Cooperative Extension Service
College of Agricultural and Environmental Sciences
The University of Georgia
George Boyhan, Extension Horticulturist-Vegetables
East Georgia Extension Center
Rosenwald Building, Third Floor
PO Box 8112
Georgia Southern University
Statesboro, GA 30460
912-681-5639 | 912-681-0376, Fax | 912-682-3481, mobile
The Vidalia Onion Committee (VOC) and the Vidalia Onion Business Council (VOBC) met on March 21, 2002, at the Captain's Corner restaurant in Vidalia. Business conducted included deciding on advertising plans as well as the grade standard to be used this year. Members decided to have a national advertising campaign rather than a regional one (east of the Mississippi River) for this year. They decided to go with the USDA No. 1 standard for grading onions this year as they did last year. They may switch to the Georgia grade (less stringent) as the crop starts coming in. The Georgia Department of Agricultura will handle grading. The department has 19-20 inspectors and are paid 1 cent per 50-pound bag.
There was some talk about petitioning the Department of Agriculture to have a red sweet onion as part of the Vidalia name. Nothing was decided at this time. Some growers have been pushing for a co-op for several years, but the Georgia Department of Agriculture said "no" to using the Vidalia name in connection with a co-op.
Acres for this year are 14,030, which is down 1,000 from last year, and the number of growers stands at 123 for this year, which is down 20 from last year.
Before the meeting adjourned, Ronnie McCloud, an onion grower, brought in some onions that he claims are not on the approved list. He said they were from two fields representing about 110 acres. The number of onion varieties that can be grown as Vidalia onions is limited to 17 varieties. Varieties that are not on this approved list cannot be grown and have the Vidalia name. The University of Georgia is charged with conducting variety trials and making recommendations concerning new varieties.
Bob Stafford, the VOBC executive director, said letters had been sent to the seed companies from the Georgia Department of Agriculture asking who they sold the seed to and what varieties they sold this year to help determine if any illegal onions are being grown. A meeting at the Vidalia Farm was held on March 25, 2002, between growers and a Georgia Department of Agriculture representative. Reid Torrance and Bill Randle attended as advisors. Suspect onions were brought in and voted on by the growers as to whether they were legal or not. This information was collected by the Georgia Department of Agriculture. No further details are available at this time as to what the Georgia Department of Agriculture will do.
I thought this might be a good time to review the work on CA storage of onions and what growers can do to maximize their pack-out. CA storage, widely adopted about 12 years ago (enough CA storage for about half the onions produced) by the Vidalia onion industry, has been a mixed blessing. It has allowed growers to extend their season well into the fall; however, the CA storage has not always worked consistently. The number one problem of CA stored onions is Botrytis neck rot (Botrytis allii), which is impossible to detect once inside the bulb at time of storage. In a bad year, with the disease undetected inside the onions, as much as 50-70 percent of the onions can be unsaleable when the rooms are opened.
A great deal of work has been done with Botrytis neck rot over the years. Early work showed that high temperature curing could reduce or eliminate Botrytis neck rot in onions. For example, Gunkel, et. al. (1972), indicated that warm air treatment at 93 degrees F for five days was effective in controlling Botrytis neck rot. Thornton, et. al. (1996), also indicated that a substantial reduction in Botrytis neck rot occurred with heat curing above 95 degrees F. In addition, Sumner showed that in culture, Botrytis neck rot was killed at high temperatures.
Purvis and Hakim (1999) have done work looking at the effect of heat curing on Vidalia onions. Most of the previous work has been done with day-long high solids onions that may stand up better to heat curing without much loss due to shrinkage. Purvis' work has shown that high temperature curing of Vidalia onions does not substantially increase the amount of electrolyte leakage up to 102-105 degrees F for 48 hours, which is a good indicator that the onions can stand the high temperatures required to reduce the pathogen. All the work with high temperature curing has emphasized the temperatures measured are within the bulbs. Although the curing equipment used in the Vidalia area is capable of producing heated air at 95 degrees F or above, I'm not sure this equipment can evenly deliver this heat throughout the onions and ensure this temperature within each bulb. Onions nearest the source of heat would easily reach this temperature, but those onions farthest from the heat would not. Increasing temperature of the heated air to compensate for this might result in cooking the onions nearest the heat source. The capacity and design of the equipment may require improvement to maximize the advantage of heat curing.
Heat curing can be a source of problems with other diseases. Late season bacterial diseases such as sour skin (Burkholderia cepacia) can be dramatically aggravated with high temperature curing. The curing process is an ideal environment for the rapid increase of these pathogens. Some highly susceptible late season varieties are particularly susceptible to this type scenario.
Purvis has also investigated how Botrytis neck rot reacts in CA storage. He found that the fungus can live and continue to grow under these conditions but it does not sporulate. What this means is that the pathogen can spread bulb to bulb as long as the bulbs are in contact with one another, but it cannot spread throughout a CA storage facility.
Field conditions apparently play a role in Botrytis neck rot incidence as well. It is a soil-borne disease, but the severity from one year to the next can be dramatically different from the same field. Severely cold winters that damage the tops of onions appear to be years in which Botrytis neck rot is worse. Damage in general will act as a place of ingress for the pathogen. Onions that are damaged in harvest, transporting, grading, etc., have places where the disease can get a foothold. There has been some work suggesting that neck length (how much remains when the top is cut) may play a role in Botrytis neck rot incidence, with longer necks having less disease. The reasoning here is that, with a longer neck, the pathogen has to travel farther to get into the bulb. If this neck is dried quickly enough, the pathogen is prevented from entering the onion. The evidence as to whether this is true is mixed. In some studies, this has not been the case.
Variety also plays a role in storability. Varieties 'Sweet Melody' and 'Savannah Sweet' have consistently performed in the top five for percent sound onions after five months of CA storage. Some varieties do very poorly in storage as well. Generally, very early or very late varieties do not do very well in storage. Mid-season varieties appear to perform best.
Based on the results of these studies, I would recommend that growers plan ahead and choose varieties that will be specifically put in storage. 'Savannah Sweet' and 'Sweet Melody' would be good choices. Harvest when mature, clip the tops with relatively long necks (1 to 1.5 inches), take care not to damage the onions in handling. Heat cure the onions at 95 degrees F but not above 105 degrees F. Cure them for three to five days under these conditions, ensuring that there is a high volume of heated air movement around all the onions. Make sure no onions with sour skin or other bacterial diseases are placed in the curing bins, since they will quickly ruin all the onions being cured. After curing, grade the onions removing any damaged or diseased onions. Do not remove the outer dry scales from the onion as these scales will help protect the onion from any contact infection in storage. Fill CA rooms quickly, two to three days rather than two to three weeks. Monitor and maintain CA conditions in storage. When you're getting ready to open the rooms, maintain atmospheric conditions (high CO2, low O2) and raise the temperature. This will prevent fungal growth that can occur from water condensing on cold onions. Once room temperature has been reached, the rooms can be opened and the atmosphere allowed to reach normal.
Gay, J.C. 1998. Botrytis neck rot management in Vidalia onions. 1996-97 Ga. Onion Res.-Ext. Rep. Pub. No. 3-98:3.
Gunkel, W.W., J.W. Lorbeer, J. Kaufman, and H.A. Smith, Jr. 1972. New developments on artificial drying. A method for control of Botrytis neck rot in bulk stored onions. Ann. Prog. Rep. NY Farm Electrification Council. 29:43-44.
Maw, B., and P.E. Sumner. 1998. Guide to harvesting and curing of Vidalia onions. 1996-97 Ga. Onion Res.-Ext. Rep. Pub. No. 3-98:19.
Maw, B., R. Gitaitis, A. Purvis, and D. Sumner. 1998. Curing Vidalia (sweet) onions. 1996-97 Ga. Onion Res.-Ext. Rep. Pub. No. 3-98:20-21.
Purvis, A., and A. Hakim. 1999. Effect of bruising on weight loss and storage quality of two onion cultivars. 1998 Ga. Onion Res.-Ext. Rep. Pub. No. 3-99:44-45.
Purvis, A., and A. Hakim. 1999. Effect of heat treatment to control Botrytis neck rot on the integrity of onion bulb scales. 1998. Ga. Onion Res.-Ext. Rep. Pub. No. 3-99:46-47.
Thornton, M.K. Mohan, M. Larkin, and V. Bijman. 1996. 'Vaquero' onion storage evaluation. Univ. of Idaho, Parma Res. & Ext. Center.
From My DeskWe will begin harvesting onions in a few weeks at the Vidalia Farm. It looks like it's going to be a good year if somewhat delayed with the late cool weather we've been having. Don't forget the Vidalia Onion Field Day, which will be April 18, 2002, from 12 noon until 3 p.m. We will have a sponsored lunch and visits to the field plots.