Onion inventory control
This paper is concerned with a periodic review inventory system with fast and slow delivery modes and regular demand forecast updates. At the beginning of each period, on-hand inventory and demand information are updated. At the same time, decisions on how much to order using fast and slow delivery modes are made. Fast and slow orders are delivered at the end of the current and the next periods, respectively. It is shown that there exists an optimal Markov policy and that it is a modified base-stock policy. This paper is concerned with a periodic review inventory system with fast and slow delivery modes and regular demand forecast updates. At the beginning of each period, on-hand inventory and demand information are updated. At the same time, decisions on how much to order using fast and slow delivery modes are made. Fast and slow orders are delivered at the end of the current and the next periods, respectively. It is shown that there exists an optimal Markov policy and that it is a modified base-stock policy.
Onions are one of the oldest vegetables in continuous cultivation dating back to at least 4,000 BCE. The ancient Egyptians are known to have cultivated this crop along the Nile River. There are no known wild ancestors, however, the center of origin is believed to be Afghanistan and the surrounding region. Onions are among the most widely adapted vegetable crops. They can be grown from the tropics to subarctic regions. This adaptation is primarily due to differing response to day length. Unlike most other species, day length influences bulbing in onions as opposed to flowering. Onions are grouped into three groups based on their response to hours of daylength. The short-day varieties bulb with daylengths of 10-13 hours, intermediate varieties bulb with day lengths of 13-14 hours and are found in the mid-temperate regions of this country. Finally, long-day onions are adapted to the most northern climes of the United States as well as Canada and bulb with daylengths greater than 14 hours.
Onions were first brought to this country by early European settlers. These onions were adapted to the temperate climate found throughout the Northeast where the first European settlements occurred. Varieties from warmer regions of the Mediterranean eventually made their way to the Southeastern United States. In particular, varieties from Spain and Italy would become important to the Vidalia onion industry. The first of these varieties came through Bermuda and were thus referred to as Bermuda onions.
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 was YB986, which was selected from Excel, which in turn was derived from White Bermuda. The Granex name is a combination of Grano and Excel, the original parents.
The Vidalia onion industry began in 1931 when a grower by the name of Mose Coleman grew the first short-day onions in Toombs County. These mild onions were immediately popular with customers. At the beginning of the depression, these onions sold for $3.50 a 50 lb. bag, a considerable amount of money at the time. Soon other growers became interested in these mild onions. The industry grew slowly and steadily for several decades.
Its growth was fueled by the fact that the city of Vidalia sat at the intersection of important roads prior to construction of the interstate highway system. In addition, the supermarket chain Piggly Wiggly maintained a distribution center in Vidalia and would buy the onions and distribute them through their stores. Slowly the industry began to gain a national reputation.
In order to help promote the onions further, onion festivals were started in both Vidalia and Glennville in the mid 1970’s. At this time, approximately 600 acres of onions were produced. Growth continued during the next decade. 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. Growers should check the Georgia Department of Agriculture website or call the Department for information about growing Vidalia onions. Growers are required to be within the defined growing regions, use specific approved varieties, and register with the state of Georgia. The Georgia Department of Agriculture website can be accessed at: http://www.agr. georgia.gov/vidalia-onion.aspx.
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 months. The adoption of the Federal market order and CA storage has allowed this industry to grow to its current level of approximately 12,000 acres.
TRANSPLANT PRODUCTION AND DIRECT SEEDING
Short-day onions can be grown from both seed and transplants; however, the majority of onions are grown from transplants.
Transplant production begins in late summer with land preparation followed by seed sowing in September. Land for transplant production should not have been in onions or related Alliums for at least three (3) years. This is not always possible with fixed center-pivot systems. Sites with a history of onion diseases and severe weed problems should be avoided, however.
Once a site has been selected, a soil test should be taken to determine the optimum level of fertility and soil pH. The University of Georgia has specific recommendations for plant bed onions. Therefore, when submitting a soil sample to the University of Georgia’s Soil Test Laboratory, make sure to indicate that they are for transplant or plant bed onion production. The site should be deep turned to bury any residue from the previous crop. Several different seeders are available for transplanting. These should be set to sow 60-70 seed per linear foot. As an example, using a Plant-It Jr. four-hopper transplanter, set the plates to No. 24. This should give the needed seeding rate for plant beds. Vacuum seeders are also a good choice and can accurately deliver seed in the amounts and to the depth required. Other seeders can be used as long as they are capable of sowing 60-70 seed per linear foot and can consistently plant the seed at the proper depth (1/4-1/2 inches).
The plant bed soil should have a pH range of 6.0-6.5 for optimum growth. Soils in Georgia are generally acidic, therefore, if your soil pH is low, applications of lime are recommended. Dolomitic lime is preferred over calcitic lime because it supplies both calcium and magnesium while adjusting the pH. Changing soil pH is a relatively slow process, therefore if low pH is suspected early soil testing and lime application is advantageous to insure the soil pH is corrected in sufficient time for planting. Soil pH can take several months to change with lime applications.
Nitrogen recommendations on Coastal Plain soils range from 100 to 130 pounds of nitrogen per acre. On Piedmont, Mountain, and Limestone Valley soils apply 90 to 120 pounds per acre. Table 1 indicates the phosphorus and potassium recommendations based on soil residual phosphorus and potassium levels.
In addition, boron should be applied at one (1) pound per acre. If zinc results are low, five (5) pounds per acre of zinc should be applied. Sulfur is critical for proper onion production. This is particularly true on the Coastal Plain soils of South Georgia that are very low in sulfur. Sulfur at a rate of 20-40 pounds per acre will be required to produce quality onion transplants on these sandy loam soils.
A typical fertility program would consist of 300-400 pounds per acre of 10-10-10 with 12% sulfur applied preplant. This would supply 30-40 pounds of N-P-K along with 36-48 pounds of S. This would be followed by additional applications of P and K according to soil test recommendations. Generally additional P will not be needed, while additional K can be supplied as potassium nitrate (13-0-44). The additional N can be supplied in one to two applications of calcium nitrate (15.5-0-0) applied at 4 and 6 weeks post seeding. It should be noted that any fertilizer that supplies the required nutrients as required by the soil test can be used to produce plant bed onions. More recent work indicates that high P applications at plant bed seeding have no effect. Phosphorus can have limited availability during periods of cool soil temperature, but plant bed seeding in September; soil temperatures are sufficiently high to avoid P deficiency; however, plant beds that have not been fertilized properly at seeding may require ‘pop-up’ fertilizer to overcome deficiencies during the cooler months of November and December.
It is critically important that seedbeds be irrigated regularly to develop a good plant stand. A tenth of an inch of water applied several times a day may be needed to insure consistent soil moisture. See section on irrigation.
Plants are ready for harvest in about eight to ten weeks. Good quality transplants will be about the diameter of a pencil when ready. Transplants are pulled and bundled in groups of 50-80 plants and tied with a rubber band.
Approximately one half of the tops are cut from the transplants, usually with a machete. Harvested transplants are transported to the field in polyethylene net or burlap bags. Onion transplants can experience a ‘heat’ in these bags, which greatly reduces transplant survival. Care should be taken with transplants so they are not stored for excessively long periods of time in these bags, nor should they be left in the sun for too long. Planning is critical, harvest only enough plants that can be reasonably transplanted that day. Overnight storage in these bags should be avoided whenever possible, but if necessary they should be removed from the field to a cool dry location.
Alternatively, onions can be directly sown in the soil for production. This eliminates all of the fertilizer and other management requirements of transplant production. Timing and seedbed preparation are critical for successfully growing onions from direct seeding.
For direct seeding, onion seed should be sown on October 15th plus or minus one week. This is later than sowing for transplant production, but is required to avoid undue seed-stem formation (flowering) in the spring. The soil should be prepared so that it is free of clods and plant residue. The soil surface should be smooth with the proper amount of soil moisture. Soil that is too wet will clog the sowing equipment. Soil that is too dry may result in the seeder riding up on the soil and not sowing the seed at the proper depth. Seed should be sown with a precision seeder such as a vacuum planter set to sow seed at 4-6 inches in-row at a depth of 1⁄4 - 1⁄2 inches deep. The plant stand will be similar to transplanted onions with four rows on a slightly raised bed with 12-14 inches between the rows. Direct sowing can save a tremendous amount on costs and labor; however, care must be exercised to correctly sow the seed since you will only have one chance to get it right.
Table 1. Recommendations for phosphorus and potassium based on soil test analysis for plant bed onion production.
VARIETY SELECTION AND CHARACTERISTICS
As mentioned earlier, the type of onion grown in South Georgia is a short-day onion that bulbs during the short days of winter (>11 hours daylength). Although limited research has been done in this area, it may be possible to grow intermediate-day onions in North Georgia; however, they would not be as mild as the south Georgia Vidalia onions.
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Figure 1. Bulb Shapes: 1. flattened globe; 2. globe; 3. high globe; 4. spindle; 5. Spanish; 6. flat; 7. thick flat; 8. Granex; 9. top (courtesy of Texas A&M University).
The Vidalia onion industry is controlled by a Federal marketing order that is administered by the Vidalia Onion Committee and the Georgia Department of Agriculture. This market order defines what type of onions can be grown and be marketed as a Vidalia onion. A Vidalia onion must be a yellow Granex type. These onions should be slightly flattened, broader at the distalend (top) and tapering to the proximal end (bottom) (Figure 1). In addition, rules have given the Georgia Department of Agriculture the authority to determine acceptable varieties for the Vidalia industry. Under these rules, the University of Georgia has been mandated to test all onion varieties for three (3) years before making recommendations to the Georgia Commissioner of Agriculture. Varieties that the Georgia Department of Agriculture have recommended to be grown as Vidalia onions are listed in Table 2.
Table 2. List of approved Vidalia onion varieties (2014).
Variety Source Season
Allison D. Palmer Seed Mid
Caramelo Nunhems USA Inc. Mid
Century Seminis Seed Mid
DP Sweet 1407 D. Palmer Seed Mid
Georgia Boy D. Palmer Seed Mid
Goldeneye Seminis Seed Mid
Gianex Yellow PRR Seminis Seed Mid
Honeybee Shamrock Early
Miss Megan D. Palmer Seed Mid
Me. Buck D. Palmer Seed Mid
Nirvana Nunhems USA Inc. Mid
Nunhems NUN1002 Nunhems USA Inc. Mid
Nunhems NUN1003 aka Vidora Nunhems USA Inc. Mid
Nunhems NUN1006 aka Plethora Nunhems USA Inc. Mid
Pirate Bejo Seed Mid
Ringo Sakata Seed Mid
Spell Sweet D. Palmer Seed Mid
Savannah Sweet Seminis Seed Mid
Candy Ann Solar Seed Early
Candy Kim Solar Seed Early
Sweet Agent Seminis Seed Early
Sweet Caroline Nunhems USA Inc. Mid
Sweet Harvest Sakata Seed Mid
Sweet Jasper Sakata Seed Mid
Sweet UNO Enza Zaden Mid
Sweet Vidalia Nunhems USA Inc. Mid
WI-129 Wannamaker Seed Early
Onion varieties grown in Southeast Georgia fall into three broad maturity categories; early, mid-season, or late. 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.
Along with maturity, varieties will perform differently on a wide range of quality attributes, as well as yield. Varieties can differ for pungency, sugar content, disease resistance, seed stem formation, double centers, bulb shape, and bulb size. Growers should consider all of these characteristics when making decisions on variety selection. Growers wishing to try new varieties should consult University of Georgia variety trial results. Trial results should be examined over several years to get a true picture of a variety’s potential. Even after evaluating trial data, growers considering new varieties should grow them on limited acreage to get a feel for their performance potential under their growing conditions. In addition, growers wishing to grow Vidalia onions should check with the Georgia Department of Agriculture for the current allowed varieties.
SOILS AND FERTILIZER MANAGEMENT
Onions grow best on fertile, well-drained soils. Tifton series 1 and 2 soils are found in the Vidalia onion area and are well suited for onion production. 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. Avoid soils with heavy clay content and coarse sandy soils. Clay soils tend to have a higher sulfur content, which can lead to pungent onions. Exceedingly sandy soils are more difficult to manage because they require more fertilizer and water.
Fertilizer and lime requirements should always be based on a recent, properly obtained soil sample. Check with your local county extension office or crop consultant regarding proper procedures for soil sampling and interpretation of results. Obtain the soil sample a few months prior to crop establishment in order to determine lime requirements and make necessary lime applications in a timely manner. If soil test results show a pH below 6.0, apply and disk in dolomitic lime two to three months before land preparation to bring the pH to the optimum range of 6.2 to 6.5. It is essential to apply sufficient lime to keep the soil pH above 6.0. Low pH can cause nutrient deficiencies during the growing season. Also, the high rates of fertilizer used in producing onions cause the pH to drop during the growing season. If the pH is not corrected at the beginning of the onion season, nutrient deficiencies could occur during the year and reduce yield. Calcium and phosphorous deficiencies can often be linked to low pH, even though soil tests indicate adequate levels. But phosphorus deficiencies due to low pH can be difficult to correct during the growing season.
Onions require more fertilizer than are used in most vegetable crops because fertilization of both plantbeds and dry bulb onions must be considered. They respond well to additional fertilizer applied 40 to 60 days after seeding or transplanting. The method of fertilizer application is very important in obtaining maximum yield with multiple applications insuring good yields. This will increase the amount of fertilizer utilized by the plant and lessen the amount lost from leaching. More recent research; however, indicates that good results can be obtained with as few as three fertilizer applications. Preplant fertilizer will vary with the natural fertility and cropping history. Proper application methods and function of various nutrients are outlined below. Table 3 shows a suggested fertilizer program for a soil testing medium in P and K.
Nitrogen (N), especially in nitrate (NO3) form, is extremely leachable. If too little nitrogen is available, onions can be severely stunted. High nitrogen rates are believed to produce succulent plants that are more susceptible to chilling or freezing injury, disease, and to production of flower stalks. Onions, heavily fertilized with nitrogen, are believed to not store well. Finally, excess nitrogen late in the growing season is believed to delay maturity and causes double centers. Make the final nitrogen application at least four weeks prior to harvest. Rates of nitrogen will vary depending on soil type, rainfall, irrigation, plant populations and method and timing of applications. For dry bulb production from transplanting or direct seeded onions should require between 125-150 lbs/acre nitrogen. It is usually best to incorporate 25% to 30% of the recommended nitrogen prior to planting and apply the remainder in two or three split applications.
Phosphorus (P) is essential for rapid root development. It is found in adequate levels in most soils but is not readily available at low soil temperatures. Because of these factors, under most conditions all of the P should be applied preplant and incorporated before transplanting. This amount should be counted as part of the total seasonal fertilizer application. Table 4 shows the recommended phosphorous to be applied based on various soil test levels.
Potassium (K), is an important factor in plant water relations, cell-wall formation and energy reactions in the plant. Potassium is also subject to leaching from heavy rainfall or irrigation. Therefore, it is best to split K applications by incorporating 30% to 50% of the recommended K before planting and splitting the remainder in one to two sidedress applications. A low K level makes plants more susceptible to cold injury. Table 4 lists recommended K applications based on soil test results.
Sulfur (S) is an essential element for plant growth. Early applications of sulfur are advisable in both direct-seeded and transplanted onions. To minimize pungency, fertilizers that contain S should not be applied after the end of January. Research conducted in Georgia on S and onion pungency has shown that pungency (pyruvate analysis) of mature onions increases with high rates of S or whenever S applications are made after late January. Therefore, S should not be applied to onions after late January unless the onions exhibit S deficiency. Do not completely eliminate S from the fertility program. Apply 40 to 60 pounds of elemental S with half incorporated at transplanting or seeding and half applied at the first sidedress application. Do not apply S in rates higher than 40-60 lb/acre.
Boron (B) is required by direct-seeded or transplanted onions in the field. If the soil test shows B levels are low, apply one pound of B per acre and incorporate prior to transplanting or seeding. Do not exceed the recommended amount since boron can be toxic to onions.
Zinc (Zn) levels determined to be low by soil testing can be corrected by applying five pounds of Zn per acre. Excessive amounts of Zn can be toxic, so apply only if needed. Zinc is usually added in the preplant fertilizer.
Magnesium (Mg) levels in the soil must be adequate for good onion growth. If dolomitic limestone is used in the liming program it will usually supply some of the required Mg. However, if soil pH is adequate and the soil-test Mg level is low, apply 25 pounds of Mg per acre in the preplant fertilizer.
Slow release fertilizers have been introduced to the Vidalia growing region. These fertilizers have performed well and can be considered in a fertility program. These fertilizers; however, have not proven satisfactory for single fertilizer application.
Table 3. Sample fertilizer recommendations for transplanted onions with a plant population of 60,000 to 80,000 plants per acre. Make adjustments for soil test levels other than medium P and medium K.
Timing Amount (lb/acre) Type Method N P2O5 K2O S
Preplant 400 !0-10-10 with 12% S Broadcast & incorporate 40 40 40 48
January
85
250
200
0-0-60
0-20-0
15.5-0-0
Broadcast
0
0
31
0
50
0
50
0
0
February 520 15.5-0.0 Broadcast 81 0 0
Total 152 90 90 48
A complete fertilizer with minor elements will provide most of the other required nutrients. Micronutrients can become toxic, if excessively applied so apply them only when needed and in precise amounts. Routine visual inspection of onion fields to watch for nutrient deficiencies is always important. However, during periods of high rainfall or frequent irrigation, be particularly aware of the potential for nutrient deficiencies to occur.
Deficiencies of major nutrients cannot be feasibly corrected through foliar nutrient applications. Therefore, it is important to properly manage soil fertility to maintain optimum growth and development. Some deficiencies of minor elements can be remedially corrected through foliar applications. However, it is always best to supply adequate amounts of these nutrients through your basic soil fertility program. Plants utilize nutrients more efficiently when the nutrients are taken up from the soil. Also, by the time you visually see a deficiency symptoms, you have probably already lost some potential yield.
Table 4. Recommended potassium and phosphorous applications based on soil test ratings of each nutrient.*
*Nitrogen recommendations: Coastal Plain Soils: 130-150 lb/acre N. Piedmont, Mountain and Limestone Valley Soils: 110-130 lb/acre N.
Plant Tissue Analysis
Plant tissue analysis is an excellent tool to evaluate crop nutrient status. Periodic tissue analysis can be used to determine if fertility levels are adequate or if supplemental fertilizer applications are required. Tissue analysis can often be used to detect nutrient deficiencies before they are visible.
Plant tissue analysis is accomplished by sampling the most recently mature leaves of the plant. A sample of 20-30 leaves should be taken from the field area(s) in question. Check with your local county extension office or crop consultant on proper tissue analysis techniques. The University of Georgia through its Plant, Soil, and Water Testing Laboratory can analyze your samples. Table 5 shows critical ranges for nutrient concentrations in onion tissue for the crop stage just prior to bulb initiation.
Table 5. Plant tissue analysis critical values for dry bulb onions.
Adapted from Vegetable Production Guide for Florida. Pub. No. SP 170. Univ. of Florida Cooperative Extension Service. 1999.
CULTURAL PRACTICES
Transplants (see Transplant Production) are generally set in November to December. They can, however, be successfully set in January. Plants set in February will generally be smaller at maturity. Consequently, they will have a smaller percent of jumbos. Early varieties should be planted prior to the end of December. If planted late, they will have lower yields and smaller bulbs because they are strongly daylength sensitive and will ‘go down’ (tops break over at the neck) or reach maturity earlier than other varieties.
Transplants are field set on slightly raised beds approximately four feet wide. Beds are six feet center to center. These beds or panels, as they are sometimes called, will have four (4) rows of onions spaced 12-14 inches apart and a spacing of 4.5 to 6 inches within the row (Figure 2). The spacing is determined by peg spacing on a pegger used to place holes in the bed surface 1 to 2 inches deep (Figure 3). Transplants are hand set in each hole.
Onions grow slowly during the cool short days of winter. Because of this, fertilizer, pesticide, and irrigation practices must minimize disease while maintaining optimum growing conditions.
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Figure 2. Typical onion field.
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Figure 3. Pegger
Harvest maturity is reached when 20-50% of the onion tops are down. In most seasons, onion neck tissue will break down when the plant is mature. Although this is a good rule-of-thumb for determining when onions mature, the tops may not go down as readily in some years or for some varieties. In addition, early varieties are very day length sensitive and usually go down early and uniformly. These early varieties should be harvested when 100% of the tops go down. They can be allowed to stay in the field for a week after tops down and will continue to enlarge. This will increase yield as the bulbs continue to increase in size. Knowing the variety and carefully inspecting the crop is the best method to determine maturity. Whether the tops go down or not, the neck tissue will become soft, pliable, and weak at maturity. Onions harvested too early may be soft and not dry down sufficiently during curing. In addition, they may begin to grow because they are not completely dormant. If the onions are harvested too late, there may be an increase in post-harvest diseases and sunscald on the shoulder of the bulb. Although F1 hybrids will have a narrow window of maturity, they will not all mature at once. Generally, a field of onions will be harvested before all the bulbs have their tops down.
Onions are prone to physiological disorders that growers should try to minimize. One such disorder is splits or doubles. This condition is caused by cultural and environmental factors as well as being influenced by genetics. Over-fertilization, uneven watering, and temperature fluctuations (particularly below 20 °F) are all believed to have an influence on double formation. Some varieties are more prone to production of doubles than others. Varieties prone to doubling should be seeded a week or so later on the plant beds as well as transplanted a bit later to minimize this disorder.
Onions are biennials forming bulbs the first year, which will act as a food source the following year when the plant flowers. The process of flowering in onions is called bolting. A seedstalk or scape will form very quickly and appear to bolt up. These flower stalks or seedstems can form in the first year if appropriate environmental conditions occur and plant size are favorable. Cool temperatures during the latter part of the growing season (March and April), when plants are relatively large, can result in a high percentage of seedstems. There also appears to be a variety component to seedstem formation.
Onions can generally withstand light to heavy frosts, but hard freezes can result in onion damage. Freeze injury may be readily detectable as translucent or water soaked outer scales of the bulbs. A day or two after the freeze event, onions should be cut transversely to see if translucent scales are present. In some cases, freeze damage may not be readily detectable for several days. In these cases, the growing point may have been affected and subsequent growth will be abnormal, increasing the incidence of doubles. Apparently, the growing point is damaged to the extent that two growing points develop. Under severe freeze conditions the plant may be killed. Control of freeze and frost injury is usually done by cultivating the fields, if such an event is anticipated. Cultivating fields results in a layer of moist soil at the surface which acts as insulation. This holds the day’s heat in the soil around the bulb and root. The downside to cultivating may increase the incidence of disease caused by throwing up contaminated soil on tender onion tissue.
Onions may develop disorders that are not associated with insect, disease, or nutrient problems. Greening is one such occurrence. This occurs when the bulb is exposed to sunlight for an extended period of time. Early fertilizer application is needed to develop a strong healthy top, which shade the bulb during development.
Sunscald will occur at the shoulder of all onions that are exposed to sunlight for an extended period of time. Bulb sunscalding can occur when maturity is reached and harvest is delayed. Harvest should occur as soon as possible after the crop has matured. Scales several layers deep will dry and turn brown. Under severe conditions the internal tissue may actually cook or, become soft and translucent.
Translucent scale is a physiological disorder similar in appearance to freeze injury. The big difference is, freeze injury will always affect the outer scales whereas translucent scale may first appear on scales several layers deep in the bulb. Translucent scale is a postharvest phenomenon caused by high CO2 in storage facilities. This is most likely to occur in refrigerated storage without adequate ventilation. CO2 levels above 8% will increase the chance of translucent scale. Growers and packers should carefully monitor storage facilities to prevent this.
Physical damage of onions may appear that may be confused with botrytis leaf blight (see disease section). This damage is usually caused by wind-blown sand or hail. Strong winds can cause flecking of leaves particularly in fields with dry sandy soils. Hail damage will usually be more severe with large (0.125-0.25 inches in diameter) white or yellow lesions on the leaves. The shoulders of the exposed bulbs will often have a dimpled feel. In severe cases, the crop can be defoliated and destroyed.
Occasionally plants may exhibit a striped appearance. If this is widespread in a field, S deficiency is the probable cause (see fertility section). If it appears on an isolated plant, it is probably a chimera. Chimeras result when a mutation occurs in the meristematic tissue (growing point) resulting in a striped plant. This should not be a concern.
IRRIGATING SWEET ONIONS IN GEORGIA
Because of the importance of water management in onions, all commercially grown onions in Georgia are irrigated. Research and extension trials in Georgia have indicated that properly irrigated onions will yield 25 to 50 percent more than dry land onions. Irrigated fields typically yield a higher percentage of 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.
Irrigation System Options
Almost all onions in Georgia are sprinkler-irrigated. The two most commonly used systems are center pivots and traveling guns.
Center pivot systems are generally one of the lowest cost systems per acre to install and require very little labor to operate. If properly maintained, they apply water very uniformly, and because of the low pressure required to operate them, they are generally very energy efficient. They are not well adapted to small irregular shaped fields, and unless the system is towable, it is restricted to use in only one field. If a farmer has a limited amount of irrigated land, this characteristic can be detrimental to desirable crop rotations.
Traveling guns are mobile systems that can be moved from field to field or farm to farm. They can be used on almost any shaped field. They do require high water pressure to operate and consequently require more fuel per acre-inch of water than other options. Traveling guns also require a considerable amount of labor to operate. These systems tend to increase soil compaction and are harsh on young plants.
Other irrigation systems can be used as long as they can supply the need water evenly over the entire field.
Irrigation Scheduling
Water use of onions varies considerably throughout the growing period and varies with weather conditions. The peak water demand for onions can be as high as 1.5 to 2.0 inches per week. Peak use generally occurs during the latter stages of bulb enlargement especially during periods of warm weather. However, there are other stages when supplemental water may be needed.
Transplanted onions should be watered very soon after setting. About one-half inch applied at this time will help establish good contact between the soil and roots and assure a good stand.
During the next two or three months the plants will be small and have a relatively shallow root system. The fall months also tend to be some of the driest months in Georgia. During this period, irrigation should be applied whenever the soil becomes dry in the top six inches. Irrigation amounts should be limited to about one-half inch per application during this stage. Irrigation applications are typically infrequent during this period, since the plants are small and water demand is relatively low.
When the bulbs begin to enlarge water demand will gradually increase as will the need to irrigate when the weather turns dry. Rooting depths at this stage are typically 12 inches or less. Because of the shallow rooting depth, irrigation applications should not exceed 1.0 inch. Typical applications should range between 0.6 and 1.0 inch, for loamy soils and for sandy soils, respectively. During dry weather, irrigate two or three times per week, especially when the weather is warm. Of course, when temperatures are cool, irrigations may be less frequent.
Unlike most other crops, onions do not generally wilt when they experience moisture stress. Since moisture stress is difficult to detect by visual inspection, it is very helpful to monitor soil moisture. This can be done by installing tensiometers or electric resistance blocks or any other moisture sensor in the soil. Install soil moisture sensors at two depths, one near the middle of the root zone and one near the bottom. Common practice is to install one at four to six inches and one at 10 to 12 inches. The ideal range for soil moisture is between (soil tension) 5 and 20 centibars for most coastal plain soils. Readings of less than five indicates saturated conditions and above 20 indicates the soil is becoming dry. If you use a center pivot or traveling gun, you should start early enough so that the last part of the field to get watered does not get too dry before the system gets there.
In general, if the system requires three days to water the entire field, then you should install at least three soil moisture stations, evenly spaced around the field. Each station will consist of two sensors, one shallow and one deep. You should monitor the readings on the soil moisture sensors at least three times per week when the weather is dry.
CHEMICAL APPLICATION
Two types of sprayers, boom and air-assisted, are used for applying insecticides, fungicides, herbicides, and foliar fertilizers. Air-assisted sprayers (Figure 4) utilize a conventional hydraulic nozzle, plus air to force the spray into the plant foliage. Boom sprayers (Figure 5) get their name from the arrangement of the conduit that carries the spray liquid to the nozzles. Booms or long arms on the sprayer extend across a given width to cover a swath as the sprayer passes over the field.
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Figure 4. Air assisted sprayer.
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Figure 5. Boom sprayer.
Pumps
Three factors to consider in selecting the proper pump for a sprayer are capacity, pressure, and resistance to corrosion and wear. The pump should be of proper capacity or size to supply the boom output and to provide for agitation 5 to 7 gallons per minute (gpm) per 100-gallon tank capacity. Boom output will vary depending upon the number and size of nozzles. Also, 20 to 30 percent should be allowed for pump wear when determining pump capacity. Pump capacities are given in gallons per minute. The pump must produce the desired operating pressure for the spraying job to be done. Pressures are indicated as pounds per square inch (psi). The pump must be able to withstand the chemical spray materials without excessive corrosion or wear. Use care in selecting a pump if wettable powders are to be used as these materials will increase pump wear.
Before selecting a pump, consider factors such as cost, service, operating speeds, flow rate, pressure and durability. For spraying vegetable crops, a diaphragm pump is preferred because of service ability and pressures required.
Nozzles
Nozzle selection is one of the most important decisions to be made related to pesticide applications. The type of nozzle determines not only the amount of spray applied, but also the uniformity of application, the coverage obtained on the sprayed surfaces, and the amount of drift that can occur. Each nozzle type has specific characteristics and capabilities and is designed for use under certain application conditions. The types which are commonly used for ground application of agricultural chemicals for onions are the fan and cone nozzles.
Herbicides
The type of nozzle used for applying herbicides is one that develops a large droplet and has no drift. The nozzles used for broadcast applications include the extended range flat fan, drift reduction flat fan, turbo flat fan, flooding fan, turbo flooding fan, turbo drop flat fan, and wide angle cone nozzles. Operating pressures should be 20 to 30 psi for all except drift reduction and turbo drop flat fans, flooding and wide angle cones. Spray pressure more than 40 psi will create significant drift with flat fan nozzles. Drift reduction and turbo drop nozzles should be operates at 40 psi. Flooding fan and wide angle cone nozzles should be operated at 15 to 18 psi. These nozzles will achieve uniform application of the chemical if they are uniformly spaced along the boom. Flat fan nozzles should overlap 50 to 60 percent.
Insecticides and Fungicides
Hollow cone nozzles are used primarily for plant foliage penetration for effective insect and disease control, when drift is not a major concern. At pressures of 60 to 200 psi, these nozzles produce small droplets that penetrate plant canopies and cover the underside of the leaves more effectively than any other nozzle type. The hollow cone nozzles produce a cone shaped pattern with the spray concentrated in a ring around the outer edge of the pattern. Even fan and hollow cone nozzles can be used for banding insecticide or fungicides over the row.
Nozzle Material
Various types of nozzle bodies and caps, including color-coded versions, and multiple nozzle bodies are available. Nozzle tips are interchangeable and are available in a wide variety of materials, including hardened stainless steel, stainless steel, brass, ceramic, and various types of plastic. Hardened stainless steel and ceramic are the most wear-resistant materials. Stainless steel tips, even when used with corrosive or abrasive materials, have excellent wear resistance. Plastic tips are resistant to corrosion and abrasion and are proving to be very economical for applying pesticides. Brass tips have been common, but wear rapidly when used to apply abrasive materials such as wettable powders. Brass tips are economical for limited use, but other types should be considered for more extensive use.
Water Rates (GPA)
The grower who plans to use spray materials at the low water rate should follow all recommendations carefully. Use product label recommendations on water rates to achieve optimal performance. Plant size and condition influence the water rate applied per acre. Examination of the crop behind the sprayer before the spray dries will give a good indication of coverage.
Other irrigation systems can be used as long as they can supply the need water evenly over the entire field.
Agitation
Most materials applied by a sprayer are in a mixture or suspension. Uniform application requires a homogeneous solution provided by proper agitation (mixing). The agitation may be produced by jet agitators, volume boosters (sometimes referred to as hydraulic agitators), and mechanical agitators. These can be purchased separately and installed on sprayer tanks. Continuous agitation is needed when applying pesticides that tend to settle out, even when moving from field to field or when stopping for a few minutes.
Nozzle Arrangements
When applying insecticides and fungicides, use a broadcast boom arrangement. Place nozzles on 10 to 12 inch centers for complete coverage of the plant.
Calibration
The procedure below is based on spraying 1/128 of an acre per nozzle or row spacing and collecting the spray that would be released during the time it takes to spray the area. Because there are 128 ounces of liquid in 1 gallon, this convenient relationship results in ounces of liquid collected being directly equal to the application rate in gallons per acre.
Calibrate with clean water when applying toxic pesticides mixed with large volumes of water. Check uniformity of nozzle output across the boom. Collect from each for a known time period. Each nozzle should be within 10 percent of the average output. Replace with new nozzles if necessary. When applying materials that are appreciably different from water in weight or flow characteristics, such as fertilizer solutions, etc., calibrate with the material to be applied. Exercise extreme care and use protective equipment when active ingredient is involved.
Table 6. Distance to measure to spray 1/128 acre. One ounce discharged equals one gallon per acre.
Nozzle Spacing (inches) Distance (feet) Nozzle Spacing (inches) Distance (feet)
6 681 20 204
8 520 22 186
10 408 24 170
12 340 30 136
14 292 36 113
16 255 38 107
18 227 40 102
To determine a calibration distance for an unlisted spacing, divide the spacing expressed in feet into 340. Example: Calibration distance for a 13” band = 340_13/12=313 feet.
From Table 6, determine the distance to drive in the field (two or more runs are suggested). For broadcast spraying measure the distance between nozzles. For band spraying, use band width. For over the row or directed use row spacing.
Measure the time (seconds) to drive the required distance; with all equipment attached and operating. Maintain this throttle setting!
With sprayer sitting still and operating at same throttle setting or engine RPM as used in Step 2, adjust pressure to the desired setting. Machine must be operated at same pressure used for calibration.
For broadcast application, collect spray from one nozzle or outlet for the number of seconds required to travel the calibration distance.
For band application, collect spray from all nozzles or outlets used on one band width for the number of seconds required to travel the calibration distance.
For row application, collect spray from all outlets (nozzles, etc.) used for one row for the number of seconds required to travel the calibration distance.
Measure the amount of liquid collected in fluid ounces. The number of ounces collected is the gallons per acre rate on the coverage basis indicated. For example, if you collect 18 ounces, the sprayer will apply 18 gallons per acre. Adjust applicator speed, pressure, nozzle size, etc. to obtain recommended rate. If speed is adjusted, start at Step 2 and recalibrate. If pressure or nozzles are changed, start at Step 3 and recalibrate.
DISEASES OF VIDALIA ONIONS
Onion diseases can cause severe losses by reducing yield and quality of marketable onions. These onion diseases can occur in seedbeds, production fields and in storage. Disease management requires a systems approach that involves practices such as rotation, sanitation, optimum fertilization, preventive fungicide/bactericide applications, harvest timing, and proper harvesting, handling, and storage. If one or more of these practices are omitted, disease management is significantly compromised.
Fungal Diseases Affecting Roots and Underground Plant Parts
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Figure 6. pink colored onion roots of onions infected with pink root (Phoma terrestris).
PINK ROOT
Pink root, caused by the fungus Phoma terrestris, is the most common and damaging 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.
Symptoms: The name of this disease is its most descriptive symptom. Roots infected by the pink root fungus turn pink or sometimes appear purplish (Figure 6). 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. Later infected plants are stunted, producing small, unmarketable bulbs.
Management Options: Utilizing a long rotation to non-related crops (3-7 years) can work as a management strategy for reducing losses to pink root; however, this may not always be possible. Also, correct soil tilth, fertility and water management will reduce stresses which enhance disease development. The optimum temperature for growth and infection by pink root is 79o F, therefore delaying planting until soil temperatures average 75o F or below will allow for roots to grow and develop prior to temperatures that enhance infection. Harvesting onions prior to soil temperatures reaching 79o F will allow onions to escape further pink root infection. Fumigation with metam sodium, chloropicrin and 1,3-D dichloropropene (Telone) have been shown to increase yields when onions have been planted to fields heavily infested with pink root. Onion varieties resistant to pink root that also have horticulturally acceptable qualities should also be considered.
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Figure 7. Onion basal plate infected with Fusarium basal rot.
FUSARIUM BASAL ROT
Fusarium basal rot is caused by the fungus Fusarium oxysporum f. sp. cepae. This disease 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: Symptoms may be observed in the field as yellowing leaf tips which 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 (Figure 7). 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.
Management Options: Like pink root, utilizing a long rotation (4 or more years) to non-related crops can be a key management strategy for reducing losses to Fusarium basal rot. Use of healthy transplants, avoiding fertilizer injury and controlling insects will reduce losses to basal rot. Storing onions at 34o F will help minimize losses. Resistance to Fusarium basal rot has been identified in some commercial onion cultivars. (check on current varieties)
Fungal Diseases Affecting Aboveground Plant Parts
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Figure 8. Gray sporulation of Botrytis neck rot.
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Figure 9. Reddish brown discoloration of onion scales caused by Botrytis neck rot.
BOTRYTIS NECK ROT
Botrytis neck rot is the most damaging fungal disease affecting onions in Georgia with severe losses occurring both in the field and in storage. The fungus causing botrytis neck rot, Botrytis allii, 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.
Symptoms: Although the bulk of losses to botrytis neck rot are instorage, severe losses can be experienced in field situations. Plants infected in the field exhibit leaf distortion, stunted growth and splitting of leaves around the neck area. A grayish sporulation of the fungus may be observed between leaf scales near the neck area (Figure 8). In storage, infection can be internal with no discernible symptoms on the onion. It is not until onions are removed from storage that the infection becomes evident. Apparently the infection enters the neck and continues to grow undetected in storage until the onions are removed. It has been demonstrated that botrytis neck rot is not capable of sporulation in controlled atmosphere storage (high CO2, low O2, refrigerated storage), but continues to grow and destroy infected onion tissue. Infected tissue is sunken, water soaked and spongy with a reddish brown color (Figure 9). The grayish fungal sporulation can be seen between scales in infected bulbs. The gray mold will later appear on the onion surface and may give rise to hard, black sclerotia.
Management Options: Harvesting healthy mature onions with a well-dried neck will greatly reduce botrytis neck rot incidence in storage. Avoid over-fertilization and high plant populations which lead to delayed maturity and reduced air movement through the canopy, respectively. Curing onions with forced air heated to 98o F will cause the outer scales to dry down and become barriers to botrytis infection. Storing onions near 34o F at approximately 70% 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. A combination of boscolid and pyraclostrobin as well as these products individually have been shown to give good control of botrytis neck rot. Using any fungicide should be integrated into a complete system of disease control. In addition, follow label direction for use. For questions on a specific program of disease control contact your local county extension agent.
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Figure 10. Pale lesions caused by Botrytis leaf blight.
BOTRYTIS LEAF BLIGHT
Botrytis leaf blight caused by Botrytis squamosa is another botrytis disease. However, this fungus 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 80o F.
Symptoms: Initial symptoms of botrytis leaf blight are small (less than .25 inches in length) whitish, necrotic spots surrounded by pale halos (Figure 10). Spots often become sunken and elongated. Severely blighted leaves may result in reduced bulb size.
Management Options: Preventive spray schedules containing the fungicides maneb, mancozeb, and chlorothalonil are the primary means used to suppress development of botrytis leaf blight. In addition, iprodione, cyprodinil and fludioxonil, bocolid, and pyraclostrobin represent other materials that are effective against this pathogen that growers may wish to integrate into their disease management program. Destruction of cull piles, deep soil turning, and long rotations are also recommended to reduce losses to this disease.
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Figure 11. Elliptical lesion characteristic of purple blotch.
PURPLE BLOTCH
Purple blotch, caused by Alternaria porri, is probably one of the most common diseases of onion and is distributed worldwide. The fungus overwinters as mycelium in onion leaf debris. During periods favorable for sporulation (leaf wetness or relative humidity of 90% 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.
Symptoms: Purple blotch symptoms are first observed as small, elliptical, tan lesions that often turn purplish-brown (Figure 11). 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.
Management Options: 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. Spray schedules which include mancozeb, chlorothalonil, and iprodione will suppress purple blotch. In addition, boscolid and pyraclostrobinare effective against this disease. These schedules should be intensified later in the season during periods of prolonged leaf wetness and high relative humidity.
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Figure 12. Dark sporulation indicative of Stemphylium leaf blight.
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.
Symptoms: 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 contain a darker, more olive brown to black color than do purple blotch lesions (Figure 12). 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.
Management Options: Practices used to suppress purple blotch will generally reduce losses to Stemphylium leaf blight. However, unlike purple blotch, the fungicide iprodione, boscolid, and pyraclostrobin are the only fungicide thought to be effective against Stemphylium leaf blight.
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Figure 13. Velvety sporulation of the downy mildew fungus. Photo by Tom Isakeit, Texas A&M University, 1995.
DOWNY MILDEW
Onion downy mildew, caused by the fungus 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 55o F, long periods of leaf wetness and/or high relative humidity (95%) are optimal for infection and spread.
Symptoms: Downy mildew may be first detected in the early morning as a violet, velvety sporulation (Figure 13). With time, infected areas of leaves become pale and later turn yellow. These lesions may girdle the leaf and cause it to collapse. Epidemics may begin in small spots in a field that will spread, mainly during periods of high relative humidity, and cause considerable defoliation.
Management Options: Management practices which ensure good airflow and adequate drainage will reduce the risk of high losses to this disease. Avoiding infected planting stock and destroying cull piles reduce available inoculum. Preventive application of fungicides provides the primary control of downy mildew in regions where it is a perennial problem. Fungicides such as mefenoxam, fosetyl-Al, chlorothalonil and mancozeb should be used at the first report of disease in the growing area.
Bacterial Diseases
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Figure 14. Collapsed leaves caused by bacterial streak.
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Figure 15. Dark green lesions caused by bacterial streak.
BACTERIAL STREAK AND BULB ROT
This bacterial disease of onion, caused by Pseudomonas viridiflava, is a problem in the southeastern U.S. onion production areas. Disease is favored by excessive fertilization and prolonged periods of rain during the cool winter months of onion production.
Symptoms: Leaf symptoms initially appear as oval lesions or streaks that later result in the total collapse of the entire leaf (Figure 14). Initially, streaks are usually green and water-soaked but later cause constricted, dark green to almost black lesions near the base of infected leaves (Figure 15). 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.
Management Options: Preventive application of fixed copper materials tank mixed with EBDC fungicides (Maneb, Mancozeb, Manzate, Dithane, Penncozeb and others) may reduce the incidence and spread of this disease. Avoiding over-fertilization with N during winter months may reduce losses to bacterial streak. Practices that reduce post harvest rot such as harvesting mature onions, curing onions immediately after clipping, and avoiding bruising or wounding will help avoid disease problems.
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Figure 16. Bleached center leaves caused by the center rot pathogen Pantoea ananatis.
CENTER ROT
Center rot, caused by Pantoea ananatis, is another bacterial disease of onions grown in Georgia. Unlike bacterial streak, warm weather favors the development of epidemics of center rot. This bacterial pathogen is also found to be present in many weed species occurring in the Vidalia onion growing region.
Symptoms: Foliar symptoms of center rot are typically observed as severe chlorosis or bleaching of one or more of the center leaves of infected onions (Figure 16). 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.
Management Options: As with bacterial streak, fixed copper materials tank mixed with EBDC fungicides are recommended to suppress infection and spread. Several onion cultivars have been documented to be more susceptible to center rot and should be avoided. Onions that mature early may avoid center rot losses by being less exposed to the higher temperatures necessary for the development of disease.
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Figure 17. Onion bulb deterioration caused by sour skin.
SOUR SKIN
Burkholderia cepacia is 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 85o F are not uncommon.
Symptoms: 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 (Figure 17). 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.
Management Options: Avoidance of overhead irrigation near harvest time will reduce losses to this disease. Also, use practices which reduce the chance of irrigation water becoming contaminated with the sour skin bacteria. Avoid damaging onion foliage prior to harvest as this provides wounds for the bacteria to enter bulbs.Do not allow mature onions to remain in fields during the warm climates associated with the later harvest season as infection and spread of this bacterium is enhanced with higher temperatures. Infected bulbs should be discarded before storing as disease can spread from infected bulbs to healthy bulbs. Infected onions should not be heat cured postharvest as this will rapidly spread this pathogen to uninfected bulbs. Storing onions in cool (32o F) dry areas will prevent bulb-to-bulb spread of sour skin.
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Figure 18. Deterioration of the core bulb scales caused by bacterial soft rot.
BACTERIAL SOFT ROT
Bacterial soft rot, caused by Erwinia carotovora pv. carotovora, is a common problem in many vegetables, usually during storage. It usually develops in onions after heavy rains or after irrigation with contaminated water. This disease is primarily a problem on mature onion bulbs during warm (68o-85o F), humid conditions.
Symptoms: 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 watersoaked 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 (Figure 18). When harvesting, the tops of infected onions will pull off leaving the rotting bulb still in the ground.
Management Options: Avoid overhead irrigation where the water source has been potentially contaminated with soft rot bacteria. Application of fixed copper products may be marginally effective in reducing spread. As with most bulb diseases, harvesting mature onions, care in handling, and storage in cool dry areas will prevent post harvest losses.
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Figure 19. Yellow bud of onion. Image by Ronald D. Gitaitis, University of Georgia, Bugwood.org.
YELLOW BUD OF ONION
Yellow bud (YB) is an emerging onion disease that has potential to severely affect Vidalia onion production. This disease was first observed in Georgia in 2007 and has since been spreading throughout the Vidalia onion-growing area in Georgia. However, to the best of our knowledge, this disease has not been reported elsewhere. The causal agent is a gram-negative, rod-shaped, aerobic bacterium that possesses all the phenotypic characteristics of Pseudomonas syringae. The yellow bud bacterium is possibly a pathovar of P. coronafaciens (as it is host-specific).
Symptoms: Symptoms of yellow bud include intense chlorosis in emerging leaves and severe blight in the older leaves. In time, yellow bud leads to stand loss, reduced bulb size, and may create possible avenues of ingress for secondary, soft rot organisms. The disease has also been observed in onion seed beds, thus infected transplants could be widely dispersed to areas throughout the Vidalia region or elsewhere. Occurrence of yellow bud in seedbeds may be an indication that the pathogen could be seedborne. There is evidence that this pathogen can be seedborne and seed transmitted in onion seeds.
Management Options: Preventive application of fixed copper materials tank mixed with EBDC fungicides (Maneb, Mancozeb, Manzate, Dithane, Penncozeb and others) may reduce the incidence and spread of this disease. This pathogen is also temperature sensitive and in most cases, yellowing symptoms subside with increase in temperature (>55°C).
Viruses
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Figure 20. Yellow spot virus (IYSV)
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Figure 21. Tomato spotted wilt virus (TSWV).
IRIS YELLOW SPOT VIRUS (IYSV) AND TOMATO SPOTTED WILT VIRUS (TSWV)
Recently these viruses have been detected in onions, but it is unclear if they are or will become a major disease in onions. TSWV has been a major disease in other crops in Georgia for many years. IYSV is known to be pathogenic on onions, which has become a major disease in other onion producing regions particularly in the western U.S. and particularly on onion seed crops. IYSV is spread by onion thrips (Thrips tabaci), which surprisingly are not generally found in Georgia. Recently, however, this virus has been detected in Tobacco thrips (Frankliniella fusca), which is widely distributed in Georgia.
These viruses can be detected in onions that are otherwise symptomless. These latent infections may never become a problem or symptoms may develop when onions are stressed such as during cold weather, during and after transplanting, or some other stress condition. It is unknown, however, if this does occur.
Symptoms: There is not enough information available to clearly identify symptoms associated with these virus infections. Small necrotic spots with green tissue remaining in the center may be symptom expression. This has not always been correlated with detection during laboratory screening.
Management Options: Since these viruses are spread by thrips, thrips control may help control infection. Typically thrips control (see insect section) has been important during late winter and early spring, but with the detection of these viruses, growers should begin scouting onions in the fall and early winter for thrips, taking necessary action when they appear. Since stress may be a factor in symptom development, care should be taken to minimize stress. Proper fertilization, water, and control of other diseases may be important. Obviously transplanting shock and cold weather are unavoidable, but it may be helpful to avoid transplanting onions just prior to colder temperatures. If cold weather is expected it may be wise to delay transplanting until the cold has passed.
Registered fungicides:
mancozeb & maneb Dithane, Manzate, etc.
chlorothalonil Bravo, Echo, Equus
iprodione Rovral
cyprodinil + fludioxonil Switch
fixed copper
mefenoxam
fosetyl-A1 Aliette
pyrimethanil Scala
mefenoxam + chlorothalonil Ridomil GoldBravo
mefenoxam + copper Ridomil Gold/Copper
mefenoxam + mancozeb Ridomil Gold MZ
mancozeb + copper ManKocide
boscolid + pyraclostrobin Pristine
boscalid Endura
dimethomorph Acrobat
azoxystrobin, pyraclostrobin Quadris/Amistar, Cabrio
Known effectiveness of registered fungicides:
1Information in this table was partly derived from ratings given at the IR-4 Bulb Vegetable Crop Group Workshop held during the 1999 American Phytopathological Society annual meeting in Montreal, Canada. Ratings for products does not necessarily indicate a labeled use.
C = When used in combination with mancozeb
V = Variable levels of control
R = Pathogen resistance (insensitivity) may be present at some locations
E = Excellent disease suppression
G = Good disease suppression
F = Fair disease suppression
P = Poor to no disease suppression
U = Unknown efficacy
VEGETABLE DISEASE CONTROL
ONION INSECTS AND THEIR CONTROL
Since onions are a winter crop in southeast Georgia, insect problems are not as severe as they would be for spring, summer, or fall crops. Preventative measures and careful scouting can minimize or eliminate any potential problems.
Soil borne insects such as cutworms, onion maggots, wireworms, and others can be controlled with preplant applications of an appropriate soil insecticide (Table 7). Application should be made just prior to seeding plantbeds as well as just prior to transplanting to final spacing.
Onion maggots (Delia antiqua) can be a severe pest in more northern states. The seed corn maggot (D. platura) is much more common in Georgia and generally does not cause as much damage as the onion maggot. The adults of both species are flies similar to, but smaller than houseflies. Adults lay their eggs in the soil near seeds or seedlings and the hatching larva feed on the developing plants. Seedcorn maggots can reduce plant stands in seedbeds, as germinating seeds and small seedlings can be killed. Once plants are established, seedcorn maggots are not likely to cause plant mortality, but may be associated with dead and decaying plants as these plants are attractive to the maggots, which will feed on most decaying plant material. It is also not uncommon to find large populations in fields shortly after severe frost damage. The frost damage results in an abundance of decaying organic matter in the fields, which is attractive to seed corn maggots. Seed corn maggot can be a problem late in the season as a contaminant in harvested bulbs. While they likely cause minimal damage to bulbs, the pupae can be tightly attached to and transported with bulbs, resulting in adult fly emergence in unwanted locations. To avoid stand loss from seed corn maggots, fields should be plowed early to reduce the amount of fresh organic matter in the soil and/or care should be taken to thoroughly treat the soil with an appropriate insecticide.
Cutworms, wireworms, and other soil insects are frequently present in fields before planting. These insects tend to be more of a problem in fields that have been fallow (with abundant weed hosts) or in turf. Proper weed sanitation and field preparation several weeks prior to planting or transplanting can reduce problems with soil insects. Where soil insect problems are anticipated, preventative treatment with a pre-plant insecticide is recommended (Table 7).
Cutworms are the larval stage of many species of moth in the Noctuidae family. These caterpillars generally feed at night and hide during daylight hours. Damage generally is detected as plants cut off near the soil line. Their nocturnal habits and cryptic coloration make them difficult to find, which is required for proper diagnosis of the problem. These pests are more easily detected by examining plants very late or very early in the day. See Table 7 for appropriate control measures.
Wireworms are the larval stage of click beetles. There are several species of these insects, which may attack onions. Eggs are laid in the soil and the larva feed on below ground portions of plants. While some species have multiple generations in a year, others are capable of living as larvae for 1 to 2 years before pupating and becoming adults. See Table 7 for appropriate control measures.
Thrips are the primary insect pest of onions. 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. While these insects can appear in the fall, they are much more common in late winter and early spring as temperatures increase. Populations of thrips and the severity of this insect problem on onions can vary considerably from year to year. When considering direct damage to onions, careful scouting of plants should begin shortly after the beginning of the year. Special attention should be given to leaf folds and down in the ‘neck’ of the plant. Thrips have a strong preference for these ‘tight’ areas that provide protection and will congregate at these locations.
Spraying for thrips should begin when an average of 5 thrips are present per plant. However, research has indicated that a single spray of an effective insecticide when there is one thrip per plant can reduce subsequent thrip populations and reduce the number of subsequent insecticide sprays. Spraying within two weeks of harvest for thrips control does not appear to provide any benefit in terms of yield even if the threshold is exceeded. Thrips reduce yields in onion by reducing bulb size, thus, once the bulb has reached full size, thrips damage is inconsequential to yield. However, thrips may transmit some onion diseases and control near harvest may affect bulb quality.
Insecticide resistance in thrips populations is an ever present threat and the different species of thrips may respond differently to specific insecticides. Excessive use of insecticides or use of ineffective insecticides only increases the presence of insecticide resistance. Thus, when sprays for thrips are made, then they should only be made in response to thrips populations exceeding the threshold, and species identification should be made prior to insecticide selection. It is also important to keep track of which insecticides are currently effective.
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Figure 22. Western flower thrips.