews would be needed about 40 hours (five days) a year. Placing emitters on the irrigation main line requires hand labor, and one worker can cover between 15 and 20 acres a day. Workers setting transplants (approximately 5 days for the forty acre farm) often wear latex gloves. Workers with poles also move vines out of row middles for the lay-by fertilizer application made at mid-season. When harvesting, one person walks the field to indicate which melons to pick, at which point the cutters (one or two per row) cut and turn the melon so the white belly is apparent. Two to three pickers per row then come after the cutters and melons are handled approximately three times before being placed in a box or truck, where one or two stackers work. Pickers/stackers are often ungloved and unshirted.
Primary planting dates for watermelons in Florida range from December 15 to April 15. Planting in south Florida may extend from December 15 to March 1, in central Florida from January 15 to March 15, and in north Florida from February 15 to April 15. A small amount of acreage scattered among south, central and north Florida is planted for a fall harvest (October through December) (6,9).
Seeds are planted at a distance of 60 to 108 inches (152 to 274 cm) between rows and 24 to 72 inches (61 to 183 cm) between plants, giving a plant population of 4,356 per acre at the closest spacing. Optimum yield under irrigation is generally obtained at a spacing of 84 to 96 inches (213 to 244 cm) between rows and 24 to 36 inches (61 to 91 cm) between plants. Seeds are planted at a depth of 1.5 to 2.0 inches (3.8 to 5.1 cm). Between 80 and 100 days are required from seed to maturity and between 60 and 90 days from transplanting to maturity, depending on the variety (7,10).
Moderate amounts of fertilizer are required to achieve adequate yields in watermelon production. The plant responds better when applications of nitrogen and potassium are split, but fertilizer application methods vary according to the production and irrigation system. For example, when full-bed mulch production with overhead irrigation is utilized, fertilizers are incorporated into the bed before the plastic mulch is applied, and a liquid fertilizer injection wheel is used for any needed later applications of nitrogen and potassium. Fertilization for the second crop in a double-cropping system is achieved by use of either a liquid injection wheel or through the microirrigation system, where applicable (6,7,14,17).
In non-mulched systems with seepage irrigation, the micronutrients, all of the phosphorus, and about 15 to 20 percent of the nitrogen and potassium are incorporated into the bed before planting, with the rest of the nitrogen and potassium applied later as a sidedress band along the outside of the bed. When using drip irrigation, growers incorporate the micronutrients, all of the phosphorus, and about 20 to 40 percent of the nitrogen and potassium into the bed prior to planting and add the remaining nitrogen and potassium through the drip irrigation (6,7,17).
All types of irrigation (overhead, drip, seepage) can be used in watermelon production (11). In 1997, 75.9 percent of the watermelon acreage in Florida was irrigated, representing 62.2 percent of the watermelon producing farms in the state (3). Most watermelon production utilizes overhead sprinkler irrigation systems, except for production in south Florida, which generally utilizes seepage irrigation (6). In north-central Florida, about half of the growers use drip irrigation (10). In general, cucurbits have slightly lower water requirements than other vegetable crops (9).
Watermelons are very sensitive to the cold, and many growers in north and central Florida plant windbreaks during the winter that serve to protect spring-planted watermelon seedlings from blowing sand and wind damage. The warming effect near the plant also helps to achieve faster plant growth. Rye is the most common windbreak used, although sorghum and ryegrass are also effective, and sugarcane can be used in south Florida. The windbreak is planted in strips, and on seepage-irrigated fields, windbreaks can be created in irrigation ditch areas and roadways (10,14).
Cucurbit plants have separate male and female flowers, requiring pollen to be transferred in order for fruit development to occur. At least 1000 grains of pollen must be deposited evenly on the stigma of female flowers, and if pollen is not evenly deposited, the resulting fruit will not be uniform. Honeybees are the principal pollinators, and growers usually rent hives to place in or around their fields to achieve adequate pollination. Although natural pollinators may be present, growers cannot always rely on them to provide sufficient pollination activity, since they can be affected by weather conditions (6,18).
Research on watermelon pollination in Florida has shown that female flowers require at least eight bee visitations for adequate fruit set to occur. Sufficient pollination generally requires one strong hive for every two acres, or a bee population that provides at least one bee for each 100 flowers. Hive placement is among the many factors that may affect pollination. Hives are often placed around the perimeter of large fields for adequate distribution. Standard pollination recommendations for watermelon production in Florida are not available, and growers must work with the beekeeper to determine optimum number of colonies and placement for each field situation (6,18). Seedless watermelon varieties tend to require slightly higher bee populations (11).
Honeybee activity is greatest from an hour or two after sunrise until mid-afternoon, which coincides with the time that flowers remain open and are most amenable to setting fruit. Most watermelon pollination occurs between 7 and 11 A.M., with the greatest percentage of fruit set resulting from insect visits that occur between 9 and 10 A.M. To avoid adversely affecting the bee colonies, any pesticide applications during the flowering period should be made at dusk or later, when bees are not active, since honeybees are highly susceptible to most of the insecticides used on watermelons. Use of a written contract detailing expectations of both grower and beekeeper is recommended (6,18,19).
Seedless watermelon plants do not produce enough pollen for adequate fruit set and development and are therefore interplanted with seeded pollenizer watermelon plants. The pollenizer variety is planted in the outside row and then in every third row, or it may be planted every third plant in a row, which tends to complicate harvesting. Since up to one-third of the plants in a field will be pollenizers, the watermelons produced from them are marketed as well, and varieties are chosen that can be easily distinguished from the seedless watermelons. For pollen to be available to the seedless plants at the appropriate time, direct field seeding of the pollenizer variety usually occurs at the same time that the seedless variety is planted in the greenhouse. When icebox (smaller) varieties are used as the pollenizer, direct seeding is delayed from seven to ten days, since icebox varieties flower earlier. To assure sufficient pollen for late-blooming seedless plants, some growers make a second planting of the pollenizer variety two to three weeks after the first planting (11).
Watermelons are harvested in Florida between April 1 and July 15, with the most active harvest period between May 1 and July 1. April and May production is primarily from southwest and west-central Florida. Growers in central Florida begin harvesting in mid- to late-May, while production from north-central Florida enters the market in June (1,20). The majority of Florida’s watermelon acreage is harvested during June (6).
The fruit is harvested upon attaining an acceptable level of sweetness, which develops simultaneously with the deep red flesh color, also a desirable characteristic. Some specialty watermelon varieties produce yellow-fleshed fruits. Watermelons are harvested while still crisp, as a result of high moisture content in the intact cells of the flesh. Once cells begin to separate and form air spaces, the fruit loses its crispness. When watermelons are harvested immature, they may be firm but will not develop an adequate level of sweetness (21).
When destined for local markets, watermelons are harvested when fully ripe, but those destined for distant markets must be harvested just before full ripeness is achieved, in order to minimize damage from handling. During harvest, watermelons must be protected from sunburn by maintaining them in shade until shipment. Watermelons that are harvested early in the morning are more likely to experience bruising and splitting than those harvested later in the day. Upon harvest, watermelons are neither washed nor pre-cooled. While refrigeration is not required during transportation, temperatures between 55 and 70EF (12.8 and 21.1EC) are recommended. Decay and flesh breakdown can occur at temperatures above 90EF (32.2EC), while chilling injury can occur below 50EF (10EC), resulting in fading of redness, leakage of juice, and decay. Relative humidity should also be maintained below 90 percent, because stem-end rot is more likely under conditions of higher humidity. After harvest, containerized watermelons that will be shipped out of state may be kept in cooled rooms until later shipment in refrigerated trailers. However, much of the watermelon crop harvested in Florida is shipped under ambient conditions (6,21,22).
Watermelons are harvested by hand by cutting them from the vine. All of the watermelons within a field do not ripen at the same time. The first harvest produces greatest yield and quality, and in addition to declining size and quality, later harvests are more costly because fewer watermelons are collected over the same area. In addition, once watermelon harvest begins in other states, the value of Florida’s crop declines significantly. Therefore, although yield would be maximized by harvesting several times, low market prices of late-harvested watermelons cause many Florida growers to pick only once or twice, particularly in north Florida (4).
Florida watermelons are usually shipped loose. Damage sustained during harvesting and handling may produce cracking and lower the quality and appearance of the fruit. Watermelon crews hired at harvest time therefore carefully lay the watermelons in rows and load them into padded field trucks to be off-loaded to highway trucks or taken to the packing facility. Some watermelons, mainly icebox and seedless types, may be packed in cardboard cartons, with two to five watermelons per carton, or in pallet bins holding about 1,100 pounds of watermelons, but many are shipped in bulk (6,21,22). Pallet bins are becoming increasingly popular, since they can be moved directly into supermarkets.
At a collection point, watermelons are off-loaded by hand from the field trucks, sized and graded, and then re-loaded into highway trucks or into the fiberboard pallet bins. Watermelons can also be sorted, sized, and loaded directly into fiberboard bins in the field. Bulk shipments entail handling of watermelons at least five times from harvest until reaching the retail store. Minimization of handling throughout the process is desirable to reduce bruising and improve overall quality. Packaged or bin-shipped watermelons are priced higher than bulk-shipped watermelons, because they are less likely to have sustained damage from handling and are easier to handle at the terminal market (6,21,22).
Watermelons should be consumed within two to three weeks after harvest. While the redness and flavor of the fruit may improve over the first week when stored at or slightly above room temperature, the red color fades when stored at or below 50EF (10EC). Optimum storage conditions include temperatures between 50 and 59EF (10 to 15EC), 90 percent relative humidity, and no exposure to ethylene gases, which are released from certain fruits and vegetables (6,21,22).
Pest ManagementThe most important pest groups in Florida watermelon production are disease pathogens, particularly viral diseases, which are difficult to manage, and weeds, which will become more difficult to manage with the loss of methyl bromide. Insect pests are generally less damaging than on other cucurbits.
Watermelon producers have access to a great deal of information regarding the production of watermelons in Florida, as well as integrated pest management programs that have been developed by researchers throughout the state and promoted through the Cooperative Extension Service. Growers employ a variety of pest management techniques, particularly for disease management. In several watermelon production areas of Florida, methyl bromide is utilized in the management of soil-borne diseases (such as Fusarium and Pythium), weeds (such as nutsedge and pigweed), nematodes (such as root-knot), and insects (such as cutworms, mole crickets, wireworms, and grubs) (7).
Use of Methyl Bromide in Florida Watermelon Production
Methyl bromide is a broad-spectrum soil fumigant that has played a key role in the production of several Florida vegetables since its adoption as part of the full-bed plastic mulch production system several decades ago. Florida’s subtropical environment creates ideal conditions for a rapid increase in populations of soil-borne pests, and the use of methyl bromide has allowed the producers of certain high-value vegetables to continually crop the same land without rotating to less profitable cropping systems (7). However, as a Class I ozone depleting compound, methyl bromide is currently scheduled to be banned in the United States by the year 2005, and its present use is under restriction.
Methyl bromide has been used directly on the watermelon crop in Florida to a limited extent, since many watermelon growers are able to avoid the buildup of soil-borne pest populations by renting pasture land that has been out of watermelon production for several years (7). When applied as a preplant soil fumigant to watermelons under plastic mulch, it is combined with a low concentration of chloropicrin to detect escaping fumes and control diseases. The material is injected with pressurized nitrogen gas into the soil at a depth of six to eight inches (15 to 20 cm) (23). Cost of methyl bromide in 2002 ($1,200 to $1,800 an acre) largely prohibits the use of this gas for watermelon production. Indeed, no methyl bromide was reported as being used in this year, while the alternative (dichloropropene) was reportedly used on 14 percent of Florida production area. More often, watermelon growers rely indirectly on the use of methyl bromide, when they plant watermelons in a double-crop mulched bed system following tomato or pepper. The pest-suppressing effects of methyl bromide applications to the first crop carry over to the watermelon crop (7).
A model developed in Florida to evaluate the impact on the production of the state’s vegetables as a result of the loss of methyl bromide estimated a 40 percent drop in watermelon production and a 35 percent reduction in acreage. Furthermore, the model predicts that reduced production of most of Florida’s vegetables affected by the loss of methyl bromide will be replaced by production from Mexico and Texas, whose production does not rely on methyl bromide. However, loss of watermelon production is not thought likely to be offset by increased production from other areas, because there are currently no other significant suppliers for the May watermelon shipments, whose production relies on methyl bromide in Florida. Earlier and later Florida shipments will not change, because their production does not rely on methyl bromide. Overall, a price increase of 11.2 percent is expected for watermelons as a result of the loss of methyl bromide (7).
Alternatives to Methyl Bromide
Researchers evaluating alternatives have concluded that it will not be possible to replace methyl bromide with one management tactic, but rather a combination of tactics, which will vary according to the crop. The fumigant cannot be replaced with a single chemical, but rather a combination of fumigants and non-fumigants. For example, the mix of fumigants (such as 1,3-dichloropropene and chloropicrin) with an additional herbicide treatment is thought to give the best yield response for tomato and strawberry production in Florida (24). However, there are no herbicides available that can adequately control nutsedge, the most troublesome weed in watermelon production. Methyl bromide is currently the only effective control for nutsedge available to Florida watermelon producers. Although the polyethylene mulch with which methyl bromide is used can itself suppress weed growth, nutsedge is able to penetrate through the mulch.
Although many alternative management practices are currently under evaluation, including the use of cover crops, organic amendments, biological control agents, crop rotation, hot water/steam treatment, paper and plastic mulch, pest resistant crop varieties, solarization, natural product pesticides, supplemental fertilization, and fallowing, several have already been cited as being impractical under Florida conditions. For example, steam/hot water treatment is costly under present technology, flooding can only be used by growers in certain parts of the state, and soil solarization is not only less effective on well-drained, sandy soils, but the hottest part of the year in Florida experiences heavy rainfall, resulting in water accumulation above the plastic that impedes soil heating (7,24). In general, alternative cultural controls such as flooding, fallowing, and soil solarization are carried out during the summer months, in preparation for fall-cropped vegetable production, but watermelons are not planted until the winter and spring months.
Several of the proposed alternatives to methyl bromide show promise as part of a comprehensive Integrated Pest Management (IPM) program. However, the conditions under which each will be most effective must still be worked out. Substantial research is required to determine which combinations of the proposed alternatives are most effective for the management of soil-borne pests in watermelon production in different regions of the state. Evaluations of alternatives for watermelon production are needed both for the crop when planted alone and when double-cropped following a higher value crop (24).
The principal insect and mite pests on watermelons in Florida are aphids, rindworms (caterpillars feeding on the rind, which can include beet and fall armyworms, cabbage looper, tobacco budworm, corn earworm, saltmarsh caterpillar, and others), whiteflies, and thrips. Occasional or minor arthropod pests include seedcorn maggot, cutworms, leafminers, cucumber beetles, mole crickets, wireworms, white-fringed beetle larvae, mites, and flea beetles. Leafhoppers and fleahoppers, grasshoppers, omnivorous leafrollers, plant bugs (including lygus bugs and stink bugs), and squash bugs may occasionally be seen on watermelons but are not economically damaging (8,20,25-27).
APHIDS [melon aphid (Aphis gossypii), green peach aphid (Myzus persicae), cowpea aphid (Aphis craccivora), spirea aphid (Aphis spiraecola), Aphis middletonii, and Uroleucon pseudambrosiae, among others]. Aphids constitute the principal insect pest of watermelons in Florida, primarily because of their role in virus transmission. Melon aphid, green peach aphid, and cowpea aphid feed and reproduce on watermelons and other cucurbits. The other aphids listed are important virus vectors but do not colonize watermelons. The most abundant aphid that colonizes watermelons in Florida is the melon aphid, which is a major pest of cucurbits and cotton and also attacks many other plants, including eggplant, pepper, potato, citrus, okra, and a range of ornamentals and weed species (27).
Aphids feed by piercing plant tissue with their needle-like mouthparts (stylets) and sucking out water and nutrients from the vascular system of the plant. Feeding damage and injury from toxins in the saliva that are injected into the plant tissue during feeding result in thickening, crumpling, and downward curling of leaves. Heavy aphid attack may kill very young plants. Research in Texas has shown that direct losses from aphid feeding injury on watermelons are avoided when populations are maintained below ten aphids per leaf. Aphids also deposit large amounts of honeydew on the plant surface, which encourages the growth of sooty mold. A short life cycle and reproduction by live birth allow aphid populations to increase rapidly in Florida (28).
Aphids damage watermelon plants not only directly by feeding, but also indirectly by transmitting viruses. The three principal viruses affecting watermelons in Florida (papaya ringspot virus type W, watermelon mosaic virus 2, and zucchini yellow mosaic virus) can be transmitted not only by aphids that colonize watermelons, but also by a number of aphid species that do not reproduce on watermelons. These include A. middletonii, A. spiraecola (spirea aphid, also known as green citrus aphid), and U. pseudambrosiae (29).
These viruses are transmitted by aphids in a stylet-borne, nonpersistent manner, meaning that an aphid can pick up virus particles in its stylet from an infected plant and transfer them immediately to a healthy plant without the virus circulating through the body of the aphid. There is no delay from when the aphid acquires the virus to when it can transmit it, but the aphid is able to transmit the virus for only a short period of time. An aphid determines the suitability of a plant as a food source by probing (inserting its stylet into the plant’s tissue to test it). As a result, viruses can be transmitted by aphids that do not actually feed on the plant but only land on it momentarily to probe. Aphid vectors that do not feed on watermelons will move from plant to plant within a field, probing each one and spreading the virus. Aphids that do feed and reproduce on watermelons may also move from one plant to another within the field under crowded conditions, when winged individuals are produced. Virus transmission to watermelon plants can occur within 10 to 15 seconds (29).
RINDWORM COMPLEX. Any caterpillars that feed on the rind of watermelons are called rindworms, including beet armyworm (Spodoptera exigua), fall armyworm (Spodoptera frugiperda), yellow-striped armyworm (Spodoptera ornithogalli), cabbage looper (Trichoplusia ni), tobacco budworm (Heliothis virescens), corn earworm (Helicoverpa zea), saltmarsh caterpillar (Estigmene acrea), and granulate cutworm (Feltia subterranea). Many rindworms also feed on the foliage and stems of the plant, although greatest damage occurs from feeding on the rind, which directly reduces quality and marketability of the fruit. Rindworms generally scar the surface of the fruit in irregular patterns, rather than boring into it. Larger caterpillars inflict greater damage, and management is generally easier when the caterpillars are smaller. Rindworms are often found on the underside or least exposed parts of the watermelon fruit (29).
Beet armyworm and cabbage looper are presently the most abundant rindworms on watermelons in Florida. The cabbage looper has a wide host range, which in addition to watermelons includes cabbage and other crucifers, potato, spinach, tomato, cucumber, cotton, and soybeans. After overwintering in Florida and nearby states, it migrates north each year. Females lay their eggs on both leaf surfaces of host plants, and each of the emerging larvae (caterpillars) feeds for two to four weeks, then spins a cocoon and pupates. Between ten days and two weeks later, the adult moth emerges. There are several generations each year in Florida (29).
Beet armyworm is active throughout the year in south Florida, where many generations occur annually. A monitoring program has existed in south Florida for several years, and trap captures of beet armyworm moths are highest from late March through mid-June, corresponding to the later part of the leafy vegetable season in the region. During this time, the population rises and falls in cycles of three to five weeks. Another smaller increase in beet armyworm population generally occurs from mid-August through October and is likely related to the presence of late summer weeds (30). From south Florida, beet armyworm moths migrate into north Florida and other southeastern states each year. Females can lay over 600 eggs each, usually in groups of about 100. Eggs are laid on the underside of lower leaves. The larvae feed from one to three weeks, in groups when younger and scattered on the plant when larger. After pupating in a cocoon, the adult emerges after about one week (29).
WHITEFLIES (silverleaf whitefly (Bemisia argentifolii), sweet potato whitefly (Bemisia tabaci), and greenhouse whitefly (Trialeurodes vaporariorum). The silverleaf whitefly, sweet potato whitefly, and greenhouse whitefly can all infest watermelons. However, the most prevalent in Florida is the silverleaf whitefly, previously known as strain B of the sweet potato whitefly, which is a major pest of cucurbits and other vegetable crops in the state. The first U.S. outbreak of the silverleaf whitefly was recorded in south-central Florida in 1986, and its aggressive establishment and associated insecticide resistance have led to heavy infestations in the state's vegetable and ornamentals industries since then (31).
With a host range of over 500 species of plants, the silverleaf whitefly has been observed to reproduce on at least 15 crops and 20 weed species in Florida. Whitefly populations commonly peak on the state’s crops at the time of harvest, as the whitefly migrates from crop to crop throughout the year. In south Florida, populations build on fall vegetables and move directly to overlapping spring crops. In south-central Florida, the whitefly uses cabbage and other winter crops to survive until the spring. Over the summer fallow period, whitefly populations are low, because during that time whiteflies are limited to weeds, such as water primrose, hairy indigo, and spurge. Weeds are poor hosts to the whitefly and usually harbor many natural enemies that reduce populations further (31,32).
Whiteflies damage watermelon plants by removing plant sap, depleting the plant of needed nutrients. On some hosts, whiteflies cause severe damage by transmitting plant viruses. On watermelons, whiteflies have been reported to transmit squash leaf curl and lettuce infectious yellows, but these viruses do not presently occur in Florida. The principal damage inflicted by whiteflies to watermelons in Florida is due to feeding, which under heavy infestations can greatly reduce the plants’ vigor (29).
Adult females produce an average of 160 eggs each, depositing them on the lower surface of host plant leaves. The first nymphal (immature) stage, the crawler stage, attaches itself to the leaf near the empty egg case. The whitefly passes through three more sedentary nymphal stages, appearing like transparent scales, before molting to the adult stage. Whiteflies feed by sucking the plant’s sap through their needle-like piercing-sucking mouthparts. Like aphids, they extract large amounts of the plant’s sap (phloem), excreting the excess liquid as honeydew, upon which sooty mold can grow (29,31,33).
Also like aphids, whiteflies are found mainly on the lower leaf surface, and only the first nymphal stage and the adults can move, so the effectiveness of insecticidal control is limited if complete coverage is not achieved. Alternation of insecticides is also important to minimize resistance development. Management of whiteflies requires a combination of tactics, with an emphasis on cultural control.
THRIPS [tobacco thrips (Frankliniella fusca) and melon thrips (Thrips palmi). Tobacco thrips is a problem on watermelons in central and north Florida and is rarely found in south Florida. They damage watermelons primarily in the seedling stage, with little or no damage observed once plants begin to grow rapidly. Feeding, which mainly occurs in the terminal buds, produces scarred leaves with silvery or clear areas that look as if they were damaged by blowing sand. Crinkling of tissue and the appearance of chlorotic areas can also result. Heavy infestations can stunt watermelon plants (26,29).
Melon thrips is an important insect pest on watermelons in south Florida and is only found in that part of the state, where it is a relatively new pest, having been first reported in early 1991 in the Homestead area (Miami-Dade County). It has caused large economic losses in several vegetable crops, particularly snap bean, pepper, eggplant, cucumber, winter melon, and squash. In addition to those crops and watermelons, melon thrips also feeds on cantaloupe, potato, tobacco, soybean, broad bean, spinach, and amaranth. Both adult and immature thrips feed in groups on leaves, stems, flowers and fruits, removing sap with their sucking mouthparts. Feeding by melon thrips on watermelons results in a bronzed appearance on leaves. However, greatest damage occurs from feeding at the tips of vines, which restricts canopy development (29,34-36).
Melon thrips is present throughout the growing season in south Florida but is most abundant between December and April. Completion of the life cycle requires about 20 days at 86°F (30°C) and about 80 days at 59°F (15°C). Each female thrips produces an average of 50 eggs, which are deposited in slits the female makes in the leaf tissue. Larvae usually feed on older leaves, concentrating along the leaf midrib and veins. After passing through two larval instars in about four to five days (at 79 to 90°F, or 26 to 32°C), the larva drops to the ground, where it passes the prepupal and pupal stages in a soil chamber it has constructed. After three to four days (at 79 to 90°F), the adult emerges and seeks out new host plant leaves on which to feed (29).
The relationship between early damage from thrips and watermelon yield has not been determined. Melon thrips is resistant to many insecticides. In addition, the use of broad-spectrum insecticides may increase populations of melon thrips by killing off its natural enemies, which are thought to contribute to thrips management in the field. Use of insecticides also requires thorough spray coverage and is not effective against the egg and pupal stages, which are protected (28,29,36).
SEEDCORN MAGGOT (Delia platura). Seedcorn maggot often reduces plant stand in central Florida by attacking young emerging seedlings. Adults of seedcorn maggot are small flies that become active early in the spring, laying their eggs where organic matter is high, such as in manure or where cover crops have been plowed under. Eggs are also laid at the base of transplants. Upon hatching, the maggots enter the seed or the stem of the developing seedling. Injury from seedcorn maggot feeding results in wilting and eventually death of the plant. Maggots feed on organic matter for two to three weeks, passing through three larval stages (instars) during that time. As the weather warms, the adult flies seek wooded areas, where the insect oversummers in the pupal stage. The flies are most active during cool, wet springs. Transplanting or seeding into soils with temperatures of at least 72EF (21.3EC) at a depth of 4 inches (10 cm) will reduce likelihood of damage from seedcorn maggot (29).
CUTWORMS [granulate cutworm, (Feltia subterranea), and others]. Cutworms are stout caterpillars with a dull, greasy appearance. Several cutworms, particularly granulate cutworm, can attack watermelons in Florida (25). Cutworms are exclusively a problem for young watermelon seedlings. Retreating to the soil or under surface litter during the day, they become active at night. They often cut the main stem of the seedling at the base and can greatly reduce stands of newly emerged watermelon plants. Cutworms can also climb the stem of the plant to feed on foliage. Being more prevalent in spring and fall, they overwinter in the larval stage, often in grasslands. Baits applied for cutworm control should be laid out in the afternoon or early evening for maximum effectiveness (29).
LEAFMINERS (Liriomyza sativae and Liriomyza trifolii). Leafminers are occasionally a major pest of watermelons in south Florida. The adult fly punctures the upper leaf surface while feeding, and the female inserts eggs into the puncture holes. When each larva (maggot) emerges, it feeds between the upper and lower leaf surfaces, creating a tunnel or mine that winds across the leaf and becomes larger as the maggot grows. When larval development is complete, after about two weeks, the maggot cuts through the leaf surface and emerges from its mine, dropping to the soil to pupate. Where watermelons are grown on plastic mulched beds, leafminer pupae can be seen on the surface of the plastic. Leafminer populations can increase rapidly in Florida, where the life cycle can be as short as 18 to 21 days (29).
Infestations are often more severe late in the season. Under heavy infestations, leaves may be nearly covered with mines, and defoliation late in the season can result in sunscald damage on the fruit. Adequately managing leafminer populations in watermelon seedlings is particularly important, because the mines can serve as entry points for the fungus that causes gummy stem blight, an important watermelon disease in Florida (15,29).
CUCUMBER BEETLES [banded cucumber beetle (Diabrotica balteata), striped cucumber beetle (Acalymma vittata), spotted cucumber beetle (D. undecimpunctata howardi)]. Cucumber beetles occasionally feed on watermelons in Florida to the degree that control measures become necessary. Banded cucumber beetle is more prevalent in south Florida, while in north Florida, spotted cucumber beetle is the most common. Striped cucumber beetle is seen only occasionally, primarily in western and northern Florida (37).
Cucumber beetles prefer to feed on cucurbit plants like squash, cucumber, and cantaloupe in addition to watermelon, but can also feed on a wide range of crops, including corn, beet, pea, sweet potato, okra, lettuce, onion, cabbage, potato, tomato. Spotted cucumber beetle, whose larvae are known as southern corn rootworm, is the most general feeder of the three, having been recorded from over 200 species of crops, grasses, and weeds. Feeding damage from adult cucumber beetles results in ragged holes in the leaves, and the beetles may also feed on stems. The larvae, which are found in the soil, may feed on the roots of the vines and the underside of the watermelon fruits laying on the soil. Greatest effect on watermelon yield occurs when plants are attacked at the seedling stage. Older plants can withstand higher amounts of feeding damage. When cucumber beetles are found on ten percent of seedlings, control measures should be instituted (37-39).
Cucumber beetles have many generations each year. The life cycle of the banded cucumber beetle may be as short as 45 days under optimal conditions. Over a period of two to eight weeks, each female banded cucumber beetle deposits between two and 15 clusters of up to 100 eggs each in soil cracks, with up to 850 eggs being deposited by each female. Striped cucumber beetle females lay approximately 100 eggs each, at the base of host plants. Five to nine days later, the larvae hatch. After passing through three instars in approximately 11 to 17 days for banded cucumber beetles (somewhat longer for striped and spotted cucumber beetles), during which time the larvae feed on roots and tunnel through stems of host plants, pupation occurs in the soil. Adult banded cucumber beetles emerge after four to six days. About 16 days after adults emerge, females begin to lay eggs. Adults live for 17 to 44 days, with average longevity of 26 days (37,38).
MOLE CRICKETS (Scapteriscus spp.). Mole crickets occasionally present a problem to watermelon producers in Florida. They damage young seedlings by feeding on and tunneling in and around the roots, often girdling the stems at their base. Their presence can be confirmed easily by the tunnels they make just below the soil surface. Mole crickets are active at night, particularly on warm nights when soil is moist, and they are rarely seen during the day (40).
Mole crickets overwinter as adults or older nymphs, often a foot or more below the soil surface. Adults deposit their eggs in chambers they make in the soil, usually four to twelve inches below the surface. Nymphs hatch after about three weeks, and they feed for many weeks before reaching the adult stage. Although mole crickets may attack many vegetable crops, they prefer pasture and grass, where they are generally found in great numbers in Florida. When pastureland is rotated to watermelons, a common practice in north Florida, mole crickets are more likely to be a problem (40,41).
WIREWORMS (Conoderus spp.). Wireworms, which are larvae of click beetles, are among the most destructive of soil insect pests. Conoderus spp. wireworms, including C. rudis, C. amplicollis (Gulf wireworm), C. falli (southern potato wireworm), and C. verpertinus (tobacco wireworm), are the most common on vegetable crops in Florida. Wireworms cause greatest damage to germinating seeds, and transplants are generally less susceptible. In addition to attacking seeds before or at germination, wireworms can bore into the tap root and tunnel up the stem or feed on smaller roots of seedlings. They can cause sudden stand reductions by quickly attacking young seedlings, causing wilting, stunting, and death. Click beetles lay their eggs in the soil near plant roots, and upon hatching, the wireworms feed on the nearby roots. Wireworms may spend up to several years in the soil in the larval stage and may be present at a soil depth of 1 to 5 feet (0.3 to 1.5 meters) (10,28).
Like mole crickets, wireworms are more numerous in pasture and grass, so watermelons planted to land previously in pasture is likely to experience greater wireworm problems. In addition, soil insects such as wireworms are likely to increase in importance when methyl bromide is no longer available. Chemical controls for wireworms must be applied before or at planting.
WHITE-FRINGED BEETLE (Graphognathus spp.). White-fringed beetle is an occasional pest on watermelons and other crops in north Florida. It prefers cotton, peanut, okra, soybean, cowpea, sweet potato, beans, and peas, but most vegetable and field crops are attacked by this pest, which has a host range of over 385 species. Larvae (grubs) feed on roots, tubers, underground stems, and dead plant matter, and can inflict serious damage to the taproot by boring into it. In addition to directly killing the seedling, grub damage to roots leaves the plant susceptible to root infections. White-fringed beetle grubs may damage plants in only a few scattered areas within a field or may affect nearly all plants within a field. Adults feed on leaves of host plants, cutting out notches along the leaf margins. They lay eggs in the soil at the base of the plant, and the grub stage can live up to two years in the soil. There are currently no insecticides effective against the grub, although several insecticides are available for adult white-fringed beetles. The presence of white-fringed beetle grubs is best determined by the crop history of the land. Watermelons should not be planted to land infested with white-fringed beetle grubs (42).
SPIDER MITES. Spider mites, the most common type of mite affecting crop plants, are occasionally pests on watermelons in Florida. They feed on the underside of leaves, primarily along the midribs and lateral veins. After piercing the leaf surface, they suck up the plant’s sap. Watermelon plants turn pale, then yellow, and eventually brown, and the plant appears dusty as a result of mite feeding. Under severe infestations, foliage dries up and dies. When many mites are present, silken webs and white flakes of molted skin can be observed on the leaf surface. Mites move from one plant to another by parachuting through the air on their webs, using the wind. When their path is interrupted by an obstacle, they crawl down and seek out host plants. Infestations therefore begin along the edges of fields and close to high objects such as fences or trees.
Mites prefer hot, dry conditions, under which their populations may increase rapidly. Each female may produce up to 19 eggs per day and a total of up to 100 eggs. The larvae hatch after six to 19 days and begin to feed. Mites experience a resting period after the larval stage, then pass through two nymphal stages, with another resting period after each one. Maturity into adults may take as few as five days or as many as 20 days, depending upon the temperature. Under warm Florida conditions, the life cycle from egg to adult is often seven days. Few chemical controls effective against mites are available for use on watermelons. However, when considering the use of chemical controls, the age structure of the population should be determined. When many nymphs are present, the population is probably increasing, whereas if many adults are present, particularly males, it is probably declining. In addition, if a miticide is used, a second application is necessary in Florida between five and seven days after the first. The second application kills the mites that escaped the first application because they were in the egg stage. Timing is critical to prevent those mites from maturing and laying eggs (28).
In 2002, Florida growers applied insecticides/miticides totaling 12,700 pounds of active ingredient to 75 percent of the state’s watermelon acreage. During the years in which usage data have been collected, between 38 and 75 percent of watermelon acreage has been treated with insecticides each year, with total annual usage ranging from 12,700 to 47,400 pounds of active ingredient (43-48). The published applied insecticides/miticides on Florida watermelons in 2002 were B.t., endosulfan, esfenvalerate, permethrin, and carbaryl. There was also reported use of abamectin, azinphos-methyl, bifenthrin, diazinon, dimethoate, imidacloprid, malathion, methomyl, oxamyl, and spinosad. Other insecticides registered for watermelon in Florida in 2003 were azadirachtin, Beauveria bassiana, cyromazine, cryolite, carbofuran, dicofol, fenpropathrin, kaolin, oils, oxydemeton-methyl, pymetrozine, pyrethrins + rotenone or silicon dioxide, soaps, sulfur, and thiamethoxam.
Bacillus thuringiensis. The biopesticide Bacillus thuringiensis (B.t.) is an important management tool for Florida watermelon growers, who use it every year in the management of the rindworm complex (beet and fall armyworms, cabbage looper, cutworms, corn earworm, and other caterpillars that feed on the rind of the melon). B.t. is a naturally occurring soil bacterium that produces spores and crystalline bodies that act as stomach poison to the insects that consume it. The most common formulations are highly specific for lepidopterous larvae (caterpillars) and therefore do not harm beneficial organisms. However, it is most effective against smaller larvae. The median price of B.t. is $160.00 per pound of active ingredient (49). B.t. may be applied up to the day of harvest (PHI=0 days), and the restricted entry interval (REI) under the Worker Protection Standard is 4 hours.
In 2002, Florida growers applied B.t. to 30 percent of their watermelon acreage, an average of 5.0 times. During the years in which usage data have been collected, watermelon growers in Florida have applied B.t. to between 12 and 31 percent of their watermelon acreage, each making an average of 3.4 to 7.9 applications per year (43-48). Information on average rate and total pounds of active ingredient applied is not available, because amounts of active ingredient are not comparable among products.
ENDOSULFAN. Endosulfan is a cyclodiene chlorinated hydrocarbon insecticide and miticide that acts as a contact poison and is effective against a range of pests, including aphids, whiteflies, rindworms, cabbage looper, leafminers, cucumber beetles, squash bug, squash vine borer, flea beetles, mites, and omnivorous leafroller (25). Florida watermelon growers most commonly use endosulfan as a foliar spray to control colonizing aphids. Although pesticide applications to control aphid-vectored viruses in other crops have been demonstrated to actually increase virus spread by disturbing aphids, the use of endosulfan on watermelons in Florida has been shown not to increase or decrease the spread of aphid-transmitted viruses. However, sprays of insecticides such as endosulfan are not able to kill aphids quickly enough to prevent virus spread, due to the nonpersistent manner of virus transmission by the aphids (50).
The median price of endosulfan is $11.67 per pound of active ingredient, and the approximate cost per labeled application (1.0 lb ai/A) was $11.67 per acre (51,52). Endosulfan may be applied up 2 days before harvest (PHI=2 days), and the restricted entry interval (REI) under the Worker Protection Standard is 24 hours. No more than six applications may be made per acre per year, with a maximum of 3.0 pounds of active ingredient per acre each year (52).
In 2002, Florida growers applied an average of 0.68 pounds of active ingredient per application to 20 percent of their watermelon acreage, an average of 1.5 times. Total usage was 5,200 pounds of active ingredient. During the years in which usage data have been collected, watermelon growers in Florida have applied endosulfan at an average rate ranging from 0.39 to 0.89 pounds of active ingredient per acre at each application, to between 7 and 20 percent of their watermelon acreage. Growers have made an average number of applications ranging from 1.5 to 5.2 each year, totaling between 4,900 and 21,600 pounds of active ingredient annually (43-48).
PERMETHRIN. Permethrin is a broad-spectrum, synthetic pyrethroid insecticide. Watermelon growers in Florida use permethrin during most years, mainly in the management of aphids, leafminers, cutworms, and rindworms. It can also be used for melonworm, pickleworm, cucumber beetles, cabbage looper, plant bugs (including lygus bugs and stink bugs), leafhoppers, squash bug, and squash vine borer (25). The median price of permethrin is $56.32 per pound of active ingredient, and the approximate cost per labeled application (0.2 lb ai/A) was $11.26 per acre (51,53). Permethrin may be applied up to the day of harvest (PHI=0 days), but the REI under the Worker Protection Standard is 12 hours. A maximum of 1.6 pounds of active ingredient may be applied per acre during each season (53).
In 2002, Florida growers applied an average of 0.11 pounds of active ingredient per application to 15 percent of their watermelon acreage, an average of 6.5 times. Total usage was 2,800 pounds of active ingredient. During the years in which usage data have been collected, watermelon growers in Florida have applied permethrin at an average rate ranging from 0.10 to 0.19 pounds of active ingredient per acre at each application, to between 11 and 22 percent of their watermelon acreage. Growers have made an average number of applications ranging from 2.3 to 8.4 each year, totaling between 700 and 9,800 pounds of active ingredient annually (43-48).
ESFENVALERATE. Esfenvalerate is another broad-spectrum, synthetic pyrethroid insecticide used on watermelons in Florida during most years, primarily in the management of cutworms, corn earworm, and cabbage looper. It also helps in managing cucumber beetle adults, leafhoppers, grasshoppers, plant bugs (including lygus bugs and stink bugs), squash bugs, and squash vine borers (25). The median price of esfenvalerate is $163.64 per pound of active ingredient, and the approximate cost per labeled application (0.05 lb ai/A) was $8.18 per acre (51,54). Esfenvalerate may be applied up to 3 days before harvest (PHI=3 days), and the REI under the Worker Protection Standard is 12 hours. A maximum of 0.25 pounds of active ingredient may be applied per acre during each season (54).
In 2002, Florida growers applied an average of 0.03 pounds of active ingredient per application to 18 percent of their watermelon acreage, an average of 1.8 times. Total usage was 300 pounds of active ingredient. During the years in which usage data have been collected, watermelon growers in Florida have applied esfenvalerate at an average rate ranging from 0.03 to 0.07 pounds of active ingredient per acre at each application, to between 4 and 18 percent of their watermelon acreage. Growers have made an average number of applications ranging from 1.8 to 4.7 each year, totaling between 100 and 1,300 pounds of active ingredient annually (43-48).
CARBARYL. Carbaryl is a broad-spectrum, carbamate insecticide used occasionally by Florida watermelon growers in the management of cutworms, armyworms, cucumber beetles, flea beetles, leafhoppers, and squash bug (25). The median price of carbaryl is $7.17 per pound of active ingredient, and the approximate cost per labeled application (1.0 lb ai/A) was $7.17 per acre (51,52). Carbaryl may be applied up to 3 days before harvest (PHI=3 days), and the REI under the Worker Protection Standard is 12 hours. A maximum of 6 pounds of active ingredient may be applied per acre during each season. When carbaryl is applied to watermelon plants under Florida conditions, a phytotoxic reaction causing burning may result (25).
In 2002, Florida growers applied an average of 0.38 pounds of active ingredient per application to 3 percent of their watermelon acreage, an average of 2.4 times. Total usage was 700 pounds of active ingredient. During the years in which usage data have been collected, watermelon growers in Florida have applied carbaryl at an average rate ranging from 0.38 to 1.63 pounds of active ingredient per acre at each application, to between 3 and 5 percent of their watermelon acreage. Growers have made an average number of applications ranging from 2.4 to 3.0 each year, totaling between 700 and 12,900 pounds of active ingredient annually (43-48).
Over the past few years, there have been a number of new "reduced risk" materials registered for use on watermelon. These materials are generally more selective that those of historic use. These active ingredients include compounds such as pymetrozine and thiamethoxam. These materials are currently being assessed for fit into established IPM systems.
Planting early is one of the most effective cultural controls for most insect pests of watermelons. Since early production is an important goal of Florida watermelon growers as a result of market considerations, this practice is common. Early planting even for the fall crop is useful, because aphid populations tend to decline in mid-summer and do not build up again until later in the season (25).
To reduce aphid populations and the resulting spread of aphid-vectored viruses, watermelons planted during the summer for a fall harvest should be planted far from fields that were in cucurbit production the previous spring. In addition, weeds in and around the field that may harbor aphids should be eliminated. The use of mineral oil (stylet oil) can disrupt the transmission of nonpersistent viruses by interfering with the transfer of virus particles to and from aphid mouthparts. Oil sprays on watermelons in Florida applied at four to five day intervals beginning early in the season have been shown to delay primary infection, thereby slowing the start of viral epidemics in the crop and reducing viral spread. Stylet oil is most effective when virus incidence is low (less than 20 percent). Low virus incidence occurs under conditions of limited inoculum, so stylet oil is most effective for spring-planted watermelons in Florida. Economic analysis has shown that the cost of 12 to 15 applications of stylet oil over the season is offset if the oil increases yield by 5 to 7 percent (14,50).
The use of reflective mulch, usually black plastic painted with aluminum, or white mulch, has also been attempted to repel aphids, preventing them from landing on the watermelon plants. However, the effectiveness of reflective mulches has been limited, particularly as the season progresses and the plants’ foliage covers the mulch. In addition, white or reflective mulches delay warming of the soil in the early spring, a key concern for watermelon producers in Florida (25).
Populations of aphids, whiteflies, and thrips are all affected by surrounding crops and vegetation, and proper management of weeds and crop residues is an important management tactic. Destruction of crop residues in and around fields can also prevent movement of leafminers, which can be particularly damaging when entering late-planted watermelon fields. In addition to destroying crop residues and avoiding planting near infested crops, cultural controls for whiteflies include early planting, rotating with non-susceptible crops, using reflective mulches, and using physical barriers such as floating row covers (29,31).
Turning the soil several times to expose wireworms to sunlight is commonly practiced by Florida vegetable growers. Populations of all soil insects can be reduced by deep plowing at least 30 days before planting, by exposing the insects to bird predation in addition to harsh environmental conditions, as well as denying them grass and weed sources that serve as alternate food. White-fringed beetle grubs can best be managed by planting infested land to grass or grains for extended periods (29).
Aphid predators (larvae of ladybird beetles, syrphid flies, and lacewings) and parasitic wasps have been observed to reduce aphid populations in north-central Florida later in the season. However, aphid natural enemies are not present in great enough numbers during the early part of the season to effectively manage aphid populations alone (55).
A number of predators, pathogens, and parasitic insects naturally attack rindworm complex caterpillars in Florida. For example, cabbage loopers have been found to be infected with nuclear polyhedrosis virus and attacked by a number of parasitic insects, including wasps and tachinid flies. A protozoan (Microsporidium sp.) has been observed to attack granulate cutworm, and beet armyworm is attacked by both fungal pathogens and nuclear polyhedrosis virus (29).
Leafminer populations in watermelons are kept down by a number of parasites. However, natural enemies are susceptible to toxic pesticides used on the crop (29). Likewise, in the absence of broad-spectrum insecticides, whitefly populations are naturally managed by several types of organisms. Predators include green lacewing larvae and ladybird beetles. Whiteflies are also killed by parasitic wasps such as Encarsia spp. and Eretmocerus spp., as well as by disease-causing fungi such as Beauveria, Paecilomyces and Verticillium. Insect pathogens for management of the silverleaf whitefly are currently being studied under field conditions, but commercial formulations are not yet available (31).
Several natural control agents of melon thrips have been evaluated for their potential to manage the pest in Florida, including the eulophid wasp Ceranisus menes and the predatory mite Neoseiulus cucumeris, but research on the effectiveness of these and other natural enemies in managing thrips on the watermelon crop has yet to be undertaken (56,57). Several fungal pathogens are also known to infect melon thrips, including Neozygites parvispora, Verticillium lecanii, Hirsutella sp., Beauveria bassiana, and Paecilomyces fumosoroseus. Preliminary studies of the latter two species in Florida show potential for thrips management, but more research is needed to determine the effect of these fungi on thrips populations under field conditions (58).
Severe damage to Florida turfgrass, pasture grass and vegetables from mole crickets has resulted in research efforts on several biological controls. Presently, the most effective biological control agent for mole crickets is a steinernematid nematode, which was first brought to Florida from Uruguay in 1985 and identified as Steinernema scapterisci (59). This parasitic nematode has shown promise for managing mole crickets in pasture and turf in Florida, and is available commercially for mole cricket control in turf. It has been shown to be highly effective against tawny mole crickets and less effective against short-winged mole crickets. It is most effective as a biocontrol agent where mole cricket populations are highest, as in pastures. The nematode can also be applied as a biopesticide where mole cricket populations are lower, and it shows residual activity. The nematode disperses well when applied and has been recovered from infected mole crickets years after its application. One of the reasons for its effectiveness is that mole crickets in north Florida have only one generation each year, while the nematode, under appropriate conditions, could in theory produce a new generation every ten days during eight months of the year. Populations of the nematode have become established in small areas of several Florida counties. If it becomes established in pastures surrounding vegetable crop production areas, it is expected to keep mole cricket populations below damaging levels (41,60).
Diseases, particularly those produced by viruses, constitute the most serious pest problem on watermelons in Florida. The state’s warm, moist climate, as well as the overlapping progression of watermelon plantings during the season from south Florida to the northernmost regions, create conditions ideal for disease development. The incidence of the various diseases affecting watermelons in Florida varies by region, given the climatic differences within the state. Intensity of management efforts also varies by region. Growers in south Florida generally experience greater disease pressure, but earlier harvests in that region provide higher market prices, which give them the resources to aggressively manage diseases (61).
Three potyviruses that infect watermelon (papaya ringspot virus, watermelon mosaic virus 2, and zucchini yellow mosaic virus) occur annually in Florida. The viruses are spread by aphid vectors from infected watermelon plants or weeds to healthy watermelon plants (61). Since the aphids acquire and spread the viruses in a nonpersistent manner, as discussed in the insect management section, insecticidal control is not effective. Generally, growers have observed spray efforts to be futile and instead concentrate their management efforts on other tactics. Cucurbit viruses are among the most difficult of Florida’s pests to manage. Loss to the Florida watermelon crop as a result of viruses varies each year and by place. For example, little damage occurred from viruses in 1998 as a result of their late arrival that year, but in any year, fields with losses of 50 to 100 percent are not uncommon (61).
The most important fungal diseases are gummy stem blight (caused by Didymella bryoniae/Phoma cucurbitacearum) and downy mildew (caused by Pseudoperonospora cubensis). Some consider gummy stem blight to be the most troublesome disease on watermelons in Florida. Other diseases that are occasional or minor problems include phytophthora blight (caused by Phytophthora capsici), bacterial fruit blotch (caused by Acidovorax avenae subsp. citrulli), alternaria leafspot (caused by Alternaria cucumerina), seedling blight (caused by Pythium spp., Rhizoctonia solani, and Fusarium spp.), fusarium wilt (caused by Fusarium oxysporum f.sp. niveum), angular leafspot (caused by Pseudomonas syringae), anthracnose (caused by Colletotrichum orbiculare), rind necrosis (usually caused by Erwinia spp.), and powdery mildew (caused by Erysiphe cichoracearum). Blossom end rot, a physiological disorder related to calcium deficiency and water stress, is also an occasional problem. Finally, Cercospora leafspot (caused by Cercospora citrullina) and southern blight (also called southern stem rot or white mold and caused by Sclerotium rolfsii) are only rarely seen on watermelons in Florida (6,8,61).
PAPAYA RINGSPOT VIRUS TYPE W (PRSV-W). Formerly referred to as watermelon mosaic virus 1, papaya ringspot virus type W is a greater problem in south Florida, where it can severely damage the crop. Epidemics of papaya ringspot virus have been linked to weeds as the primary source of inoculum (61). Papaya ringspot virus has been shown to overwinter in wild cucurbits such as wild balsam apple and creeping cucumber, from which it is spread to spring-planted watermelons in south Florida. During most years, the virus reaches central and north Florida in early summer (50).
WATERMELON MOSAIC VIRUS 2 (WMV-2). Watermelon mosaic virus 2 is another potyvirus causing regular problems for watermelon production, particularly in central and north Florida during the spring production season. Incidence of watermelon mosaic virus 2 in central Florida rarely exceeded five percent during the 1960s and 1970s. However, beginning in the late 1980s, the virus has caused severe losses to the spring watermelon crop in central and north Florida, with incidence in fields of up to 100 percent. Infection early in the season results in yield loss and reduced fruit quality due to blemishes, particularly rings and spots on the watermelon rind. However, if the virus does not enter a field until the time of fruit set, little to no yield difference is likely (27,50).
The virus is transmitted in a nonpersistent manner by numerous species of aphids, which in Florida include Myzus persicae, Aphis spiraecola, Aphis middletonii, Aphis illinoisensis, and Uroleucon pseudambrosiae (62-64). During the summer and fall, watermelon mosaic virus 2 builds up on many cucurbit and leguminous summer annuals, including hairy indigo (Indigofera hirsuta), showy crotalaria (Crotalaria spectabilis), alyceclover (Alysicarpus vaginalis), and Florida beggarweed (Desmodium tortuosum). These hosts serve as sources of inoculum for the fall watermelon crop, and their presence is probably the reason that fewer aphids are necessary to initiate virus epidemics on the fall crop than on the spring crop. The source of inoculum for the spring watermelon crop has not yet been identified. The major factor in the appearance of spring epidemics of WMV-2 in Florida is secondary spread of the virus, since primary infections in watermelon fields are limited (50).
ZUCCHINI YELLOW MOSAIC VIRUS (ZYMV). Zucchini yellow mosaic virus, the third potyvirus affecting watermelon production in the state, was first observed in Florida in 1981 and identified in 1984. At that time, researchers demonstrated that it caused mosaic symptoms in watermelons and other cucurbits and could be transmitted by the aphids Myzus persicae and Aphis spiraecola (65). The incidence of zucchini yellow mosaic virus is generally lower than that of the other two potyviruses that affect watermelons. In central Florida, zucchini yellow mosaic virus is most common in late spring and fall. The virus produces more severe symptoms than does watermelon mosaic virus 2, with resulting discoloration and distortion of fruits (50).
GUMMY STEM BLIGHT (caused by Didymella bryoniae/Phoma cucurbitacearum). Gummy stem blight is one of the most important diseases of watermelons in Florida. It can affect most above-ground parts of the watermelon plant. Symptoms, which may be difficult to distinguish from other foliar diseases, include brown to black leaf spots, stem cankers, or fruit spots. Lesions eventually develop black spots, which are the fruiting bodies of the fungus (pycnidia). A brown, gummy material may be produced on the surface of lesions. On watermelon fruit, the disease is referred to as black rot. It produces water-soaked spots on the fruit, from which gummy material leaks out (61,66).
The fungus causing gummy stem blight can be found on crop debris, in the air (as airborne spores), in and on contaminated seed, and on volunteer and wild cucurbits. The principal source of inoculum for the disease is infected debris from other cucurbit plants. Airborne spores (ascospores), which are important in field-to-field spread of the fungus, have been detected throughout the year in Florida, with a peak during June and July. The pycnidiospores, produced in the pycnidia, spread the pathogen from plant to plant within a field, principally in splashing rain. At least one hour of free moisture on leaves is required for infection to occur, and lesions continue to expand only when leaves are wet. Disease development is therefore most rapid when frequent rains occur (61,66).
DOWNY MILDEW (caused by Pseudoperonospora cubensis). Downy mildew, the other key fungal disease of watermelons in the state, occurs principally in south Florida, and its likelihood of occurrence in the state decreases with distance to the north. It occurs every year in south Florida, while in north Florida its presence is sporadic. Ideal conditions for the development of downy mildew include nighttime temperatures between 55 and 75EF (12.8 and 23.9EC) and relative humidity above 90 percent. Therefore, in south Florida, fall and spring planted watermelons may be infected with downy mildew as early as the appearance of the first true leaves (61).
Downy mildew attacks foliage, producing lesions on the leaves that can cause the plants to wilt and can kill them if they are severely infected early in their development. Initial symptoms include small yellow spots, which may be angular and which grow to one-half inch (1.3 cm) or more in diameter and turn brownish in color. Leaves of watermelon plants may curl upward dramatically as a result of the lesions. The disease can reduce yield and lower fruit quality. The principal source of inoculum in south Florida is probably volunteer watermelon plants. Spores are produced mainly on the underside of the leaves, within the lesions, and are dispersed by wind to other plants, primarily from late morning to midday. In the presence of moisture, spores that have landed on a leaf germinate and enter the leaf tissue. New lesions are produced within four to seven days, and downy mildew is able to spread rapidly if not controlled (61).
PHYTOPHTHORA BLIGHT (caused by Phytophthora capsici). Phytophthora blight occurs sporadically in Florida, but during weather favorable to the fungus, the disease can spread rapidly, causing serious losses. In 1998, the disease was widespread and severe on several vegetable crops in Florida. In the southwest region of the state (Lee, Collier, and Hendry Counties), 25 percent of watermelon plants in some surveyed fields were found to have the disease, while disease incidence on watermelons in Manatee County was 36 percent. Previously, phytophthora blight had affected only the fruit of watermelons, but during the 1998 outbreak, many watermelon plants were affected and died in Manatee County, regardless of plant age. In untreated test plots at the University of Florida Southwest Research and Education Center in Immokalee, 100 percent plant mortality occurred on watermelon plants of the variety ‘Regency’ (67,68).
The fungus causes a seed rot and seedling blight (damping-off). Seedlings, which may be discolored at the base, fall over. Under moist conditions, the white fungal growth can be seen. In mature watermelon plants, foliar symptoms appear as water-soaked (greasy looking) blotches, which later dry out and turn brown. Some runners may experience dieback. Although such symptoms may appear under highly favorable conditions, as occurred in 1998, Phytophthora blight primarily produces a fruit rot on watermelons. Irregular brown lesions on the fruit expand rapidly to round or oval lesions that may contain concentric rings. The fungal body (mycelium) appears as a cottony, white or gray growth in the center of the rotted tissue, surrounded by brown, water-soaked areas. The disease causes the entire fruit to eventually decay (67,68).
Phytophthora capsici spreads from plant to plant by wind or water, in the form of swimming spores (zoospores), which require sufficient surface moisture to infect host tissue. Disease development is therefore most rapid during warm, wet weather (75 to 90EF or 24 to 32EC) and in low, waterlogged areas of the field or during excessive rainfall. During the 1998 epidemic, disease development continued rapidly even after rainfall had ceased, suggesting that moisture on leaf surfaces from dew and fog is sufficient for spread of spores. When ideal conditions are present, symptoms of Phytophthora blight may be observed within three to four days after infection. The fungus is able to survive on seed and in the soil on host plant debris in the form of thick-walled spores (oospore). These survival structures, which can survive in the soil for at least two years, can serve as the source of inoculum for later crops (66-68).
BACTERIAL FRUIT BLOTCH (caused by Acidovorax avenae subsp. citrulli). Bacterial fruit blotch occurs sporadically, affecting only a limited number of fields, but it can cause severe losses where it occurs. Also called greasy fruit spot or watermelon fruit blotch, this bacterial disease is a relatively new problem in Florida watermelon production. In 1989, the first year it was observed in the state, losses of 50 to 90 percent of marketable fruit occurred in some fields. When an outbreak occurs early in the season, no marketable fruit may be harvested (61,69-71).
The pathogen produces both a leafspot and a fruit spot, and symptoms can occur on seedlings, leaves, and fruit. When it attacks seedlings, water-soaked lesions form, and the seedling can collapse and die. Lesions on leaves, which are light to reddish brown and found principally along the major veins, can occur throughout the season. Although they are generally inconspicuous and do not usually contribute to defoliation, leaf lesions serve as reservoirs of the bacteria that later infect the fruit. On the watermelon fruit, lesions start out as very small, water-soaked areas that usually do not appear until close to harvest. Later, they enlarge, and within two weeks, they may cover the entire upper surface of the fruit. Cracks may later appear on the rind, which may show internal discoloration, and the whole fruit may rot within, as secondary pathogens enter the open lesions (61,69,70,72,73).
Bacterial fruit blotch is transmitted by seed, the principal source of epidemics in Florida. In addition to infected seed, the disease can enter watermelon fields on infected transplants, infected volunteer watermelons, and possibly from infected wild cucurbit plants. For example, wild citron, a common weed throughout the southeastern U.S., is highly susceptible to the disease. Conditions in transplant production facilities, including a warm, humid environment and the use of overhead irrigation that can splash bacteria to nearby seedlings, can contribute to a high level of secondary spread. In the field, the pathogen spreads among plants and from leaf lesions to watermelon fruit. Seeds from infected fruit that has rotted in the field can fall to the soil, producing infected volunteer plants during the next season that are a source of local inoculum (71,72).
ALTERNARIA LEAF SPOT (caused by Alternaria cucumerina). Alternaria leaf spot is a minor disease on Florida watermelons. Symptoms begin on the upper surface of older leaves as very small yellow or tan spots that may be surrounded by light green or yellow halos or by a water-soaked area. The spots later grow to up to three-quarters of an inch (2 cm) in diameter and turn brown in color. The lesions, which are similar in appearance to gummy stem blight, are the source of spores that are spread primarily by the wind. Under severe infestations, the disease produces leaf curling, defoliation (which can leave the fruit susceptible to sunscald), and premature ripening. Lower yields, lower fruit sugar, and fruit deformity may occur (61).
The fungus can survive on or in crop debris, with debris on the surface more likely to spread spores because of exposure to the wind. Volunteer cucurbit plants and weeds, such as balsam apple, are also sources of inoculum. When watermelons are planted successively for multiple harvest dates, older infested plants located upwind can also contribute to disease spread. Although wind is the main vehicle for spore dispersal, movement by rain splash and mechanical means can also occur. As in other fungal diseases, spores require moisture to germinate and enter the leaf tissue, while spore release from the plant is best achieved under dry conditions. The optimum temperature for infection is 68EF (20EC), and within three to twelve days from spore penetration, the next group of spores is released.
SEEDLING BLIGHT (caused by Pythium spp., Rhizoctonia solani, and Fusarium spp.). Seedling blights can kill seedlings before or after they emerge. Rot symptoms, either wet or dry, are observed in the presence of this disease. The lesions resulting from seedling blight caused by Rhizoctonia solani are reddish-brown to orange and appear sunken. Pythium spp. cause shoots or roots to appear gray and water-soaked. The incidence of seedling blight is higher when watermelons are planted in cool soils, since seedlings that emerge slowly are more susceptible to pathogenic fungi (61).
FUSARIUM WILT (caused by Fusarium oxysporum f.sp. niveum). The fungus responsible for fusarium wilt of watermelons is soil-borne and is widespread in many fields. Fusarium wilt occurs commonly in Florida when resistant varieties are not used. However, even with the use of resistant varieties, the disease often occurs. Resistance to fusarium wilt is not complete, meaning that some plants will be susceptible even with the use of resistant varieties. In addition, resistance to only two of the three races of the fungus has been incorporated into resistant watermelon varieties (74). Increases of soil levels of the third race (race 2), which is highly aggressive, have been shown to follow monocultures of resistant varieties of watermelons in Florida. Therefore, long rotations are required for the management of fusarium wilt, even when using resistant varieties (75).
The disease causes wilting and decline, which may occur in entire plants or in individual runners. Most commonly, the entire plant wilts quickly, without yellowing, and then turns brown and dies. A longitudinal cut in the lower stem of infected plants will reveal yellow, orange, or brownish streaks in the vascular tissue. In Florida, the disease usually occurs before fruit set. Infection rarely occurs at temperatures above 86EF (30EC), and optimum temperature for infection is about 80EF (27EC) (61,74)
ANTHRACNOSE (caused by Colletotrichum orbiculare). In the past, anthracnose was a serious disease in Florida watermelon production, but the use of resistant varieties has limited its impact. Three races of anthracnose are known (races 1, 2, and 3), and some varieties show resistance to some races and not others. Anthracnose race 2 has caused serious damage to watermelons in the southeastern U.S. When it does occur, anthracnose can destroy the entire field if not controlled, particularly after several days of warm, rainy weather (4,61).
All above-ground plant parts may be affected by this fungal disease, and infected plants may die under severe conditions. Early symptoms of the disease include angular, brown to black leafspots on older leaves, similar in appearance to those of gummy stem blight or downy mildew. Tan, oval shaped lesions may appear on the stems. Spores from leaf and stem lesions later infect the fruit, producing sunken, water-soaked spots. Disease development is greatest during humid, rainy weather. Spores are spread by wind, splashing rain, people, and machinery. The fungus, which can be seedborne, survives between crops on infected plant debris and volunteer plants (4,61,66).
In 2002, Florida growers applied fungicides totaling 205,800 pounds of active ingredient to 96 percent of the state’s watermelon acreage. During the years in which usage data have been collected, between 86 and 96 percent of watermelon acreage has been treated with fungicides each year, with total annual usage ranging from 205,800 to 315,500 pounds of active ingredient (43-48). The published applied fungicides on Florida watermelons in 2002 were azoxystrobin, chlorothalonil, copper hydroxide, mancozeb, and maneb. There was also reported use of copper sulfate, fosetyl-Al, mefenoxam, propiconazole, sulfur, thiophanate-methyl, and trifloxystrobin. Other fungicides registered for watermelon in Florida in 2003 were dimethomorph, pyraclostrobin, zoxamide, phosphoric acid, and carbonic acid.
The use of methyl bromide, as discussed in the pest management section, aids in the management of damping-off (caused by Pythium spp., Fusarium spp. and Rhizoctonia spp.) and fusarium wilt (7).
MANCOZEB. Mancozeb is an ethylene(bis) dithiocarbamate (EBDC) fungicide which is used prophylactically, primarily for the management of downy mildew. It may also be effective in managing gummy stem blight, anthracnose, alternaria leaf spot, and cercospora leaf spot (76). The median price of mancozeb is $3.67 per pound of active ingredient, and the approximate cost per labeled application (2.4 lb ai/A) was $8.81 per acre (51,77). Mancozeb may be applied up to 5 days before harvest (PHI=5 days), and the REI under the Worker Protection Standard is 24 hours. A maximum of 19.2 pounds of active ingredient (or EBDC fungicide in general) may be applied per acre for each crop (77).
In 2002, Florida growers applied an average of 0.91 pounds of active ingredient per application to 78 percent of their watermelon acreage, an average of 6.5 times. Total usage was 115,800 pounds of active ingredient. During the years in which usage data have been collected, watermelon growers in Florida have applied mancozeb at an average rate ranging from 0.91 to 1.56 pounds of active ingredient per acre at each application, to between 20 and 78 percent of their watermelon acreage. Growers have made an average of 3.8 to 7.2 applications per year, totaling between 81,500 and 244,900 pounds of active ingredient annually (43-48).
CHLOROTHALONIL. Chlorothalonil is a broad-spectrum chloronitrile fungicide used as a protectant on Florida watermelons. Growers primarily apply it for the management of gummy stem blight, downy mildew, and leaf spot, although it may also be effective in managing anthracnose and alternaria leaf spot (76). The median price of chlorothalonil is $10.32 per pound of active ingredient, and the approximate cost per labeled application (2.23 lb ai/A) was $22.99 per acre (51,53). Chlorothalonil may be applied up to harvest and the REI under the Worker Protection Standard is 12 hours. It should not be applied to watermelons under conditions conducive to sunburn (high levels of heat and sunlight, drought, or poor vine canopy).
In 2002, Florida growers applied an average of 1.29 pounds of active ingredient per application to 62 percent of their watermelon acreage, an average of 2.6 times. Total usage was 51,600 pounds of active ingredient. During all of the years in which usage data have been collected, watermelon growers in Florida have applied chlorothalonil at an average rate ranging from 1.26 to 1.72 pounds of active ingredient per acre at each application, to between 45 and 70 percent of their watermelon acreage. Growers have made an average of 2.6 to 3.8 applications per year, totaling between 51,600 and 160,900 pounds of active ingredient annually (43-48).
AZOXYSTROBIN. Azoxystrobin in a member of the strobilurin class of fungicides. It is used in the management of anthracnose as well as gummy stem blight, downy mildew, and leaf spot (76). The median price of azoxystrobin is $118.76 per pound of active ingredient, and the approximate cost per labeled application (0.25 lb ai/A) was $29.69 per acre (51,53). Azoxystrobin may be applied up to one day before harvest and the REI under the Worker Protection Standard is 4 hours.
In 2002, Florida growers applied an average of 0.12 pounds of active ingredient per application to 34 percent of their watermelon acreage, an average of 1.3 times. Total usage was 1,400 pounds of active ingredient. During all of the years in which usage data have been collected, watermelon growers in Florida have applied azoxystrobin at an average rate ranging from 0.12 to 0.20 pounds of active ingredient per acre at each application, to between 31 and 34 percent of their watermelon acreage. Growers have made an average of 1.1 to 1.3 applications per year, totaling between 1,400 and 2,200 pounds of active ingredient annually (43,44).
COPPER HYDROXIDE. Watermelon growers in Florida use copper hydroxide for the management of downy mildew and bacterial fruit blotch (76). The median price of copper hydroxide is $2.11 per pound of active ingredient, and the approximate cost per labeled application (1.54 lb ai/A) was $3.25 per acre (51,78). Copper hydroxide may be applied up to the day of harvest, but the REI under the Worker Protection Standard is 24 hours.
In 2002, Florida growers applied an average of 0.69 pounds of active ingredient per application to 28 percent of their watermelon acreage, an average of 2.3 times. Total usage was 11,700 pounds of active ingredient. During the years in which usage data have been collected, watermelon growers in Florida have applied copper hydroxide at an average rate ranging from 0.50 to 1.09 pounds of active ingredient per acre at each application, to between 6 and 34 percent of their watermelon acreage. Growers have made an average of 2.3 to 3.3 applications per year, totaling between 9,500 and 18,800 pounds of active ingredient annually (43-48).
MANEB. Maneb is the other EBDC fungicide used by watermelon growers in the management of downy mildew, gummy stem blight, anthracnose, and leafspot, although its use in Florida is only occasional. The median price of maneb is $4.00 per pound of active ingredient, and the approximate cost per labeled application (1.6 lb ai/A) was $6.40 per acre (51,78). Maneb may be applied up to 5 days before harvest (PHI=5 days), and the REI under the Worker Protection Standard is 24 hours. No more than 12.8 pounds of active ingredient per acre may be applied during a season.
In 2002, Florida growers applied an average of 1.14 pounds of active ingredient per application to 8 percent of their watermelon acreage, an average of 4.0 times. Total usage was 9,300 pounds of active ingredient. During the years in which usage data have been collected, watermelon growers in Florida have applied maneb at an average rate ranging from 1.06 to 1.78 pounds of active ingredient per acre at each application, to between one and 17 percent of their watermelon acreage. Growers have made an average of 2.2 to 8.0 applications per year, totaling between 2,200 and 59,400 pounds of active ingredient annually (43-48).
In addition to two new strobilurin fungicides (trifloxystrobin and pyraclostrobin), several other new fungicides have been registered for use in watermelon. Zoxamide and dimethomorph are materials that are just now being examined for overall utility in Florida watermelon production.
Incidence of viral diseases can be reduced on fall-planted watermelons by planting as far as possible from fields that were planted to cucurbits the preceding spring (14). In addition, removing volunteer watermelon and other cucurbit plants from around the field can further reduce the spread of viruses (61). Weeds should be removed before watermelons are planted, because senescence of weeds during the growing season may cause insect pests such as aphids, thrips, and whiteflies to move into the crop. This practice is useful principally in south Florida. Researchers have been investigating the use of reflective mulches to repel aphids from landing on watermelons. Additionally, some growers are using purified mineral oil sprays (stylet oil) to prevent virus transmission, as described in the section on insect management.
Weed management, in addition to reducing potential sources of virus, can improve efficiency of fungicide sprays, since weed presence can interfere with adequate spray coverage. Weeds near watermelon plants tend to increase moisture on the crop, which is necessary for the germination of fungal spores that initiates disease development. Therefore, proper management of weeds in and around the field is an essential component of overall disease management in watermelons (61).
The use of resistant varieties and rotation to land that has been out of watermelon production for at least eight years have been the primary means of managing fusarium wilt of watermelon in Florida. Even when using resistant varieties, an interval of at least four to five years between watermelon plantings should be followed. Rotation also aids in the management of gummy stem blight and of seedling blights, in addition to fusarium wilt. The best rotation crops for watermelons are grasses, such as corn or pastures. Growers had been using land previously in pasture for watermelon production, but declining availability of land, particularly in south Florida, has limited this practice. Maintaining soil pH between 6.5 and 7.5 by liming, combined with using nitrate rather than ammonium forms of nitrogen fertilizer, has also been shown to reduce fusarium wilt of watermelon in Florida, as do the use of healthy transplants, removal or turning under of crop residues after harvest, and delayed thinning (4,6,7,79).
In addition to varieties resistant to fusarium wilt, watermelon varieties with resistance to anthracnose race 1 are also available in Florida (80). Although most varieties in use today carry resistance to both fusarium wilt and to anthracnose, that resistance is not complete, with some plants always being susceptible. Researchers in Florida are developing two transgenic varieties of virus-resistant seedless watermelons (81).
The minimum recommended rotation time for gummy stem blight is at least two, and preferably four, years. In addition to crop rotation, turning under old crop debris aids in the management of gummy stem blight. Spores serving as the primary inoculum of the disease survive on watermelon, muskmelon, cucumber, squash, and pumpkin debris, and deep plowing prevents those spores from being spread by wind into active watermelon fields. Deep plowing also aids in the management of southern blight and leaf spots, because burying the pathogens causing these diseases inhibits their movement. Furthermore, the use of drip irrigation and planting on a well-drained field with good air circulation can minimize periods of leaf wetness that intensify the development of the disease (61).
Phytophthora blight is most effectively managed by controlling excess water (planting in well-drained fields and employing appropriate irrigation practices). Since the fungus can spread in contaminated soil, hands and shoes should be washed before workers move from one field to another (68).
Since bacterial fruit blotch can be seed-borne, it is important to use only seed that is free of bacterial fruit blotch inoculum. Researchers evaluating several seed treatments to eliminate the fruit blotch bacterium determined that fermentation of seeds in watermelon juice and debris followed by washing and drying reduced seed transmission to under one percent. The most effective treatment was found to be fermentation of seeds for 24 to 48 hours, followed by soaking of washed seeds in one percent hydrogen chloride or calcium hypochlorite for 15 minutes before washing and drying (70). Most companies selling watermelon seed now test their seed lots to declare them free of the fruit blotch pathogen but do not guarantee complete absence of the pathogen. Changes in the ways that seeds are handled and tested have led to great increases in the price of watermelon seeds for the grower (71).
Several cultural practices other than crop rotation can also aid in the management of damping-off (seedling blight), including turning under or composting crop residues, avoiding planting in cool soils, and using healthy transplants or seed treatments. Furthermore, transplants should not be set too deeply, and they should not be exposed to production fields during transport, which can be avoided by using covers on transport vehicles (61).
Research on suppressiveness of soils to various plant pathogens has been conducted in Florida for years. The development of soils suppressive to fusarium wilt of watermelon has been found to be stimulated by monoculture production of the variety ‘Crimson Sweet’ (79). As a result of inducing fusarium wilt-suppressive soils, successive planting of ‘Crimson Sweet’ watermelons on the same land reduces the severity of the disease.
‘Crimson Sweet’ is the only variety that has been found to promote this buildup of suppressiveness, but suppressive soils promoted by ‘Crimson Sweet’ monocultures effectively reduce fusarium wilt on all other varieties. The factor responsible for the suppressiveness was shown to be sensitive to fumigation with methyl bromide and to 30 minute treatment with moist heat (158EF or 70EC) (82). In studies to determine the specific cause of fusarium wilt suppression in soils planted successively to ‘Crimson Sweet’ watermelons, researchers have found the ‘Crimson Sweet’ variety to favor the growth of nonpathogenic populations of indigenous F. oxysporum and of overall populations of nonpathogenic bacteria (83). Nonpathogenic populations of the fungus have been determined to be the principal antagonists responsible for the suppressive nature of the soils. Further studies of these antagonists are underway, with a long-term goal of developing them as biological control agents (84). However, no commercially feasible biological control agents are currently available for watermelon diseases.
Post-harvest decays and their management
When post-harvest diseases of watermelons occur, the most common are black rot and stem end rot. Bruising during shipment can reduce quality, in addition to contributing to post-harvest decay. Decay organisms require entry points such as mechanical damage or weakened tissue, although they may enter the fruit through contaminated water or from other infected plant parts. Post-harvest decay is minimized by following appropriate cultural practices, minimizing handling during harvest and shipping, and managing temperature during shipping and storage (21).
Black rot is a significant post-harvest disease of watermelons. By the time infested fruits arrive at the market, they have developed green to black spots on the rind, which enlarge as the decay enters the interior of the watermelon. Black rot can leave the fruit susceptible to secondary bacterial and fungal infections (21). After infection occurs in the field, the disease can develop during shipment, and it is more severe under warm, humid conditions (66).
Stem end rot begins as the stem shrivels and turns brown, and the infection enters the fruit at the point of attachment with the stem. The watermelon tissue becomes water-soaked and softened, with accompanying browning and shriveling. When the disease is severe, black pycnidia (spores) and the gray mycelium (body of the fungus) can be seen (21).
While these diseases may occur, post-harvest disease problems on watermelons in Florida are generally minimal unless mechanical damage has been sustained. The move to shipment in pallet-sized bins, rather than loading in bulk, should reduce post-harvest disease incidence, since bulk loading entails greater potential for bruising and cracking (21).
Nematodes are microscopic roundworms living in the soil which feed on the plants’ roots and damage the tissue. Watermelons are highly susceptible to nematode injury, as are all cucurbits. Damage can be particularly severe on sandy soil, which is the predominant soil type in Florida watermelon production. Typical above-ground symptoms of nematode feeding on watermelon roots include stunting, premature wilting, leaf yellowing, and related symptoms characteristic of nutrient deficiencies. Stunting and poor stand development tend to occur in patches throughout the field as a result of the irregular distribution of nematodes within the soil. Damage and yield loss are directly related to the level of infestation by nematodes. The principal nematode pests on watermelons in Florida are root-knot (Meloidogyne spp.) and sting (Belonolaimus longicaudatus). Reniform nematodes (Rotylenchulus spp.) can be a problem in south Florida (85,86).
ROOT-KNOT NEMATODE (Meloidogyne spp.). All of the common species of root-knot nematodes (M. incognita, M. javanica, M. arenaria) can be damaging to watermelons in Florida. However, root-knot damage has historically been low, particularly due to the practice in north and central Florida of growing watermelons in old pasture land or after long grass rotations to reduce damage from fusarium wilt (85,86). In southwest Florida, the lack of reported damage from root-knot nematodes is attributed to the use of methyl bromide.
Root-knot nematodes enter the host plant root as second stage juveniles and settle within the root to establish a feeding site. At the feeding site, secretions from the nematode cause the surrounding plant cells to enlarge and multiply, producing the characteristic galls associated with root-knot attack. On watermelons, galls measuring from one-eighth to one-quarter inch (0.3 to 0.6 cm) in diameter begin to form as soon as the roots are infested. As more nematodes enter the root and feeding continues, the galls fuse to form large tumors on the roots. Within the root, the developing female molts several times before developing into a swollen, pear-shaped adult. The adult may live in the host plant for several months, laying hundreds to several thousand eggs that are released into the soil. Low temperatures or very dry soil conditions can cause eggs to hatch more slowly (86-88).
Root deformation and injury caused by root-knot nematodes reduce root area and interfere with water and nutrient uptake. Resulting symptoms include stunting, wilting, chlorosis and yield loss. In addition to expending the plant’s resources, the gall tissue is more susceptible to secondary infections such as root rots (87).
STING NEMATODE (Belonolaimus spp.). Sting nematodes are ectoparasites, remaining outside the plant root and feeding superficially at or near the root tip by penetrating the root deeply with their long needle-like mouthparts (stylets). Affected root tips turn yellow and later necrotic, with cavities forming and the root tip swelling slightly. Damage from sting nematode feeding inhibits root elongation and causes roots to form tight mats and appear swollen, resulting in a "stubby root" or "coarse root" appearance. Under severe infestations, new root growth is killed in a way that resembles fertilizer salt burn (86,88,89).
Sting nematodes are especially damaging to seedlings and transplants. In north Florida, they are most abundant in April and May. Sting nematodes prefer sandy soils (with 84 to 94 percent sand) and are most abundant in the upper 12 inches (30 cm). Optimum soil temperature for this nematode is between 77 and 90EF (25 and 32EC), and optimum soil moisture is about seven percent (86,88,89).
RENIFORM NEMATODE (Rotylenchulus spp.). Reniform nematodes feed within the plant root, entering as second-stage juveniles and settling at a feeding site. By releasing growth regulators into the surrounding tissue, the nematodes cause the plant to redirect nutrients to the cells around the feeding site, using up energy and disrupting the vascular system. However, they do not produce galls on the root tissue (88,89).
Oxamyl, dichloropropene, and methyl bromide are the principal pesticides used in the management of nematodes on Florida watermelons (43-48). Additional nematicides registered for use on watermelons are azadirachtin and metam sodium.
DICHLOROPROPENE. Dichloropropene is a fumigant which only kills nematodes, in contrast to methyl bromide, which kills, nematodes, fungi, and weeds. The median price of dichloropropene is $1.24 per pound of active ingredient, and the approximate cost per labeled application (159 lb ai/A) was $197.31 per acre (49,90). Agricultural workers must be restricted from the treatment area for five days after application.
In 2002, Florida growers applied an average of 90.21 pounds of active ingredient per application to 14 percent of their watermelon acreage, an average of 1 time. Total usage was 318,000 pounds of active ingredient. This was the first year that dichloropropene use was numerically reported (43).
Crop rotation is an important cultural management tool and is frequently used by north Florida watermelon growers, who often plant watermelons on land after a long rotation of poor or nonhost pasture grass. However, as new land or pasture land is increasingly unavailable, the rotation time is correspondingly shortened or eliminated, and increased nematode problems have been observed. When the use of cover crops or rotational crops is considered, the choice of crop depends on which nematode populations are present. While several grasses, including corn, sorghum, bahiagrass, bermudagrass, and pangola digitgrass have successfully reduced populations of root-knot nematodes, other nematode species may increase on those crops. Therefore, a prior nematode assay should be performed to determine the species of nematodes in the field (7).
Several other cultural measures also aid in the management of nematodes in watermelon production. Rapid destruction or removal of crop roots after harvest reduces the material on which nematodes may continue to feed and reproduce. In addition, maintaining fields weed-free between crops and practicing frequent cultivations during the season to reduce nematode-harboring weeds further eliminates the source of continued infestation. In most instances, it is not possible to manage nematodes without simultaneously implementing an effective weed management program. Finally, using strong, nematode-free transplants gives watermelon plants a healthy start, which may enhance their ability to tolerate nematode attack (7).
Numerous predators and parasites of nematodes are known, including fungi, bacteria, other nematodes, and mites, and many have been found in most of the soils that have been surveyed. Attempts have been made to use organic amendments to create soil conditions more conducive to the development of these nematode antagonists. Results of organic amendment studies have been controversial, in part because the amendments can affect plant growth directly, exclusive of any effect on the nematode population. Efficacy of organic amendments depends on numerous factors whose combined effects need further study (91,92).
Mass production and field release of nematode antagonists has also been investigated, with the bacterium Pasteuria penetrans being the most promising biological control agent, particularly for root-knot nematodes. While studies of the biology of Pasteuria species have shown its potential as an effective management tool for nematodes, two major problems continue to make it impractical for commercial use. Most Pasteuria spp. isolates are very host specific, so numerous products would have to be developed for use against the range of nematode pests affecting agricultural crops. In addition, methods for low-cost mass rearing of the bacteria, which require a full understanding of its nutrient requirements, have not yet been developed. P. penetrans has been found to be common in soils throughout Florida, and researchers are also investigating its soil ecology to find ways to increase its presence in soils. Therefore, while Pasteuria has shown promise in reducing nematode populations, its use as a biological control agent for nematode management has not yet left the research stage (91-94).
Competition from weeds can be severe in watermelon production, due to the slow growth rate of the crop early in the season, as well as its low planting density and low vining habit. Early season weed management is therefore essential. Weeds late in the season can reduce the efficiency of harvest and incur losses, but yield loss from competition does not occur when weeds emerge later in the growth of the watermelon crop. When weed populations are high, watermelon yield can be reduced by 50 to 100 percent in the absence of weed management. When weeds are managed, the potential yield loss from weed competition still reaches an estimated 15 percent (6,95-97).
A variety of weeds are problematic for Florida watermelon producers, including nutsedges (yellow and purple), grasses (crabgrass, goosegrass, and Texas panicum, for example), and broadleaf weeds, such as bristly starbur, Florida pusley, and purslane. Amaranths are particularly troublesome, and nutsedge is becoming a major problem. The specific weeds present will vary by region within the state and by previous land use. For example, on recently cleared land or land that has been in grass production for a long period, weed pressure is usually low. On the other hand, pressure from broadleaf and grass weeds can be quite high on recently cultivated land (95).
NUTSEDGE (Cyperus spp.). Yellow nutsedge (C. esculentus) and purple nutsedge (C. rotundus) are among the greatest weed problems in Florida watermelon production and will pose a greater dilemma without the use of methyl bromide. Both of these perennial sedges are found in disturbed habitats throughout Florida and the southeast U.S. Yellow nutsedge may produce some seed but reproduces primarily by rhizomes and tubers. The parental plant develops rhizomes, which end in bulbs or tubers that produce new plants. Tuber production is favored by low nitrogen levels and high temperatures (80 to 91EF, or 27 to 33EC). The plant is tolerant of high soil moisture but is intolerant of shade. Purple nutsedge is also able to reproduce from tubers when conditions are harsh, making it difficult to control. Unlike the rhizomes of yellow nutsedge, purple nutsedge rhizomes growing off the parent plant produce new plants in a series ("tuber-chains"). The plant also reproduces by seed to a limited degree. Although purple nutsedge is also intolerant of shade, it is able to survive a wide range of environmental conditions, growing well in nearly all soil types and over a range of soil moisture, soil pH, and elevation. Purple nutsedge is also able to survive extremely high temperatures (98).
Recent research in Florida has shown that the presence of 25 yellow nutsedge plants per square meter (2.32 per square foot) in the watermelon bed over the season reduces yield by 98 percent, and six yellow nutsedge plants per square meter (0.56 per square foot) reduces yield by 20 percent. Nutsedge plants emerging at least five weeks after watermelons have no effect on watermelon yield (99).
AMARANTH (Amaranthus spp.). Amaranths (pigweeds) are summer annual broadleaf herbs with erect stems that can grow to 2 meters (6.5 feet) tall. Several species of amaranth are present in Florida, but the main amaranth weeds in watermelons are smooth pigweed (Amaranthus hybridus), and spiny amaranth (Amaranthus spinosus). Amaranths or pigweeds reproduce solely by seed, producing very small, dark seeds. Smooth pigweed flowers from July to November, and spiny amaranth flowers from June to October. They prefer open areas with bright sunlight (98,100).
Research in Florida has shown that the presence of six smooth pigweed plants between watermelon plants over the whole season will reduce watermelon yield by 100 percent. However, watermelon plants maintained free of emerging smooth pigweed for four weeks suffer no yield loss as a result of competition from smooth pigweed plants emerging after that time, and a weed free period of approximately three weeks after the crop’s emergence results in a 10 to 20 percent yield loss. In addition, 10 percent yield loss results if smooth pigweed that emerges at the same time as the watermelon is removed after five days, and watermelon suffers 20 percent yield loss after two weeks (99,101).
CRABGRASS (Digitaria spp.). Crabgrasses are annual grass plants that reproduce mainly by seed, but also by spreading and rooting of stems at the base. They germinate during the summer, flowering from June or July to October and quickly establish clumps. Crabgrass thrives in moist soil (98,100). A study of competition from large crabgrass in watermelon production demonstrated that when the weed emerged at least six weeks after watermelon emergence, no reduction in yield or fruit quality occurred. However, watermelon yield was reduced by seven percent for every week prior to that in which large crabgrass was present, and yield loss from competition over the whole growing season reached more than 70 percent (99).
GOOSEGRASS (Eleusine indica). Goosegrass is similar in appearance to crabgrass, but grows more densely. It is also a summer annual, and it prefers sunny, moist conditions. Reproducing by seed, it flowers from July to October (98,100). As few as 24 goosegrass plants per 30 feet (9.1 m) of row can reduce watermelon yield (99).
TEXAS PANICUM (Panicum texanum). Texas panicum is primarily a problem in south Florida. A summer annual with erect stems, the plant produces large seeds and also roots at the nodes (98).
BRISTLY STARBUR (Acanthospermum hispidum). Bristly starbur, which is present only in the northern part of the state, is becoming a major problem for watermelon producers in that region. This highly competitive weed also appears to be spreading southward. Bristly starbur received its common name because of the bristly appearance of the flat, triangular fruits, several of which are clumped at each head. Fruits, stems, and leaves are all densely covered with hairs. Bristly starbur produces abundant seed until the plant freezes in the fall. Deep plowing is thought to aid in reducing its population, because seeds buried below 3 inches (7.5 cm) in the soil have been found to lose viability after three years (96).
FLORIDA PUSLEY (Richardia scabra). Florida pusley is a loosely branched annual that stands erect or lies flat on the ground. Its hairy stems and oppositely arranged leaves are often rough in texture, particularly along the main veins. The plant is only found in central and north Florida and is often mixed with Brazilian pusley (R. brasiliensis). Florida pusley reproduces by seed and blooms in any month in the absence of frost (96).
PURSLANE (Portulaca oleracea). Purslane is a broadleaf summer annual with a single taproot from which arise multiple branched, purplish-red stems that often form large mats. Clusters of small leaves are found at the end of its branches. The plant reproduces by seed, flowering from August to October. Being resistant to drought, it is difficult to kill. However, it is susceptible to frost injury (98,100,102).
In 2002, Florida growers applied herbicides totaling 3,900 pounds of active ingredient to 15 percent of the state’s watermelon acreage. During the years in which usage data have been collected, between 14 and 31 percent of the state’s watermelon acreage has been treated with herbicides each year, with total annual usage ranging from 3,900 to 18,300 pounds of active ingredient (43-48). The only published applied herbicide on Florida watermelons in 2002 was sethoxydim. There was reported use of ethalfluralin, glyphosate, halosulfuron, paraquat, and trifluralin. Other herbicides registered for watermelon in 2003 were clomazone, clethodim, DCPA, naptalam, diquat, pelargonic acid, and bensulide. Following label directions when applying herbicides is essential. Watermelons have a narrow range of tolerance for most herbicides, and the crop can easily be damaged by incorrect application (96).
SETHOXYDIM. Sethoxydim is a cyclohexene, post-emergence herbicide used to manage many annual and perennial grass weeds. It does not control sedges or broadleaf weeds. The median price of sethoxydim is $47.45 per pound of active ingredient, and the approximate cost per labeled application (0.28 lb ai/A) was $13.35 per acre (51,103). Sethoxydim may be applied up to 14 days before harvest (PHI=14 days), and the restricted entry interval under the Worker Protection Standard is 12 hours. A maximum of 0.56 pounds of active ingredient over the season may be applied per acre.
In 2002, Florida growers applied an average of 0.26 pounds of active ingredient per application to eight percent of their watermelon acreage, an average of 1.0 time. Total usage was 500 pounds of active ingredient. During the years in which usage data have been collected, watermelon growers in Florida have applied sethoxydim at an average rate ranging from 0.09 to 0.31 pounds of active ingredient per acre at each application, to between two and 13 percent of their watermelon acreage. Growers have made an average of 1.0 to 1.2 applications per year, totaling between 200 and 500 pounds of active ingredient annually (43-48).
Watermelon tolerances for several potential herbicides are currently being established through the IR-4 program for minor crops. However, these products are safe only at specific times in the crop’s development and under a limited range of application rates, and therefore liability risks will probably limit their labeling by the major companies. More likely, third-party registrations will have to be obtained in order to make them available to watermelon growers in Florida.
Adequate management of weeds in Florida watermelon production cannot be obtained with herbicides alone, but requires a combination of management tactics, including good field preparation and cultivation (6,97). Between-row cultivation is more common for watermelons than for other cucurbits such as cucumbers and cantaloupe, due to the wider row spacing in watermelon production. Cultivation must be shallow to avoid injuring crop roots, and it is limited to the first four to five weeks after emergence. Not only does mechanical cultivation become impractical after that time due to the running of vines, but it is no longer cost effective, because yields are reduced by weed competition primarily during the early weeks (99,101). Used early in the season, mechanical control (discing, hoeing, mowing or cultivation) is an important part of the overall weed management program for watermelons, as is the use of appropriate water and nutrient management practices to allow a heathy crop to better compete with weed species (7,96). Fallow treatments before planting are also effective in reducing weed populations. Weeds are disced under several times after emergence but before propagation, and the mechanical fallowing is often combined with herbicide treatments applied two weeks before discing (99). Finally, the use of plastic mulch is an important management tool for all weeds except nutsedges, which can grow through the mulch (7).
Michael Aerts is the assistant director of the Environmental and Pest Management Division of the Florida Fruit and Vegetable Association. He facilitates communication between commodity groups and regulatory agencies. Mr. Aerts can be reached at: FFVA, 4401 E. Colonial Drive, Box 140155, Orlando, FL 32814, (407) 894-1351, email@example.com.
Mark Mossler is a pesticide information specialist for the Food Science and Human Nutrition Department’s Pesticide Information Office at the University of Florida’s Institute of Food and Agricultural Sciences. He is responsible for providing pesticide information to the public and governmental agencies. Mr. Mossler can be reached at UF/IFAS PIO, Box 110710, Gainesville, FL 32611, (352) 392-4721, firstname.lastname@example.org.
63) Adlerz, W.C. (1978). Secondary spread of watermelon mosaic virus 2 by Anuraphis middletonii. J. Econ. Entomol. 71:531-533.
Revision Date: January, 2004