Crop Profile for Corn (Sweet) in Florida

Updated August, 1999

Prepared June, 1999

General Production Information



Production Regions

Principal fresh sweet corn production regions in Florida include the Everglades area (Palm Beach County), which during the 1997-98 season produced nearly 58 percent of the state's crop, the Central area (lands around Lake Apopka, generating almost 21 percent of the crop), the S. East/S. West area (Dade, Collier, and Hendry Counties, 15 percent of the crop), and the West/North area (Suwannee and Jackson Counties, 6 percent of the crop). Since then, production on the muck soils around Lake Apopka has ceased, with the spring 1998 crop being the last to be grown in that area (4).



Production Practices

A survey of Florida's sweet corn producers in 1993 revealed that approximately 75 percent of sweet corn acreage is produced on organic muck soils, about 17 percent on loam or sandy loam soils, and about 8 percent on rockland (limestone) soils (7).

Sweet corn seeds can be planted any time from July through May, depending on the specific production region. Usually, growers plant in north Florida from February to April, in central Florida from January to April, and in south Florida from October to March. Standard spacing allows for approximately 30 inches between rows, with seeds typically planted about one inch deep, 6-8 inches apart. Maximum plant population is approximately 24,000-32,000 plants per acre. A total of 64-90 days elapses from seeding to harvest. Sweet corn is wind pollinated, so isolation of varieties must occur to produce desired characteristics. Typically, a distance of at least 300 feet is needed to avoid cross-pollination (1,8).

Nearly two-thirds of the sweet corn growers surveyed in 1993 were using a soil test each planting season to determine nutrient needs, and another 17 percent tested soil once a year. Thirteen percent had never used a soil test result to determine fertilizer application rates. Fifty-four percent of the sweet corn growers reported using plant tissue or sap tests to estimate nutrient needs. Thirty-eight percent of the growers were applying less nitrogen and 58 percent were applying less phosphorus than they did in 1982. In terms of application method, approximately 83 percent of growers were applying preplant fertilizer in the bed broadcast incorporated, 33 percent were applying it under the row banded, 29 percent were applying it on both sides of the row banded and 17 percent were making applications on one side of the row banded (7).

Approximately 46 percent had not changed their method of fertilizer application since 1982, and of those who had changed, 50 percent had changed to banded application and 13 percent were using an injection system. Ninety-two percent of the growers considered the efficiency of use by the plant to be an important factor in how fertilizer is applied, while reduction in the potential for nutrient runoff into water was seen as important by 92 percent, ease of application by 71 percent, reduction in the cost of fertilizer by 63 percent, reduction in the need for special equipment by 50 percent, and reduction in labor by 48 percent (7).

In 1998, 100 percent of sweet corn acreage in Florida received nitrogen applications totaling 4.04 million pounds, 98 percent received phosphate applications totaling 3.28 million pounds, and 98 percent received potash applications totaling 6.61 million pounds. During the years in which usage data have been collected, between 81 and 100 percent of sweet corn acreage in Florida has received an average of 2.0 to 3.0 applications of nitrogen annually. An average range of 41 to 56 pounds of nitrogen per acre has been used at each application, with a statewide annual total ranging from 3.63 to 5.52 million pounds. An average range of 51 to 87 pounds per acre of phosphate has been applied an average of 1.2 to 1.8 times annually to between 92 and 98 percent of the sweet corn acreage, with total annual usage ranging from 3.26 to 7.06 million pounds. Potash has also been applied an average of 1.7 to 2.0 times per year to between 77 and 100 percent of the sweet corn acreage. An average range of 88 to 120 pounds of potash per acre have been used at each application, and a total of 6.61 to 10.89 million pounds have been used annually (9-13).

Adequate water is especially important in sweet corn production during periods of silking and tasseling and of ear development (8). Most of Florida's sweet corn is grown under irrigation. In 1997, 53 percent of farms and 71.5 percent of sweet corn acreage was irrigated (5). Ninety-two percent of sweet corn growers surveyed in 1993 reported that they checked soil moisture and plant need to determine irrigation needs, while 8 percent used an established schedule modified to meet plant needs. Only 8 percent were using a mechanical system to monitor soil moisture, and of those not using a mechanical system, 30 percent considered it too expensive, 30 percent reported not knowing of a good, inexpensive system, 30 percent cited limited water supply, and 20 percent said that lack of time prevented them from adopting a mechanical system (7).

When sweet corn growers were asked to rank the importance of various factors in determining the kind of irrigation system they use, 100 percent responded that suitability for the crop was important, while 92 percent rated improved efficiency of the system as important, 84 percent rated meeting anticipated water needs as important, 75 percent rated reducing water use as important, 67 percent rated reducing initial cost of the system as important, 66 percent rated meeting water use regulations as important, 58 percent rated reducing labor costs as important, and 58 percent rated reducing maintenance costs as important. About 17 percent of the growers surveyed reported having a meter on at least one of their irrigation wells, and of those, 75 percent said they kept track of their water use more than half of the time (7).

Sweet corn harvest can occur from September 25 to July 15, with the most active harvest period occurring from November 15 to June 15 (1). Sweet corn ears are harvested only once, using either hand or mechanical methods. Harvested ears may be packed on a harvest aid or taken to an assembly area in the field or packinghouse for grading and packing. Greater selection for marketable ears can occur with hand harvesting, which usually utilizes self-propelled packinghouses with conveyors. Mechanical harvesters cut the part of the stalk that contains the ears, which are then removed from the stalk by "strippers." During grading, cull ears and trash are removed. The principal packing container used in Florida is a wire-bound crate that holds 4.5-5 dozen ears of sweet corn, weighing approximately 42 pounds. Sweet corn is sometimes packed in a waxed fiberboard carton with the same volume as the wire crate. While wire crates are most adapted to hydro-cooling, fiberboard cartons are best adapted to cooling with liquid ice (14).

Maintenance of appropriate temperature during the post-harvest period is essential for sweet corn, because quality is determined by sugar content and flavor volatiles, which decrease rapidly at room temperature. Particularly important is the pre-cooling process of removing field heat from the sweet corn. Maximum quality is retained when the corn is pre-cooled to near 32°F (0°C) within an hour of harvest and held at that temperature during marketing. However, several factors such as of the volume of sweet corn harvested in Florida, availability of equipment, and economic and marketing considerations all contribute to these ideal conditions not usually being achieved. At 32°F, sweet corn remains marketable for 5 to 8 days (and up to three weeks with the supersweet varieties), but no more than 2 days when held at 50°F (10°C) (14-16).

The most common method of pre-cooling sweet corn in Florida is hydro-cooling by showering or immersion in water. Corn in crates may be hydro-cooled for over an hour to bring it to a temperature of 41°F (5°C). Hydro-cooling is more efficient when done on bulk corn rather than crated corn, because it allows greater contact between the water and the corn. An alternate pre-cooling treatment involves the use of slush icing (package icing) in fiberboard cartons. Corn intended for local markets remains marketable for up to 2 days without pre-cooling, as long as it is kept cool after harvest (14,15). Researchers in Florida are also developing procedures for preparing and handling fresh-cut sweet corn kernels, which they believe to be a potentially successful marketing alternative for sweet corn, given the increasing popularity of fresh-cut vegetables (16).

The entire sweet corn crop in Florida is sold on the open market. Approximately 79 percent of sweet corn producers reported they or their business as being a member of the Florida Fruit and Vegetable Association, and 71 percent reported being a member of another trade association (7).

When sweet corn growers were asked about the importance of information sources to them, 84 percent said that extension agents were important sources of information, 79 percent considered vendor representatives as important, 74 percent saw trade associations as important, 71 percent felt that other University of Florida or IFAS employees were important sources, 67 percent saw other growers as important, and 42 percent said that consultants were important sources of information. Of the 33 percent of growers who reported having tried to use the supplement to the IFAS Pest Control Guide, 67 percent said they used it regularly. Almost 46 percent of the sweet corn growers surveyed were not aware of its existence (7).

Growers were also asked about their record keeping activities. Ninety-two percent reported that they kept records of yield by field, 88 percent said that they kept records of pesticide application by field and type, 87 percent kept records of fertilizer applications by field, 79 percent kept records of soil test results, 44 percent kept records of water test results, and 31 percent kept records of water use (7).

The perceptions of growers with respect to the future of sweet corn production were evaluated as well. Ninety-six percent of growers rated cost of production and government labor regulations as important factors in the future of sweet corn production. Governmental environmental regulations were considered important by 92 percent of growers, rising property taxes were rated important by 83 percent, labor scarcity by 78 percent, rising land values by 71 percent, encroaching human populations by 67 percent, government purchase of farm land by 63 percent, water scarcity by 58 percent, and increased foreign competition by 50 percent. In addition, pesticide and pest management costs, costs of fertilizers, and low prices for sweet corn were all ranked by 96 percent of respondents as important factors for the profitability of sweet corn operations. Investments required to meet environmental safety standards were considered important by 91 percent of responding growers, cost of record keeping by 71 percent, increased foreign competition by 55 percent, and investments in new irrigation systems by 50 percent (7).



Pest Management

Florida's warm, humid climate is ideal for the development of pest populations. Sweet corn grown in Florida is subject to damage from numerous insect, weed, disease, and nematode pests. When Florida sweet corn growers were surveyed in 1993 about their general pest management practices, all of the growers surveyed responded that cost and effectiveness were important factors in deciding which pesticides to use. Additionally, approximately 96 percent felt that the potential for causing environmental pollution was important, and 83 percent felt that the impact on beneficial insects was important (7).

The 1993 survey indicated that 96 percent of sweet corn growers were scouting over half of their sweet corn acreage, and 83 percent of growers reported that they regularly scouted over 75 percent of their total acreage. Fifty-four percent of respondents said that the growers themselves did the scouting, while 33 percent utilized commercial scouts and 21 percent utilized "in-house," full-time personnel. When asked to what degree they used the scouting reports in making pest management decisions, all of the respondents reported using scouting reports more than half of the time in decisions about disease, nematode and weed control, while 96 percent used the reports more than half of the time in decisions relating to insect control. Early detection and accurate identification were ranked as the most important benefits of scouting. Next in importance was increased net return, followed in decreasing order of importance by improved levels of pest control, extended useful lifetime of pesticides, and reduced environmental pollution (7,17,18).

According to growers' responses in the sweet corn survey, approximately 12 percent of pesticides were applied by the owner or manager of the farm, approximately 12 percent by hired labor, and nearly 77 percent by a custom applicator. On farms in which the owner, manager, or hired labor applied more than 25 percent of the pesticides, 78 percent reported that a full suit was worn more than half the time, 77 percent reported that a respirator and rubber gloves were worn more than half the time, and 55 percent reported that rubber boots were worn more than half the time. Seventy-nine percent of growers reported providing training in pesticide use for hired labor periodically, while 13 percent provided training as needs arose and 8 percent trained at the time of hiring (7).

Growers were also asked if pesticides were loaded into application equipment and/or mixed on their farms. Approximately 63 percent of the growers reported that they did not have a special facility for loading and/or mixing pesticides. Of those who reported having facilities, 33 percent had a concrete floor, 25 percent had a containment system and 13 percent had a roofed mixing and loading facility, with some growers having more than one type of facility (7).






Insect Pests

Insect/Mite Pests

The most important insect pests on sweet corn in Florida are the fall armyworm, corn earworm, cornsilk fly and lesser cornstalk borer. Wireworms and the corn delphacid planthopper are minor pests. Other insect and related pests that may be found on sweet corn occasionally include aphids (corn leaf aphid, bird cherry-oat aphid, melon aphid, rust plum aphid, and potato aphid), corn rootworms, cutworms (black cutworm and granulate cutworm), grasshoppers (American grasshopper, migratory grasshopper and redlegged grasshopper), corn leafhopper, corn blotch leafminer, twospotted spider mite, tarnished plant bug, fourspotted sap beetle, slugs, stink bugs (brown stink bug, onespotted stink bug and southern green stink bug), grass thrips, weevils (maize weevil, southern corn billbug and southern corn weevil), and white grubs (19,20,21). Insect pest management tactics are ever changing in order to incorporate new technologies and to adapt to new pests that are introduced into Florida.

FALL ARMYWORM (Spodoptera frugiperda). Several types of armyworms may feed on the foliage of Florida sweet corn. However, the fall armyworm, which is the most important insect pest on sweet corn in Florida, is the most destructive, not only because it is a more consistent problem, but also because the resulting damage is more direct. Fall armyworm larvae, in addition to feeding on the leaves, will burrow into the growing point of the whorl, producing a characteristic row of long holes on the inner leaves. They may also enter the husk and feed directly on the kernels. Increase of fall armyworm populations is greatest when cool, wet springs are followed by warm, humid weather in the southern part of the state, where the pest overwinters (22).

Adults can be seen along the north Florida coast during all months but are most abundant from April to December. The fall armyworm does not enter diapause and cannot survive extended periods of low temperatures, instead maintaining populations in warmer areas from which adults move northward in the spring. The female moth is highly mobile, migrating each spring from frost-free areas in the southern part of the state and spreading throughout the southeast region of the country. Eggs are laid in masses of 100 to 150, and each moth may lay over a thousand eggs in total. Control at the egg stage is extremely difficult, due to the protective covering over the mass and its position on the underside of leaves. Although they may also feed on foliage, silks and ears, the larvae feed most often on the tender tissue of the whorl. Protected from insecticide sprays there, they can cause severe damage. When feeding on the ears, fall armyworm larvae enter from the side, feeding throughout the ear. After approximately two to three weeks, each larva drops to the ground and forms a pupa in the soil, at a depth of about 1 to 3 inches (2 to 8 cm). In Florida, the pupal stage lasts about eight to nine days during the summer and about 20 to 30 days during the winter. Although the life cycle of the fall armyworm can be completed in about 30 days during the summer, it can take 60 days in the spring and fall and up to 90 days during the winter (22,23).

Monitoring of fall armyworm populations is important for adequately timing control measures (23). For example, researchers have shown that early infestation of fall armyworm (during the pre-whorl to mid-whorl stages of the corn plant's development) does not produce loss of marketable sweet corn ears (24). However, as larvae grow and enter the later stages of development, damage to the whorl increases significantly. Mean densities of 0.2 to 0.8 larvae per plant during the late whorl stage have been found to reduce yield by 5 to 20 percent in Florida sweet corn (25). Results of further field studies show that insecticidal treatments are less effective in reducing numbers of older fall armyworm larvae in sweet corn whorls than in reducing numbers of younger larvae (26).

CORN EARWORM (Helicoverpa zea). Another of the most important insect pests of sweet corn in Florida is the corn earworm. These caterpillars, also called tomato fruitworms and cotton bollworms, attack a wide variety of vegetable and field crops. Although they occasionally feed on foliage, the principal damage that corn earworms inflict results from boring into and feeding upon the ears. After migratory flights, moths arriving on corn plants lay eggs on the silks. Upon hatching, the larvae feed on the silk and then enter the ear to feed directly on kernels, primarily at the silk end of the cob. Greatest economic damage occurs when larval feeding peaks during the silking stage. Florida sweet corn has sustained damage to as much as 60 percent of ears when time of moth flights coincides with silking. The cannibalistic habit of the larvae limits them to 1 to 2 individuals per ear, still sufficient to inflict substantial cosmetic damage. There are 2 to 3 generations per season, and timing of control measures is critical, since this pest cannot be controlled once it enters the ear. Insecticide management is important, since the corn earworm has a history of developing resistance to various insecticides in several classes. Additionally, the use of broad-spectrum insecticides against this pest has led to many cases of pest outbreaks by what were previously minor pests, one example being mites (23,27).

CORN SILKFLY (Euxesta stigmatis). The corn silkfly is a major pest on sweet corn, particularly in south Florida. In untreated fields, it can cause losses to the crop of up to 95 percent, and economic losses have been reported even after insecticide treatments have been applied. Heavy infestations produce corn with no market value, and light infestations can reduce the grade of the product. Additional host plants of the corn silkfly in southern Florida include field corn, sorghum, sugarcane, guava, banana, orange, atemoya, orchid and potato, but it is most abundant on sweet corn (28,29).

Development time from egg to adult has been found in the laboratory to be 28.3 0.6 days at 86°F (30°C) and 33.8 1.6 days at 77°F (25°C). Adult flies were found to live an average of 26.7 8.0 days at room temperature. In the field, adult corn silkflies gather on tassels to mate, mainly at dawn and dusk. The female lays its eggs in groups of 2 to 40 at the tip of the ear, within the silks. Larvae develop and feed inside the corn ear, moving down into the ear in groups upon hatching from the eggs. Therefore, foliar insecticide applications are not effective against eggs and larvae. In addition, during the later part of the growing season, the thick canopy inhibits adequate insecticide coverage, and by the time ears are mature, few insecticides are available because of the required pre-harvest intervals. Given the difficulty in applying chemical controls to this pest, researchers are in the process of developing a clearer understanding of the corn silkfly's biology in Florida, in order to develop a more effective management strategy (28,29).

LESSER CORNSTALK BORER (Elasmopalpus lignosellus). A serious pest in some years on Florida sweet corn, the lesser cornstalk borer also attacks over 60 other plant species in the state. Stalk borer populations can build up on previous grass or sod crops. The pest is favored by sandy soil as well as hot, dry weather. Symptoms on corn plants include distorted, wilted, or curled plants, and entrance holes and tunnels at the base of the plant indicate feeding activity of the larvae, which wriggle strongly when disturbed (23,30).

After emerging from the soil and mating, the female moth deposits its eggs at the base of the corn plant, with each female laying about 125 eggs. Within a week the eggs hatch, and the newly emerged caterpillars feed on the foliage. Later, they burrow into the stem, forming tubes at or just beneath the soil surface. The silk tunnels around their feeding sites protect them during the two to three weeks it takes for them to complete larval development. They pupate in the soil or tunnels, and after another two to three weeks, the adults emerge. There are several generations per year, and overlap of generations occurs in late summer. Timing of control is important, since borers cannot be controlled after entering the stem. Soil insecticides used for stalk borer control must be applied at or just after planting (23,30).

WIREWORMS (Family Elateridae). Several species of wireworms may be present in Florida corn fields, including corn wireworms [Melanotus communis (Gyll.)], eastern field wireworms, Gulf wireworms [Conoderus amplicollis (Gyll.)], southern potato wireworms (C. falli Lane) and tobacco wireworms [C. verpertinus (Fabricius)]. While the gulf wireworm is the most common in Florida, C. rudis is one of the most important and is found throughout the state. Conoderus spp. wireworms are present in all sweet corn fields in Florida (31).

Wireworms, which are larvae of click beetles, are among the most destructive of soil insect pests. Eggs are laid in the soil near plant roots, and upon hatching, the larvae feed on the nearby roots. The larvae, which can live up to several years, may be present at a soil depth of 1 to 5 feet (0.3 to 1.5 meters). Wireworms damage sweet corn by eating the seeds and feeding upon the roots or stems of seedlings. They may occasionally bore into larger roots. Attacking a wide variety of crops, they can strike quickly and can leave poor stands and weak seedlings. Applying soil insecticides a few weeks before planting is the most common control measure for wireworms in sweet corn (20,23,32).

CORN DELPHACID (Peregrinus maidis). The corn delphacid, also referred to as the corn planthopper, has historically been a severe pest of sweet corn in Florida, particularly on late-planted corn, because it is more abundant in the summer. Once corn plants have reached the tasselling stage, they are less susceptible to planthopper damage. Corn delphacids tend to aggregate in corn leaf axils, with both adults and the immature nymphs feeding together. In addition to direct feeding damage, the corn delphacid is the only known vector of maize mosaic rhabdovirus (MMV) and of maize stripe tenuivirus (MStV), which have the potential to be serious problems in tropical and sub-tropical production areas. Maize stripe tenuivirus was first reported in Florida in 1975, and south Florida experienced a serious outbreak in 1979-1980. The corn delphacid is also a major pest of sorghum and can develop on goosegrass, barnyardgrass and gamagrass. Itch grass, a serious weed pest, is the most important alternate host for the corn delphacid between growing seasons in south Florida and is also a host of maize stripe tenuivirus (32-34).

APHIDS. Several types of aphids can colonize sweet corn in Florida, most commonly the corn leaf aphid (Rhopalosiphum maidis), but also the bird cherry-oat aphid (Rhopalosiphum padi), the melon aphid (Aphis gossypii), the rusty plum aphid (Hysteroneura setariae), the potato aphid (Macrosiphum euphorbiae), and the green peach aphid (Myzus persicae). The corn leaf aphid frequently infests the sheaths of sweet corn ears. Aphids feed by inserting their needle-like mouthparts into plant tissue and sucking up plant juices. In addition to depleting the plant of nutrients, they can transmit viral diseases and inject toxins that produce abnormal plant growth. For example, several aphid species including the melon aphid and the green peach aphid can transmit maize dwarf mosaic virus, which occasionally affects Florida sweet corn (21,23,35).

Throughout the state, aphid populations are exclusively female and are able to reproduce abundantly. The immature nymphs feed as well, and within just a few days they mature and begin producing more young. As a result of this rapid reproduction, aphid populations can increase dramatically. When populations are high, winged aphids begin to be produced and fly to new plants. Despite the potential for rapid increase in population, control of aphids is not difficult. Since aphids remain on the exterior of the plant and do not lay eggs, all stages are susceptible. When applying insecticides, complete coverage is necessary, since aphids tend to be more numerous on the underside of leaves and in protected areas of the plant. In addition, there are several natural enemies active in sweet corn fields that help to maintain aphid populations at low levels (23).

>Chemical Control

A total of 181,100 pounds of insecticides (excluding B.t.) was applied to 98 percent of sweet corn acreage in 1998 (9). Ninety-eight to 99 percent of the sweet corn acreage was treated with insecticides totaling 283,200 and 350,800 pounds of active ingredient in 1996 and 1994, respectively (10,11). Insecticides commonly used on Florida sweet corn include methomyl, lambda-cyhalothrin, chlorpyrifos, thiodicarb, permethrin, esfenvalerate, and carbaryl. Bacillus thuringiensis (B.t.), phorate, terbufos, and diazinon are used occasionally (9-13).

METHOMYL (Lannate®). Methomyl is the insecticide sweet corn growers utilize and depend upon the most in their broad-spectrum insect pest management programs right up until harvest. It is a carbamate insecticide applied by ground or air, used to manage earworms, armyworms, corn rootworms, flea beetles, aphids, and cutworms. The median price of methomyl is $23.33 per pound of active ingredient, and the average cost per application in 1998 was $6.77 per acre (36). Methomyl may be applied up to and including the day of harvest (PHI=0), but the restricted entry interval (REI) for methomyl under the Worker Protection Standard is 48 hours.

Methomyl, which is primarily a contact insecticide, provides little residual control of fall armyworm on sweet corn. Therefore, appropriate spray intervals depend mainly on the quantities of fall armyworms present, with frequent applications necessary at higher population densities (37). Researchers have shown methomyl to be less effective against older fall armyworm larvae than against younger larvae and have recommended that information on both the larval stage of the insect and the number of plants infested during the whorl stage of the plant's development be used for spray decisions (26).

In 1998, Florida growers applied an average of 0.29 pounds of active ingredient of methomyl per acre at each application to 79 percent of their sweet corn acreage, an average of 8.6 times each. Total usage was 83,400 pounds of active ingredient. During the years in which usage data have been collected, sweet corn growers in Florida have applied methomyl at an average rate ranging from 0.29 to 0.33 pounds of active ingredient per acre at each application, to between 79 and 97 percent of their sweet corn acreage. Growers have made an average number of applications ranging from 8.6 to 16.5 each year, totaling between 83,400 and 252,800 pounds of active ingredient annually (9-13).

LAMBDA-CYHALOTHRIN (Warrior®/Karate®). Lambda-cyhalothrin is a recently registered synthetic pyrethroid insecticide used to manage armyworms, earworms, cutworms, stalk borers, leafhoppers, corn rootworms, sap beetles, aphids, grasshoppers, and mites. The median price of lambda-cyhalothrin is $325.70 per pound of active ingredient, and the average cost per application in 1998 was $6.51 per acre (36). Lambda-cyhalothrin may be applied up to 1 day before harvest (PHI=1 day), and the restricted entry interval (REI) under the Worker Protection Standard is 24 hours.

In 1998, Florida growers applied an average of 0.02 pounds of active ingredient of lambda-cyhalothrin per application to 74 percent of their sweet corn acreage, an average of 2.9 times each. Total usage was 2,100 pounds of active ingredient (9). In 1996, Florida sweet corn growers applied lambda-cyhalothrin to 41 percent of the state's total sweet corn acreage at an average rate per application of 0.02 pounds of active ingredient per acre, an average of 3.4 times during the season. Total lambda-cyhalothrin usage on sweet corn in that year was 1,400 pounds of active ingredient (10).

CHLORPYRIFOS (Lorsban®). Chlorpyrifos is an organophosphate insecticide primarily used to manage armyworms and corn earworms. Growers apply it by either ground or aerial application equipment. The median price of chlorpyrifos is $12.20 per pound of active ingredient, and the average cost per application in 1998 was $8.66 per acre (36). Under provisions set by a Florida 24(c) Special Local Need Registration in 1994, the pre-harvest interval for chlorpyrifos has been reduced to 7 days from that of the Section 3 label's 21 days. The restricted entry interval (REI) for chlorpyrifos under the Worker Protection Standard is 24 hours.

In 1998, Florida growers applied an average of 0.71 pounds of active ingredient of chloropyrifos per application to 59 percent of their sweet corn acreage, an average of 1.5 times each. Total usage was 27,000 pounds of active ingredient. During the years in which usage data have been collected, sweet corn growers have applied chloropyrifos to a range of 33 to 59 percent of their total sweet corn acreage, at an average rate per application ranging from 0.56 to 0.78 pounds of active ingredient per acre. The average number of applications per year has ranged from 1.5 to 4.7. Total chlorpyrifos usage on sweet corn has ranged from 27,000 to 94,000 pounds of active ingredient annually (9-13).

THIODICARB (Larvin®). Thiodicarb is a broad-spectrum carbamate insecticide used to aid in the management of earworms, armyworms, and cutworms. The median price of thiodicarb is $15.31 per pound of active ingredient, and the average cost per application in 1998 was $7.35 per acre (36). Applications of thiodicarb may be applied up to and including the day of harvest (PHI = 0), buty the restricted entry interval (REI) for thiodicarb under the Worker Protection Standard is 12 hours.

Thiodicarb provides a greater degree of residual control of the fall armyworm than does methomyl. Research has shown residual control by thiodicarb of up to 9 days under south Florida conditions. Therefore, duration of the spray interval would depend not only on the fall armyworm population level, but also on the rate of plant growth and rainfall patterns (37).

In 1998, Florida growers applied an average of 0.48 pounds of active ingredient of thiodicarb per application to 55 percent of their sweet corn acreage, an average of 2.6 times each. Total usage was 28,300 pounds of active ingredient. During the years in which usage data have been collected, sweet corn growers in Florida have applied thiodicarb to a range of 16 to 73 percent of their total sweet corn acreage, at an average rate per application ranging from 0.48 to 0.62 pounds of active ingredient per acre, an average of 2.5 to 6.8 times each. Total annual thiodicarb usage on sweet corn has ranged from 15,000 to 119,400 pounds of active ingredient (9-13).

PERMETHRIN (Ambush®/Pounce®). Permethrin is a synthetic pyrethroid insecticide used every year to manage earworms, armyworms, corn rootworms, and cutworms. The median price of permethrin is $55.00 per pound of active ingredient, and the average cost per application in 1998 was $8.80 per acre (36). Permethrin may be applied up to 1 day before harvest (PHI=1 day), and the restricted entry interval (REI) under the Worker Protection Standard is 24 hours.

In 1998, Florida growers applied an average of 0.16 pounds of active ingredient of permethrin per application to 8 percent of their sweet corn acreage, an average of 1 time each. Total usage was 500 pounds of active ingredient. During the years in which usage data have been collected, sweet corn growers in Florida have applied permethrin at an average rate ranging from 0.11 to 0.16 pounds of active ingredient per acre at each application, to between 8 and 20 percent of their sweet corn acreage. Growers have made an average number of applications ranging from 1.0 to 4.2 each year, totaling between 500 and 3,200 pounds of active ingredient annually (9-13).

ESFENVALERATE (Asana®). Esfenvalerate is a synthetic pyrethroid insecticide used in most years to manage earworms, armyworms, cutworms, corn rootworms, sap beetles, aphids, and grasshoppers. The median price of esfenvalerate is $206.76 per pound of active ingredient, and the average cost per application in 1998 was $10.34 per acre (36). Esfenvalerate may be applied up to 1 day before harvest (PHI=1 day), and the restricted entry interval (REI) under the Worker Protection Standard is 12 hours.

In 1998, Florida growers applied an average of 0.05 pounds of active ingredient of esfenvalerate per application to 16 percent of their sweet corn acreage, an average of 1 time each. Total usage was 300 pounds of active ingredient. During 4 of the 5 years in which usage data have been collected, sweet corn growers in Florida have applied esfenvalerate at an average rate ranging from 0.05 to 0.06 pounds of active ingredient per acre at each application, to between 3 and 16 percent of their sweet corn acreage. Growers have made an average number of applications ranging from 1.0 to 2.0 each year, totaling between 200 and 500 pounds of active ingredient annually (9-13).

CARBARYL (Sevin®). Carbaryl is a broad-spectrum carbamate insecticide used in most years to aid in the management of armyworms, earworms, corn rootworms, and sap beetles. The median price of carbaryl is $6.50 per pound of active ingredient, and the average cost per application in 1998 was $6.63 per acre (36). Applications of carbaryl may be applied up to and including the day of harvest (PHI = 0), and the restricted entry interval (REI) under the Worker Protection Standard is 12 hours.

In 1998, Florida growers applied an average of 1.02 pounds of active ingredient of carbaryl per application to 1 percent of their sweet corn acreage, an average of 3.6 times each. Total usage was 800 pounds of active ingredient. During 4 of the 5 years in which usage data have been collected, sweet corn growers in Florida applied carbaryl to a range of less than 1 percent to 4 percent of the state's total sweet corn acreage, at an average rate per application ranging from 0.77 to 1.38 pounds of active ingredient per acre, an average of 1.1 to 3.6 times. Total carbaryl usage on sweet corn has ranged from 100 to 2,700 pounds of active ingredient annually (9-13).

BACILLUS THURINGIENSIS (B.T.) (Javelin®/Dipel®). B.t. is a microbial insecticide that acts as a stomach poison and must therefore be eaten by the insect to be effective. Sweet corn growers in Florida use it occasionally, primarily in the management of armyworms. The median price of B.t. is $140.16 per pound of active ingredient (36). B.t. may be applied up to the day of harvest (PHI=0), and the restricted entry interval (REI) under the Worker Protection Standard is 4 hours. In order to be most effective, B.t. must be applied to the foliage before the appearance of armyworm larvae (22).

During 3 of the 5 years in which usage data have been collected, B.t. was applied to a range of less than 1 to 6 percent of sweet corn acreage. Average number of applications during the crop year has ranged from 2.2 to 7.4. Rates and statewide totals are unavailable, because the amount of active ingredient is not comparable among products (9-13).

PHORATE (Thimet®). Phorate is an organophosphate insecticide used occasionally by sweet corn producers, primarily to manage wireworms. It can also aid in the management of corn rootworms, white grubs, and mites. Phorate granules are never exposed, because they are always incorporated into the soil when applied. Applications can take place only at planting and/or during one cultivation over the course of the growing season. The median price of phorate is $9.75 per pound of active ingredient, and the average cost per application in 1996 was $10.34 per acre (36). Phorate may be applied up to 30 days before harvest (PHI=30 days), and the restricted entry interval (REI) under the Worker Protection Standard is 48 hours.

During 3 of the 5 years in which usage data have been collected, sweet corn growers applied phorate to a range of 20 to 36 percent of the state's total sweet corn acreage, at an average rate per application ranging from 1.1 to 1.6 pounds of active ingredient per acre, an average of 1.0 to 1.6 times. Total phorate usage on sweet corn has ranged from 14,600 to 21,900 pounds of active ingredient annually (9-13).

TERBUFOS (Counter®). Terbufos is an organophosphate insecticide primarily used to manage wireworms, corn rootworms, and corn stalk borers. It can also be used to control white grubs, billbugs, thrips, symphylans, and nematodes. Terbufos granules are never exposed, because they are always incorporated into the soil when applied. Applications can take place only at planting or during one cultivation over the course of the growing season. The median price of terbufos is $13.67 per pound of active ingredient, and the average cost per application in 1994 was $10.25 per acre (36). Terbufos may be applied up to 30 before harvest (PHI=30 days), and the restricted entry interval (REI) under the Worker Protection Standard is 48 hours.

During 3 of the 5 years in which usage data have been collected, Florida sweet corn growers applied terbufos an average of 1.0 to 2.0 times annually to 14 to 22 percent of the state's total sweet corn acreage, at an average rate per application ranging from 0.75 to 1.25 pounds of active ingredient per acre. Total annual usage has ranged from 7,000 to 15,100 pounds of active ingredient (9-13).

DIAZINON (D.z.n.®). Diazinon is an organophosphate insecticide used on occasion by sweet corn producers. It is either applied before planting and incorporated into the soil for management of cutworms and wireworms, or it is foliar applied as needed to manage corn earworms, corn rootworm adults, aphids, grasshoppers, and mites. It can be applied by either ground or aerial application equipment. The median price of diazinon is $8.40 per pound of active ingredient, and the average cost per application in 1994 was $10.42 per acre (36). Diazinon may be applied up to 7 days before harvest (PHI=7 days), and the restricted entry interval (REI) under the Worker Protection Standard is 12 hours.

During 2 of the 5 years in which usage data have been collected, sweet corn growers in Florida applied diazinon to between 22 and 25 percent of the state's total sweet corn acreage at an average rate per application of 1.24 to 2.31 pounds of active ingredient per acre, an average of 1.4 times. Total annual diazinon usage on sweet corn has ranged from 17,200 to 41,700 pounds of active ingredient (9-13).

Use of Chemicals in IPM Programs

For several years, the Everglades Research and Education Center at Belle Glade, Florida has been monitoring moth activity of several lepidopterous pests through the use of pheromone traps. Data is maintained on fall armyworm and corn earworm activity and is available to growers and scouts to aid in forecasting populations of these pests (38).

Of the 83 percent of sweet corn growers reporting in 1993 that they were regularly scouting over 75 percent of their total acreage, 96 percent scouted for insect pests. Seventy percent of those growers regularly based their insect pest management decisions on scouting reports. Ninety-six percent used the information in scouting reports more than 50 percent of the time to make decisions about insect management, and 67 percent almost always used the information (7,17,18).

Cultural Control

Crop rotation is recommended for a variety of pests, particularly soil insects such as wireworms, grubs, and the lesser cornstalk borer. Stalk borer populations can also be reduced by removing weeds in the fields and along fence rows, often the source of borer populations. Soil insect populations can also be reduced by early destruction of previous crop residues and by thorough soil preparation. For example, turning the soil several times reduces wireworm populations by exposing them to the sunlight, a common practice among sweet corn growers in Florida. Pests like armyworms and the corn earworm, which prefer warmer weather, can be avoided to some extent by manipulating planting dates to coincide with cooler weather. In addition to early planting, the use of early maturing varieties can aid in avoiding the higher densities of those pests that occur later in the season (19,22,30,31,39).

Biological Control

Eggs of the corn silkfly are eaten by earwigs in the field, as well as by certain mites and hemipteran bugs. Predation in the field has been observed on eggs at both the base of corn plants and at the ear tip (29). Numerous biological control agents are known to cause mortality to other sweet corn insect pests in Florida as well, particularly to corn earworm and fall armyworm. The most important parasites and predators of the corn earworm are those that attack the egg stage. Attempts to rear and release parasites of the corn earworm in other parts of the country have had very limited success. A nematode that attacks the corn earworm has also been released, but whether or not it became established is unknown. Biological control for fall armyworm has been attempted as well. Releases of several fall armyworm pathogens into the southeastern U.S., including the fungus Nomuraea rileyi, as well as egg parasite releases in Florida, have had as little success as those for the corn earworm (40,41).

As is true for the corn earworm, several important egg and larval parasites naturally reduce overwintering populations of fall armyworm in south Florida. Some of those recorded from Florida include the braconid wasp parasites Cotesia autographae (Muesebeck), C. marginiventris (Cresson) and Chelonus insularis (Cresson); the eulophid wasp parasites Euplectrus comstockii Howard and E. plathypenae Howard; the ichneumonid wasp parasites Enicospilus merdarius Gravenhorst, Campoletis flavicincta (Ashmead), and Ophion sp.; and the tachinid fly parasites Euculetoria armigera (Coquillett) and Lespesia archipivora (Riley) (35). Fall armyworm predators, which are generalists that attack many other caterpillars, include ground beetles, the striped earwig [Labidura riparia (Pallas)], the spined soldier bug [Podisus maculiventris (Say)], and the insidious flower bug [Orius insidiosus (Say)]. The most significant pathogens of the fall armyworm are the S. frugiperda nuclear polyhedrosis virus (NPV) and the fungi Entomophaga aulicae, N. rileyi, and Erynia radicans (22).

The most effective immediate strategy for biological control of sweet corn insect pests in the state may be the careful management of chemical controls to minimize mortality of native biological control agents, thereby maintaining some degree of natural control. Additionally, any mass release of biological control agents to reduce field populations to below threshold levels would have to be accompanied by careful selection of minimally employed chemical controls. Compatibility of biological agents and chemical applications may be improved in the future with computer-based monitoring systems. Presently, biological control agents do not play a significant role in the management of insect pests in Florida sweet corn.



Diseases

Disease Pathogens

Major diseases of sweet corn in Florida include common and southern rust, Northern corn leaf blight, and Southern corn leaf blight. Bacterial leaf spot, damping off, and smut are minor diseases, while bacterial stalk rot, Fusarium stalk rot, charcoal root rot, the viral disease maize dwarf mosaic, brown spot, and crazy top are only occasionally seen (42,43).

RUST (caused by Puccinia spp.). Although both common rust (caused by Puccinia sorghi) and southern rust (caused by Puccinia polysora) occur on sweet corn in Florida, common rust has become the most important disease on the crop in recent years. These two rust diseases are often difficult to differentiate in the field, but they usually occur at different times of the year. Common rust occurs every spring on sweet corn, and southern rust occurs nearly every fall. Symptoms can vary depending on the corn variety, strain of the fungus, and environmental conditions such as temperature and light. Common rust produces small yellow spots on both leaf surfaces that later turn rusty or cinnamon brown, while the equivalent pustules produced by southern rust are generally limited to the upper leaf surface. The spores released from these characteristic pustules are easily spread by the wind, although rain and insects can play a role (42-44).

Common rust occurs under cool weather conditions, with optimum temperatures for spore germination occurring at 59 to 63°F (15 to 17°C). It is therefore considered to be a spring sweet corn disease, because the cooler temperatures of late fall, winter and early spring allow inoculum levels to build up considerably, while the hot weather occurring from the late spring through early fall in South Florida inhibits disease development. Upon reaching the plant, spores can germinate within 1 to 6 hours, and within 1 to 2 days spots may be seen on the foliage. Generation time ranges from 5 to 16 days, depending on temperature, with most rapid pustule formation occurring at 59 to 68°F (15 to 20°C). Oxalis spp. weeds serve as an alternate host for the common rust fungus, but the significance of their role in the life cycle of P. sorghi in Florida is not well understood (43,44).

Southern rust, which is potentially more serious than common rust in the northern part of the state, is a greater problem in warmer weather. It is considered a fall sweet corn disease, because development of significant levels of inoculum require high temperatures, which are present in early fall throughout Florida. In north Florida, where sweet corn is planted late within a double cropping system, the disease may therefore be more severe. Optimum temperatures for spore germination are between 81 and 82°F (27 to 28°C). Spots can be seen on the leaf in 6 to 7 days, and within 9 to 14 days, pustule formation occurs. Pustule development stops when temperatures reach 90°F (32°C) (43,44).

Rust diseases are generally not serious on Florida sweet corn unless husks are infected. Severe rust infections are rarely seen, particularly in south Florida, possibly due to the control of potentially serious leaf blights with fungicides that are also effective against rust organisms (42,44).

NORTHERN CORN LEAF BLIGHT (caused by Helminthosporium turcicum). Northern corn leaf blight was for many years considered the most important sweet corn disease in southern Florida, incurring losses of up to 80 percent. It is still significant, occurring every spring and occasionally at the end of the fall. However, varietal improvements in host plant resistance have reduced its impact. Initial symptoms of the disease include yellow spots that develop on the foliage, eventually enlarging to form tan or straw-colored dead areas about 4 to 6 inches long and one-half inch wide. Progressively moving higher on the plant, the disease does not infect ears, although it may produce lesions on the husk. If the disease begins before silking and is severe, yield losses of up to 50 percent may occur, but later initiation of the disease produces little yield loss. Northern corn leaf blight is a greater problem on spring sweet corn, because the pathogen requires the cooler temperatures of late fall, winter and early spring to build inoculum levels. It develops most rapidly at temperatures of 64 to 80°F (18 to 27°C) and stops development at the high temperatures commonly experienced in south Florida from late spring through early fall. Large amounts of spores can form in humid weather, during which time fungicide applications are recommended (42,43,45,46).

SOUTHERN CORN LEAF BLIGHT (caused by Helminthosporium maydis). Southern corn leaf blight may occur simultaneously with northern corn leaf blight, but can be distinguished by the lesions produced, which are smaller, lighter in color, and more parallel sided than those of northern corn leaf blight. In south Florida, it occurs nearly every fall and occasionally at the end of the spring. Plants infected with this disease may be more susceptible to stalk rot because of the early death of leaves. Southern corn leaf blight is favored by warm, moist conditions. Therefore, long periods of dry, sunny weather between rains inhibit development of the disease. The fungus survives in crop debris from season to season. Like southern rust, it is considered to be a fall sweet corn disease, because it requires the high temperatures of early fall, and disease development is inhibited by the cooler temperatures occurring during the late fall. Southern corn leaf blight is generally less severe than northern corn leaf blight, unless weather conditions are very favorable for disease development and the variety is susceptible (42,44-46).

BACTERIAL LEAF SPOT (caused by Pseudomonas avenae). Bacterial blight occurs sporadically on sweet corn in Florida, and its presence appears to depend on the sweet corn variety and the occurrence of rainy weather during the appropriate crop stage. Corn that is just pushing the tassel through the whorl appears most susceptible, but any corn in the whorl stage may exhibit new infections. Once the corn stalk has fully expanded and leaves have had time to harden off, bacterial blight is rarely observed (43).

The bacterium responsible for leaf spot can also cause a stalk rot and a basal ear rot. The disease produces dark spots on leaves emerging from the whorl that later turn white or straw-colored and elongate to several inches. Under severe disease conditions, leaves may shred easily. Among other plants attacked by the disease, vaseygrass has been found to be the primary source of inoculum in Florida. The pathogen does not survive, however, in plant debris or soil. Warm, rainy conditions cause the disease to progress most rapidly (42,45).

DAMPING-OFF (caused by Fusarium spp., Pythium spp., and Rhizoctonia solani). Several soil-borne fungi can affect corn seeds and seedlings, causing rot and softening of seeds and seedling damping-off. The greatest damage from these pathogens occurs during the pre-emergence stage, causing poor germination of seeds or poor emergence of seedlings. Damping-off produces chlorosis of the seed leaf, and while plants infected when older will seldom die, they often experience dark lesions on roots, slower growth, and reduced yields. Captan, fludioxonil, and thiram are recommended for use as seed treatment in Florida sweet corn (42). In general, seedling diseases such as damping-off have become less important in recent years due to the effectiveness of chemical seed treatments and improved varieties with good early vigor. When the shrunken-2 varieties were first introduced, seedling damping-off was a very significant problem. Now, while some does occur, particularly following saturated field conditions, it is not as prevalent (43).

CORN SMUT (caused by Ustilago maydis). Although sweet corn is generally more susceptible to smut than other corn varieties, losses from the disease are variable. While plants infected when very young will produce no yield, and individual fields may be completely destroyed, overall losses are usually no greater than 2 to 5 percent. Temperatures of 79 to 93°F (26 to 34°C) are optimal for development of the disease. Infection can occur wherever spores land on the plant. Galls, which begin to form one to several weeks after infection, can be produced on all aboveground parts, but yield loss is greatest when they occur on or above the ears. At first greenish-white in color, the galls develop into large (up to 6 inches (15 cm) in diameter) masses of dark powdery spores appearing like soot. Frequently, infected plant parts may be distorted. The fungus survives in the soil, and smut on Florida sweet corn is often more severe in soils with high nitrogen content. Mechanical plant injury from cultivation or from blown sand or hail can also make corn plants more susceptible to smut (42,45,47).

Chemical Control

The most common fungicides used on Florida sweet corn are the ethylene bisdithiocarbamates (EBDCs) mancozeb and maneb, which are protectants and therefore need to be in place prior to infection to be effective. Due to their low cost, they are the predominant fungicides in sweet corn (43). Sweet corn growers also apply propiconazole to a significant percent of acreage, while chlorothalonil is used to a limited extent. Seventy-six percent of Florida's sweet corn acreage was treated in 1998 with a total of 105,300 pounds of fungicides. Ninety-seven percent of Florida's sweet corn acreage received applications of insecticides totaling 148,400 pounds of active ingredient in 1996, while in 1994, 178,000 pounds of fungicide active ingredient were applied to 88 percent of the sweet corn acreage (9-11).

MANCOZEB (Dithane®/Manzate®/Penncozeb®). Mancozeb is the most common fungicide for control of northern and southern leaf blight, as well as rust. The median price of mancozeb is $3.87 per pound of active ingredient, and the average cost per application in 1998 was $3.64 per acre (36). Mancozeb may be applied up to 7 days before harvest (PHI=7 days), and the restricted entry interval (REI) under the Worker Protection Standard is 24 hours.

In 1998, Florida growers applied an average of 0.94 pounds of active ingredient of mancozeb per application to 61 percent of their sweet corn acreage, an average of 3.2 times each. Total usage was 76,800 pounds of active ingredient. During the years in which usage data have been collected, sweet corn growers in Florida have applied mancozeb at an average rate ranging from 0.90 to 0.97 pounds of active ingredient per acre at each application, to between 53 and 72 percent of their sweet corn acreage. Growers have made an average number of applications ranging from 3.2 to 6.5 each year, totaling between 76,800 and 186,300 pounds of active ingredient annually (9-13).

MANEB (Maneb®/Manex®). Maneb is a second EBDC used occasionally to control the same sweet corn diseases as mancozeb. Maneb may be applied up to 7 days before harvest (PHI=7 days), and the restricted entry interval (REI) under the Worker Protection Standard is 24 hours.

During 3 of the 5 years in which usage data have been collected, Florida sweet corn growers applied maneb an average of 1.8 to 3.2 times per year to a range of 10 to 35 percent of the crop's acreage. Florida sweet corn growers have annually used an average of 0.65 to 1.11 pounds of active ingredient per acre at each application and a range of 11,900 to 25,400 pounds in total (9-13).

PROPICONAZOLE (Tilt®). Propiconazole, a systemic ergosterol biosynthesis inhibiting fungicide, is the third compound used to control northern and southern leaf blight and rust. Being locally systemic, it will slow down disease development even following infection, and has been shown to have greater efficacy than the broad-spectrum protectant fungicides typically used, such as mancozeb and chlorothalonil. However, because of its high cost, propiconazole is generally used only when disease conditions are judged most favorable (43,48). The median price of propiconazole is $95.83 per pound of active ingredient, and the average cost per application in 1998 was $12.46 per acre (36). Propiconazole may be applied up to 14 days before harvest (PHI=14 days), and the restricted entry interval (REI) under the Worker Protection Standard is 24 hours.

In 1998, Florida growers applied an average of 0.13 pounds of active ingredient of propiconazole per application to 43 percent of their sweet corn acreage, an average of 1.8 times. Total usage was 4,400 pounds of active ingredient. During the years in which usage data have been collected, sweet corn growers in Florida have applied propiconazole at an average rate ranging from 0.13 to 0.16 pounds of active ingredient per acre at each application, to between 34 and 61 percent of their sweet corn acreage. Growers have made an average number of applications ranging from 1.8 to 2.6 each year, totaling between 4,400 and 8,200 pounds of active ingredient annually (9-13).

CHLOROTHALONIL (Bravo®). Chlorothalonil is a broad-sprectrum nitrile fungicide used occasionally as a protectant to control northern and southern leaf blights and common rust. Although chlorothalonil has basically the same disease spectrum as the EBDCs on corn, it is used much less, because the EBDCs are less expensive (43). The median price of chlorothaloni is $8.85 per pound of active ingredient, and the average cost per application in 1992 was $9.91 per acre (36). Chlorothalonil may be applied up to 14 days before harvest (PHI=14 days), and the restricted entry interval (REI) under the Worker Protection Standard is 48 hours.

During 2 of the 5 years in which usage data have been collected, sweet corn growers in Florida have applied chlorothalonil to only 3 to 5 percent of Florida's sweet corn acreage, an average of 1.5 to 3.0 times annually. Growers have used an average of 0.86 to 1.12 pounds of active ingredient per acre at each application, with an annual total of 4,100 to 4,900 pounds of active ingredient applied to sweet corn acreage during the years in which it was used (9-13).

Chemical Alternatives

The strobilurin fungicides are thought to be an extremely promising alternative from an efficacy standpoint. They are effective against both rust diseases (common as well as southern rust) and fungal leaf blights (northern corn leaf blight and southern corn leaf blight) (43).

Use of Chemicals in IPM Programs

Of the 83 percent of sweet corn growers reporting in 1993 that they were regularly scouting over 75 percent of their total acreage, 92 percent scouted for disease pests. Seventy-seven percent of those growers regularly based their disease management decisions on scouting reports. All reported that they used scouting reports greater than 50 percent of the time for decisions on disease management, and 42 percent said that they almost always used the reports (7,17,18).

Cultural Control

A few varieties of sweet corn are now available with good resistance to both northern corn leaf blight and to common rust. These varieties allow growers to reduce fungicide inputs significantly. Resistant varieties have also reduced the severity of damping-off diseases (43). Varieties resistant to smut are also available and can be used where practical. Many sweet corn hybrids, however, do not exhibit resistance to the southern rust fungus (42,44). When sweet corn growers were asked to rank several factors in their consideration of variety selection, yield received the highest ranking, followed in descending order by market acceptability, disease resistance, adaptability, lodging, and cost of seed (7).

There are currently no practical cultural control tactics recommended for bacterial leaf spot. However, minimization of fall armyworm and corn earworm populations is thought to help, because bacterial leaf spot has been observed to be severe following extensive attack of those insects. Additionally, the disease has been observed to spread into sweet corn fields from ditch banks, particularly those on which vaseygrass is growing, so control of vaseygrass may slow the spread of bacterial leaf blight (42).

Hastening emergence of sweet corn seedlings by avoiding planting in cold and wet soils may reduce the severity of damping-off. Appropriate soil fertility management may be helpful in reducing the severity of smut, which has been observed to be greater in fields with high soil nitrogen. Also, reducing crop debris can aid in minimizing initial infection from leaf blight fungi. Finally, a disease forecasting system is available for the Belle Glade area (south Florida), which can be useful in optimizing sprays for northern corn leaf blight (42).

Post-harvest diseases

When sweet corn is graded and pre-cooled properly, post-harvest diseases are not significant. The need to store sweet corn under cool conditions to prevent reduction in quality from loss of sugar ensures adequate post-harvest handling (14).



Nematodes

Nematode Pests

Plant injury from nematode damage is difficult to diagnose, and as a result nematode problems may go undetected. Nematodes injure sweet corn by reducing corn root growth, stalk height and stalk diameter. In most cases, plants weakened by nematodes produce smaller and fewer ears, and plants that are heavily parasitized may produce no ears, resulting in up to 100 percent crop loss (49,50).

Nematodes affecting sweet corn in Florida include sting, stubby root, stunt, spiral, lesion, lance, and occasionally root-knot nematodes. Corn yield reduction from these nematodes is generally higher in sandier soils, and the most damaging nematode, the sting nematode, is limited to soils with a very high sand content. Stubby root, stunt, and root-knot nematodes are the principal nematode pests in organic soils. General symptoms of nematode injury include stunting, wilting, and nutrient deficiency symptoms, often in patches throughout the field due to irregular distribution of nematodes (49-51).

STUBBY ROOT NEMATODES (Trichodorus spp., Paratrichodorus spp.). One of the most important nematode pests of Florida sweet corn, stubby root nematodes are found throughout the state, especially in sandier soils. These externally feeding species have a wide host range that includes vegetable, fruit and grain crops. Their life cycle, spent entirely in the soil, is relatively short, and populations can build up quickly. Their numbers can also decline rapidly, complicating diagnosis through soil sampling. By attacking the growing root tip of the host plant, stubby root nematodes cause severe stunting. Although rarely killed by stubby root nematode attack, corn plants are highly susceptible, and yields can therefore be significantly reduced (45,51).

STING NEMATODES (Belonolaimus spp.). The sting nematode Belonolaimus longicaudatus has been shown to be severely damaging to sweet corn in Florida when the crop is grown on sandy soils. Sting nematodes are external plant feeders, remaining in the soil throughout their life cycle. They feed at or near the root tip, interfering with root elongation. Feeding damage results in reduced root systems and stubby root symptoms. Plants experience stunting, premature wilting and leaf chlorosis, and plant death may occur. Small populations of the sting nematode can cause considerable plant damage, because while feeding, they inject a toxic enzyme into the root. Since these nematodes are confined to very sandy soils, they are found primarily in peninsular Florida, often occurring together with stubby root nematodes. Constant moisture, such as that encountered under irrigation, may contribute to higher populations of the sting nematode. Yield loss as a result of sting nematode damage to sweet corn may range from limited to total yield loss (45,51,52).

ROOT-LESION NEMATODES (Pratylenchus spp.). The lesion nematode Pratylenchus zeae is an important nematode pest of corn, as well as numerous other crops, including rice, sugarcane, peach, and sorghum. Its host range includes many other crops and weeds, such as tobacco, turfgrass, crabgrass, onions, wheat, rye, barley, citrus, soybean, sweet potato, and strawberry. Among the smallest of plant feeding nematodes, root-lesion nematodes enter the root to feed and reproduce within. Corn is tolerant to low populations of these nematodes, but at high population levels, stunting, chlorosis, nutrient deficiency symptoms, and blackened roots may be evident (45,53).

ROOT-KNOT NEMATODES (Meloidogyne spp.). Root-knot nematodes enter the host plant root as immatures and settle within to feed. At the feeding site, their secretions cause the surrounding plant cells to enlarge, producing the characteristic galls associated with root-knot attack. Gall size varies by host plant, and although on some hosts galls can reach a diameter of greater than one inch, galls on corn and other grasses are usually very small. In addition to expending the plant's resources, the gall tissue is more susceptible to secondary infections such as root rots. Within the root, the female continues to feed, after several molts 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 (45).

Chemical Control

Nematicides are not commonly used on Florida sweet corn, but nematodes are always a potential problem. There are several nematicides available when severe nematode pest problems occur. Chemical treatment of highly infested fields with nematicides has commonly increased sweet corn yields, particularly in fields containing sting nematode. The fumigant nematicide 1,3-dichloropropene (Telone II®/Telone C-17®) is effective against all but stubby root nematodes and is particularly effective in sandy soils. The non-fumigant, organophosphate nematicides/insecticides terbufos (Counter®) and ethoprop (Mocap®) are less useful against root-knot nematodes. They are applied at or before planting and are most effective in band applications. The use of terbufos for rootworm control by sweet corn growers has contributed somewhat to reducing the need for nematode management on the crop (49,51).

Use of Chemicals in IPM Programs

Of the 83 percent of sweet corn growers reporting in 1993 that they were regularly scouting over 75 percent of their total acreage, 25 percent scouted for nematode pests. Sixty-seven percent of those growers regularly based their nematode pest management decisions on scouting reports. All stated that they used the reports for decisions on nematode control over half of the time, and 8 percent used the reports almost always (7,17,18).

Cultural Control

Although nematode resistant germplasm has been identified and released, most commercial sweet corn hybrids, as well as tropical corn hybrids and open-pollinated cultivars, have shown to be excellent hosts for the principal nematode pests of sweet corn (root-knot, sting, and stubby root nematodes). Other useful cultural practices include timely destruction of crops and weeds, particularly roots, and avoiding the spread of nematodes to uninfested fields through contaminated soil or plant parts on machinery. Summer fallow, complemented by periodic cultivation, has proved effective for reducing soil populations of some nematode species such as root-knot, but is of limited value for control of other nematodes, such as sting and stubby root (49,50).

The wide host range of most of the nematodes attacking corn makes crop rotation difficult. However, when practical, rotation with non-host plants can improve the effectiveness of nematicides by maintaining pest nematode populations at a low level. For example, in the Sanford area (central Florida), sting and root-knot nematodes have been observed to be greatly reduced by planting hairy indigo as a summer cover crop before fall vegetables. Use of sorghum sudangrass and other cereal grains in cool winters has suppressed root-knot populations but increased others, such as sting and stubby root. In general, the host range of nematodes that attack sweet corn includes most pasture legumes and grasses, cereals, and most commercial fruit, vegetable, and agronomic crops. Therefore, the number of useful rotation crops for sweet corn production is limited. In many cases, crop rotation, like fallow, does not reduce nematode population densities below economically damaging levels after a single nonhost crop, necessitating the integration of other tactics (49,50).

When Florida sweet corn growers were surveyed in 1993, they were asked to rate the importance of several practices in minimizing damage from nematodes. Ninety-five percent of the growers rated flooding as important, 77 percent saw crop rotation as important, nearly 70 percent said that destruction of crops and weeds was important, 18 percent ranked fumigation of the previous crop as important, and 12 percent believed that preplant fumigation was important (7).



Weeds

Weed Pests

Weeds compete with sweet corn for light, water, and nutrients, and may in some cases provide refuge to insect and disease pests. In addition, they may complicate harvest operations. Florida sweet corn growers may encounter a variety of grass (crabgrass, goosegrass, Johnsongrass, sandbur, Texas panicum, annual ryegrass), broadleaf (bristly starbur, cocklebur, Florida beggarweed, Florida pusley, morningglories, pigweed, ragweed, sicklepod) and sedge (purple and yellow nutsedge) weeds (54). However, the major weed pests in sweet corn in the state are amaranths (pigweeds), grasses and purslane (55).

PIGWEED (Amaranthus spp). Pigweeds (amaranths) are summer annual broadleaf herbs with erect stems that can grow to 2 meters (6.5 feet) tall. Several species of amaranth are among the most common weeds in Florida sweet corn. The species present in Florida include smooth pigweed (Amaranthus hybridus), spiny amaranth (Amaranthus spinosus), and occasionally livid amaranth (Amaranthus lividus). 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. Pigweeds prefer open areas with bright sunlight (56,57).

PURSLANE (Portulaca oleracea). Purslane is a taprooted summer annual with multiple branched stems that often form large mats. It reproduces by seed, flowering from August to October. Being resistant to drought, it is difficult to kill (56,57).

JOHNSONGRASS (Sorghum halepense). Originally introduced from the Mediterranean region and cultivated as a hay and pasture crop, Johnsongrass is one of the most difficult perennial grasses to control because of its thick rhizome system. The plant can grow to approximately six feet and can form dense clumps. It is most common in fields, fence rows, and ditch banks, and it may be toxic to livestock (58).

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. The plant thrives in moist soil (56,57).

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 (56,57).

Chemical Control

Although mechanical cultivation remains common, herbicides have partially or completely replaced cultivation, except where surface crusting or herbicide resistant weeds make it necessary (54). A total of 73,300 pounds of herbicides was applied to 79 percent of the state's sweet corn acreage in 1998 (9). Eighty-seven percent of Florida's sweet corn acreage received applications of herbicides totaling 63,300 pounds of active ingredient in 1996, while in 1994, 75,400 pounds of herbicide active ingredient were applied to 82 percent of the sweet corn acreage (10,11). The most commonly applied herbicides are atrazine (Aatrex) and metolachlor (Dual). Usage data is unavailable for herbicides more recently adopted by sweet corn growers, such as EPTC (Eradicane®) and pendimethalin (Prowl®) + atrazine, which is more commonly used in north Florida (55).

ATRAZINE (Aatrex®). Atrazine, a widely used selective herbicide, is applied either pre-emergence or post-emergence to control annual broadleaf and grassy weeds. Atrazine has a high potential for leaching into the groundwater and is therefore not to be applied to very permeable soils or areas with a high water table (54). The projected approximate price of atrazine in Florida during 1999 is $4.10 per pound of active ingredient, and the average cost per application in 1998 was $4.88 per acre (59). The restricted entry interval (REI) for atrazine under the Worker Protection Standard is 12 hours.

In 1998, Florida growers applied an average of 1.19 pounds of active ingredient of atrazine per application to 78 percent of their sweet corn acreage, an average of 1 time each. Total usage was 40,900 pounds of active ingredient. During the years in which usage data have been collected, sweet corn growers in Florida have applied atrazine at an average rate ranging from 1.14 to 1.55 pounds of active ingredient per acre at each application, to between 64 and 81 percent of their sweet corn acreage. Growers have made an average number of applications ranging from 1.0 to 1.4 each year, totaling between 39,800 and 67,300 pounds of active ingredient annually (9-13).

METOLACHLOR (Dual®). Metolachlor is used to control annual grasses, yellow nutsedge, and some broadleaf weeds. It can be applied pre-plant incorporated, pre-plant as a surface treatment, pre-emergence, or post-emergence as a directed spray (54). The projected approximate price of metolachlor in Florida during 1999 is $9.07 per pound of active ingredient, and the average cost per application in 1998 was $10.79 per acre (59). The restricted entry interval (REI) for metolachlor under the Worker Protection Standard is 12 hours.

In 1998, Florida growers applied an average of 1.19 pounds of active ingredient per application to 20 percent of their sweet corn acreage, an average of 1 time each. Total usage was 9,700 pounds of active ingredient. During the years in which usage data have been collected, sweet corn growers in Florida have applied metoloachlor at an average rate ranging from 1.00 to 2.22 pounds of active ingredient per acre at each application, to between 6 and 24 percent of their sweet corn acreage. Growers always make one application per year, and state-wide usage has totaled between 3,600 and 15,200 pounds of active ingredient annually (9-13).

EPTC (Eradicane®). EPTC is a selective thiocarbamate herbicide used in the management of annual grasses, perennial weeds and some broadleaf weeds (55). The projected approximate price of EPTC in Florida during 1999 is $3.78 per pound of active ingredient (59).

Usage data are unavailable for this herbicide, because sweet corn growers have just recently switched to it from butylate (Sutan), which is no longer available. When in use, Sutan had been applied to 8 to 13 percent of sweet corn acreage in Florida, with between 20,200 and 31,200 pounds of active ingredient applied annually (9).

Use of Chemicals in IPM Programs

Of the 83 percent of sweet corn growers reporting in 1993 that they were regularly scouting over 75 percent of their total acreage, 71 percent scouted for weed pests. Eight-three percent of those growers regularly based their weed management decisions on scouting reports. All reported that they used scouting reports in greater than 50 percent of weed management decisions, and 71 percent almost always used those reports (7,17,18).

Cultural Control

Crop rotations may favor reduced weed growth and concurrently reduce populations of other types of pests, as can destruction of crop residues and alternate host plants. Crop management practices that improve the ability of sweet corn to compete with weeds, such as maintaining good soil fertility and pH and optimizing corn plant stand, are important weed management strategies (54). Cultivation is also used in sweet corn production to control weeds between rows (60).



Contacts

Michael Aerts
Florida Fruit and Vegetable Association
4401 E. Colonial Drive
Box 140155
Orlando, FL 32814
(407) 894-1351
email: maerts@ffva.com

Pat Cockrell
Florida Farm Bureau
Box 147030
Gainesville, FL 32614
(352) 378-8100 ext. 1545
email: pcockrell@sfbcic.com

Gregg Nuessly
University of Florida
Everglades Research and Education Center
P.O. Box 8003
Belle Glade, FL 33430-8003
(561) 996-3062
email: gsn@gnv.ifas.ufl.edu

Richard Raid
University of Florida
Everglades Research and Education Center
P.O. Box 8003
Belle Glade, FL 33430-8003
(561) 996-3062
email: rnr@gnv.ifas.ufl.edu

Thomas Schueneman
Palm Beach County Extension Office
2976 State Rd. 15
Belle Glade, FL 33430-5205
(561) 996-1655
email: tjsch@gnv.ifas.ufl.edu

Dakshina Seal
University of Florida
Tropical Research and Education Center
18905 SW 280 St.
Homestead, FL 33031-3314
email: dseal@gnv.ifas.ufl.edu



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48) Raid, R.N. (1991). Fungicidal control of foliar sweet corn diseases in the presence of high inoculum levels. Proc. Fla. State Hort. Soc. 104:267-270.

49) Personal communication with Joseph Noling, Nematologist, University of Florida Citrus Research and Education Center, Lake Alfred. April 5, 1999.

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59) Colvin D.L. (1998 October). Weeds in the Sunshine: Approximate Herbicide Pricing - 1999. Agronomy Department SS-AGR-16. Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Available: http://edis.ifas.ufl.edu/WG056

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Acknowledgments

The following individuals served as reviewers of the sweet corn profile: John Capinera, Entomologist and Chair, Entomology and Nematology Department, University of Florida, Gainesville; Pat Cockrell, Director of Commodity Activities, Florida Farm Bureau, Gainesville; Gary Leibee, Entomologist, University of Florida Central Florida Research and Education Center, Sanford; Joseph Noling, Nematologist, University of Florida Citrus Research and Education Center, Lake Alfred; Richard Raid, Plant Pathologist, University of Florida Everglades Research and Education Center, Belle Glade; Dakshina Seal, Entomologist, University of Florida Tropical Research and Education Center, Homestead; William Stall, Weed Scientist, Horticultural Sciences Department, University of Florida, Gainesville.

Information used in the profile was also provided by Susan Halbert (Entomologist, Florida Department of Agriculture and Consumer Services, Division of Plant Industry, Gainesville) and by Marilyn Swisher (Sustainable Agriculture Coordinator, Department of Family, Youth and Community Sciences, University of Florida, Gainesville).


Database and web development by the NSF Center for Integrated Pest Managment located at North Carolina State University. All materials may be used freely with credit to the USDA.