Prepared: December, 2003
The predominant onion variety in Georgia is the Vidalia onion. Yellow Granex, the standard for Vidalia onions, has its origin from Early Grano. The variety Early Grano 502 resulted in the Texas Early Grano 951C, which became one of the parents for Yellow Granex hybrid. The other parent, YB986, was selected from Excel, which in turn was derived from White Bermuda.
In 1986, the state of Georgia gave Vidalia onions official recognition and defined the geographic area where these onions could be grown. There had been some problems with onions being brought in from other areas and bagged as Vidalia onions. State recognition, however, did not give the industry the national protection it needed. Finally, in 1989, the industry was able to obtain Federal Market Order 955, which gave the industry national protection. The Vidalia Onion Committee was formed to oversee the Federal market order. Growers are required to register and give check-off funds to support the industry. The collected money is used for national and international promotional campaigns as well as for research on onion production.
In 1989, the industry began to adopt controlled atmosphere (CA) storage. CA uses a low oxygen, high carbon dioxide refrigerated environment to store onions. This has allowed the industry to expand their marketing opportunities well into the fall and winter months. The adoption of the Federal market order and CA storage has allowed this industry to grow to its current level of approximately 14,000 acres.Production Facts
13 counties (and portions of seven other counties) in southeastern Georgia. Appling, Bacon, Jeff Davis, Montgomery, Tattnall, Telfair, Toombs, Treutlen, and Wheeler counties; portions of Dodge, Jenkins, Laurens, Long, Pierce, Screven, and Wayne counties.
Nearly all commercial onions in Georgia are Vidalia onions or closely related varieties. By Federal marketing order, only yellow, Granex type onions can be grown and marketed as Vidalia onions. The Georgia Department of Agriculture recommends specific varieties, but any Granex hybrid variety, variety with Granex parentage, or similar variety types can be grown and marketed as Vidalias. Onion varieties grown in Southeast Georgia fall into three broad maturity categories: early-, mid-, or late-season. There can, however, be considerable overlap in these categories and not all varieties will perform the same as to their maturity from one year to the next.
Onions grow best on fertile, well-drained soils. However, most sandy loam, loamy sand or sandy soils will be advantageous to sweet onion production. These soils are inherently low in sulfur, which allows greater flexibility in sulfur management to produce sweet onions. Clay soils tend to have a higher sulfur content, which can lead to pungent onions. Sandy soils require more fertilizer and water. Onions are heavy feeders and require more fertilizer than most vegetable crops. Growers apply fertilizer in regularly timed applications of small amounts.
Nearly all commercial onions in Georgia are grown from transplants because of practical problems associated with direct seeding. The transplants are produced in the fall for winter season. Seed are sown beginning in September. Growers try to avoid transplant production in fields with a history of onion disease/weed problems or in fields that have had onions or related Allium species.
Transplants are generally set in November to December, but they can be set as late as early February. Plants set in February will generally be smaller at maturity and will have a smaller fraction of jumbos. Transplants are field set on slightly raised beds approximately four feet wide. Beds are six feet center to center. These panels, as they are sometimes called, will have four rows of onions spaced 14-18 inches apart and a spacing of 4.5 to 6 inches within the row. Transplants are hand set in each hole.
All commercially grown onions in Georgia are irrigated with center pivot irrigation. During the season, workers do not have to enter the fields to operate irrigation equipment. Properly irrigated onions yield 25 to 50 percent more than dry land onions. Irrigated fields typically yield a higher percentage of large and jumbo bulbs, which generally bring a higher price on the market. Irrigated onions are sweeter and less pungent than dryland onions, which is especially important for Vidalia onions.
Onions in Georgia are harvested by hand from late April through mid-June. With a controlled atmosphere system, onions can be stored for up to six months.
Potential Worker Exposure
Before planting, nearly all growers incorporate chlorpyrifos to control seed maggots, mole crickets, wireworms, and other soil pests. Typically, growers or their employees apply the pesticide with ground equipment in an enclosed cab. For most farms, the entire onion acreage can be treated in one eight-hour day.
Onion plants are set by hand. Approximately 80% of the onions are planted from November 1 through the middle of December. Four people can plant one acre per day (approx. ten hours). On larger farms, 100-200 people may be employed to set onions. The crews are typically arranged through a labor contractor.
The personnel of onion setting crews vary. Some crews only set onions for approximately ten hours per day for the entire 45-day planting period. Other crews plant onions in addition to contracting for a variety of other hand labor tasks (e.g., collecting pine straw, erecting fencing). An individual could set onions for ten hours/day for 3-4 workweeks.
Few workers are required to enter the onion fields during the season. All Georgia onions are irrigated with center pivot systems. Workers are not required to enter the field to operate or move the system. During the season, onions are not cultivated. Growers or farm employees scout the onions one or two times per week. A scout will be in the field for up to eight hours per week but probably not in single day.
Pesticides are typically applied with ground equipment with enclosed cabs. One application of herbicide is applied at the beginning of the season after the onions are planted. Insecticide or fungicide applications may require a grower or employee to be in the field for about one day (8-10 hours) per week.
Nearly all Georgia onions (80-90%) are harvested by hand over a 30-45 day period. Contracted work crews supply most of the labor for harvest. The personnel and field time for harvest is similar to the setting process described above.
In 2000, insects caused losses of more than $4.5 million (damage + cost of control). Thrips (more than $4 million in losses) and seedcorn maggot ($400,000) caused nearly all of the losses.
Thrips are a major insect pest of onions. Two species of thrips are prevalent on onions, western flower thrips (Frankliniella occidentalis) and tobacco thrips (Frankliniella fusca). These insects can be a major problem in late winter and early spring as temperatures increase. The severity of these insects on onions can vary considerably from year to year, so growers scout carefully throughout the season.
Thrips have rasping mouthparts that cause physical damage to the onion leaf. Damaged leaves are more susceptible to subsequent disease infection as well as being less efficient at photosynthesis.
Growers begin to spray for thrips when an average of five thrips are present per plant.
Seed corn maggots (Delia platura), onion maggots (Delia antiqua), and other soil insects (e.g., wireworms and mole crickets) can be especially damaging when the onion transplants are small.
Insecticides. Nearly all commercial onions are treated with insecticides.
Review recommended insecticides in the Georgia Pest Management Handbook .
Nonchemical alternatives and IPM. Nearly all commercial onion acreage is scouted. Insecticide applications are made based upon scouting information. Although scouting data are used to minimize the amount of insecticide applied during the season, insecticides play a critical role in onion IPM. No scouting thresholds are available for soil pests; nearly all growers apply a prophylactic treatment of insecticide.
Managing weeds is critical for successful onion production. Effective weed control is more difficult than for many other crops. Because onions grow slowly, they are less competitive with weeds. Additionally, the crop can be exposed to both warm and cool season weed species.
Weeds compete with onions for light, nutrients, water, and space. In addition to reducing harvestable bulbs through competition, weeds have been shown to interfere with the harvesting process by decreasing hand-harvesting efficiency. Weeds can also harbor destructive insects and diseases that can severely damage the present or proceeding crop.
Bacterial streak and bulb rot (Pseudomonas viridiflava) can use several weeds as alternate hosts, including cutleaf evening primrose, dandelion, purple cudweed, spiny sowthistle, Virginia pepperweed and wild radish. Therefore, controlling these weeds may suppress bacterial streak and bulb rot levels.
Pendimethalin (0.5-1.0 lb a.i./acre) and oxyflourfen (0.05-0.5 lb a.i./acre) are applied as a tank mix. Nearly all Georgia onion growers apply one application of this combination at the beginning of the season after the onions are set. Growers choose a rate for each herbicide based on weed history, local conditions, and economics. No other herbicides are applied during the season.
Nonchemical alternatives and IPM
Growers incorporate other techniques in an IPM program for weeds, but nonchemical methods cannot replace the critical role of herbicides in onion production. Growers do not cultivate onions during the season. Hand weeding is not common, but it may be used over small areas where herbicides failed to control weeds. Crop rotation is an important aspect of controlling some persistent weed species and problem weeds such as sicklepod, nutsedge, and cocklebur.
Review pesticides recommended for weed management in the Georgia Pest Management Handbook.
Onion diseases can cause severe losses by reducing yield and quality of marketable onions. These onion diseases can occur in seedbeds, in production fields, and in storage. Disease management requires a strategy that integrates practices such as rotation, sanitation, optimum fertilization, preventive fungicide/bactericide applications, harvest timing, and proper handling, harvesting, and storage.
Botrytis leaf blight, Stemphylium leaf blight, and purple blotch cause onion growers to apply the bulk of the fungicides applied each season. Growers also apply a significant amount of copper for center rot (a bacterial disease). Bacterial streak/bulb rot can also be a very serious disease, but it cannot be controlled with pesticide sprays.
Botrytis Leaf Blight
Botrytis leaf blight (Botrytis squamosa) is a fungal disease that infects onion foliage. This fungus survives in onion debris in the soil or in cull piles as sclerotia. The sclerotia produce conidia that become airborne and spread to foliage in production fields. Infection is greatly increased by long periods of leaf wetness and temperatures around 80oF.
Initial symptoms of Botrytis leaf blight are small (less than 0.25 inch in length) whitish, necrotic spots surrounded by pale halos. Spots often become sunken and elongated. Severely blighted leaves may cause reduced bulb size.
A preventive fungicide spray schedule is the primary means that growers use to suppress development of Botrytis leaf blight. Destruction of cull piles, deep soil turning, and long rotations are also recommended to reduce losses to this disease.
Purple blotch (Alternaria porri) is probably one of the most common diseases of onion and is distributed worldwide. This fungus overwinters as mycelium in onion leaf debris. During periods favorable for sporulation (leaf wetness or relative humidity of 90 percent or higher for 12 or more hours) inoculum becomes windborne and spreads to new foliage. Infection is highest at 77o F. Older plant tissue is more susceptible to infection by purple blotch. Thrips feeding is thought to increase susceptibility of onion tissue to this disease.
Purple blotch symptoms are first observed as small, elliptical, tan lesions that often turn purplish-brown. Concentric rings can be seen in lesions as they enlarge. A yellow halo surrounds lesions and extends above and below the actual lesion itself for some distance. Lesions usually girdle leaves, causing them to fall over. Lesions may also start at the tips of older leaves.
A regular spray schedule is critical for management of this disease. The spray schedule is intensified later in the season during periods of prolonged leaf wetness and high relative humidity. Long rotations to non-related crops, good soil drainage, and measures to reduce extended leaf wetness periods will reduce the severity of losses to purple blotch.
Stemphylium Leaf Blight
This fungal disease, caused by Stemphylium vesicarium, has become more widespread in the Vidalia onion growing region during recent years. This disease typically attacks leaf tips, purple blotch lesions, and injured or dying onion leaves and is often identified as purple blotch. Disease cycle and epidemiology are similar to purple blotch. Stemphylium vesicarium may enter purple blotch lesions, causing a black fungal growth.
Since this fungus is usually found co-infecting with Alternaria porri, symptoms are identical or at least very similar to purple blotch. However, Stemphylium leaf blight lesions appear to be a darker, more olive-brown to black color than do purple blotch lesions. In the case of Stemphylium leaf blight, lesions are often more numerous on the sides of onion leaves facing the prevailing wind. These lesions grow rapidly, coalesce, and cause severe leaf blighting during periods of prolonged leaf wetness.
Practices used to suppress purple blotch generally reduce losses to Stemphylium leaf blight.
Center rot, caused by Pantoea ananatis, is a relatively new bacterial disease of onions grown in Georgia. Warm weather favors the development of epidemics of center rot. This bacterial pathogen has recently been found to be present in many weed species occurring in the Vidalia onion growing region.
Foliar symptoms of center rot are typically observed as severe chlorosis or bleaching of one or more of the center leaves of infected onions. Infected leaves are usually collapsed and hang down beside the onion neck. In harvested bulbs, reddish, collapsed scales near the neck area have been associated with center rot.
Some growers apply a regular application of a copper-based pesticide for center rot although some experts do not consider this an effective treatment. Growers avoid onion cultivars that have been documented to be more susceptible to center rot. Some growers plant varieties that mature earlier to reduce the risk of center rot.
Fungicides. Regular fungicide applications are critical for production. Growers typically begin to apply fungicides early in the season on a 5-14 day schedule. A field may be treated with fungicide up to 15 times in a season. The length of the spray interval and the number of applications will vary with disease pressure. Growers use several different products based on the particular diseases and to manage disease resistance.
Nonchemical alternatives and IPM. Disease management in onions is based on a combination of tactics. Effective fungicides are a critical component. Chlorothalonil is particularly important because of its role in resistance management. Other products, with a more targeted mode of action, are much more susceptible to resistance problems. The availability of chlorothalonil will prolong the effective life of many other fungicides.
Long rotations with unrelated plants are commonly used to manage diseases. Harvesting mature onions and handling them carefully is the key to reduced post-harvest losses. Proper storage conditions also minimize post-harvest disease problems. For some diseases, growers avoid varieties that are particularly susceptible. Most growers modify their irrigation schedule as the season ends to reduce disease losses.
This group includes some diseases that can cause tremendous losses. Although scheduled fungicide sprays will help to suppress other fungal diseases, growers do not typically apply pesticides to control these diseases.
Bacterial Streak and Bulb Rot
This bacterial disease of onion (Pseudomonas viridiflava) is a serious problem in the southeastern United States onion production areas. The disease is favored by excessive fertilization and prolonged periods of rain during the cool winter months of onion production.
Leaf symptoms initially appear as oval lesions or streaks that later result in the total collapse of the entire leaf. Initially, streaks are usually green and water-soaked but later cause constricted, dark green to almost black lesions near the base of infected leaves. Infected leaves will generally fall off the bulb when any pressure is applied to pull them off. A reddish-brown discoloration has been observed in the inner scales of harvested bulbs.
Some growers apply copper compounds for this disease, but the practice is considered ineffective. Growers reduce losses by avoiding over-fertilization during winter. Growers also use practices that reduce post-harvest rot such as harvesting mature onions, curing onions immediately after clipping, and avoiding bruising or wounding.
Botrytis Neck Rot
Botrytis neck rot, Botrytis allii, can cause significant losses both in the field and in storage. The fungus can survive in the soil or on rotting bulbs as sclerotia. Botrytis conidia may arise from these sclerotia and be carried by wind to spread the disease.
Although severe losses can be experienced in field situations, the bulk of losses to Botrytis neck rot are in storage. Harvesting healthy mature onions with well-dried necks greatly reduces Botrytis neck rot incidence in storage. Growers avoid over-fertilization and high plant populations that lead to delayed maturity and reduced air movement through the canopy, respectively. Curing onions with forced air heated to 98oF. will cause the outer scales to dry down and become barriers to Botrytis infection. Storing onions near 34oF. at approximately 70 percent relative humidity reduces growth and spread of neck rot. Sanitation through deep soil turning and destroying cull piles helps reduce the amount of Botrytis allii inoculum in production fields.
Onion downy mildew (Peronospora destructor) is very common throughout most areas of the world; however, it is rarely observed in the Vidalia onion growing region of Georgia. This fungus can overwinter in plant debris or be brought in on sets or seed. Temperatures between 50o and 55oF., long periods of leaf wetness and/or high relative humidity (95%) are optimal for infection and spread.
Growers employ management practices that ensure good airflow and adequate drainage to reduce the risk of high losses to this disease. They avoid infected planting stock and destroy cull piles to reduce available inoculum.
Pink root (Phoma terrestris) is a common root disease of onions in Georgia. This disease is greatly enhanced by stresses imposed on plants such as heat, cold, drought, flooding, and nutrient toxicities/deficiencies. The fungus reproduces and survives indefinitely in soil; therefore, continuous production of onions in the same field results in increased losses to pink root. Roots infected by the pink root fungus turn pink or sometimes appear purplish. Infected roots eventually turn brown and deteriorate. Onions in both seedbeds and production fields can become infected. Early infected plants may die or may not produce useable bulbs. Plants that are infected later produce small, unmarketable bulbs.
Growers use a long rotation to non-related crops (3-7 years) as the key management strategy for reducing losses to pink root. Correct soil tilth, fertility and water management reduce stresses that enhance disease development. The optimum temperature for growth and infection by pink root is 79oF.; therefore, delaying planting until soil temperatures average 75oF or below will allow roots to grow and develop prior to temperatures that enhance infection. Harvesting onions prior to soil temperatures reaching 79oF will allow onions to escape further pink root infection.
Fusarium Basal Rot
Fusarium basal rot (Fusarium oxysporum f. sp. Cepae) occurs sporadically in the Vidalia area. Losses to this disease can occur in the field and later when onions are in storage. Like pink root, Fusarium basal rot can build up in soils where onions are grown year after year.
Symptoms may be observed in the field as yellowing leaf tips that later become necrotic. This yellowing and/or necrosis may progress towards the base of infected plants. Sometimes leaves of infected plants may exhibit curling or curving. Infected bulbs, when cut vertically, will show a brown discoloration in the basal plate. This discoloration will move up into the bulb from the base. In advanced infections, pitting and decay of the basal plate, rotten sloughed-off roots, and white, fluffy mycelium are all characteristic symptoms and signs of Fusarium basal rot. Sometimes, infected bulbs may not show symptoms in the field but will rot in storage.
Like pink root, growers use a long rotation (4 or more years) to non-related crops as the key management strategy for reducing losses to Fusarium basal rot. Growers set healthy transplants, avoid fertilizer injury, and control insects to help reduce losses. Storing onions at 34oF will help minimize losses. Resistance to Fusarium basal rot has been identified in some commercial onion cultivars.
Burkholderia cepaciais the causal agent of this onion bacterial disease. Sour skin primarily affects onion bulbs, but foliar symptoms may also be observed from time to time. This disease usually manifests itself during harvest when temperatures above 85oF. are uncommon.
Foliar symptoms, when observed, are similar to those of center rot. Scales of infected bulbs develop a cheesy or slimy yellow growth and brown decay. Infected scales may separate from adjacent scales allowing firmer inner scales to slide out when the bulb is squeezed. Sour skin infected bulbs usually have an acrid, sour, vinegar-like odor due to secondary organisms.
Growers avoid overhead irrigation near harvest time to reduce losses to this disease. Also, they use practices that reduce the chance of irrigation water becoming contaminated with the sour skin bacteria. Growers avoid damaging onion foliage prior to harvest as this provides wounds for the bacteria to enter bulbs. The mature bulbs are dried as soon as possible after harvest to reduce post harvest losses. Infected bulbs are discarded before storing, as disease can spread from infected bulbs to healthy bulbs. Storing onions in cool, (32oF) dry areas will prevent bulb-to-bulb spread of sour skin.
Bacterial Soft Rot
Bacterial soft rot (Erwinia carotovora pv. carotovora) is a common problem in many vegetables, usually during storage. It typically develops in onions after heavy rains or after irrigation with contaminated water. This disease is primarily a problem on mature onion bulbs during warm (68-85oF.), humid conditions.
Field symptoms are very similar to those seen with center rot in that it causes center leaves of onions to become pale and collapse. Infected scales of bulbs are initially water-soaked and later appear yellow or pale brown. In advanced stages of infection, scales become soft and watery and fall apart easily. As the interior of the bulb breaks down, a foul smelling liquid fills the core area of the bulb. When harvesting, the tops of infected onions will pull off, leaving the rotting bulb still in the ground.
Growers try to avoid overhead irrigation where the water source has been potentially contaminated with bacterial soft rot bacteria. Some growers apply fixed copper products, but this practice is considered marginally effective in reducing spread. Growers try to harvest only mature onions and handle them carefully to reduce post harvest losses.
Review recommended fungicides in the Georgia Pest Management Handbook.
Stormy Sparks (Extension) email@example.com amd
David Riley (Research) firstname.lastname@example.org, UGA Vegetable Entomologists
David Langston, UGA Vegetable Pathologist email@example.com
Stanley Culpepper, UGA Vegetable Weed Scientist firstname.lastname@example.org
George E. Boyhan email@example.com, Darbie M. Granberry firstname.lastname@example.org and
W. Terry Kelley email@example.com, UGA Horticulturists
Kerry Harrison,firstname.lastname@example.org UGA Extension Engineer
William C. Hurst, UGA Food Scientist email@example.comOnion Production Guide. [George Boyhan, Darbie Granberry, and Terry Kelley, eds.]. 2001. http://www.ces.uga.edu/pubcd/B1198.htm
Georgia Pest Management Handbook. [P. Guillebeau, ed.]. Published annually.
Prepared by Paul Guillebeau, IPM/Pesticide Coordinator, Department of Entomology, University of Georgia, Athens GA 30602 firstname.lastname@example.org