Management of Onion Diseases

Summary

Progress has been made in conjunction with collaborators in the Department of Entomology towards the further understanding the vector and transmission relationships of Pantoea ananatis and P, agglomerans, causal agents of the center rot of onion complex, with their thrips vector. The research team has determined acquisition periods necessary for successful transmission, duration of retention of the pathogen and by using fluorescent-labelled antibodies that the pathogens are non-persistent and primarily located in the midgut of the thrips. Furthermore, evidence was obtained to indicate the transmission process is passive through the contamination of feeding wounds by heavily-infested feces deposited by the thrips. In cooperation with the Department of Horticulture, novel postharvest treatments (fungicides for Botrytis neck rot and bactericides for sour skin of onion) drenching onions in a bath of different pesticides were evaluated. In addition, storing onions in an atmosphere containing sulfur dioxide was also evaluated for postharvest disease control. Briefly, sulfur dioxide had some benefit in reducing disease levels but is a very strong oxidant and has some potential to damage onions and storage facilities. The fungicide Scholar showed promise in reducing Botrytis neck rot and Kocide, a copper-containing bactericide had some benefit in reducing sour skin compared to a water control. Amazingly onions receiving the water control showed some benefit compared to onions stored dry with no other postharvest treatment. The reason for this result is not yet understood. Finally, results from a long-term study evaluating the effects of crop rotation (winter) and double-cropping (summer) onions with crops that may suppress B. cepacia populations continued to demonstrate positive trend between double-cropping onions with pearl millet in an effort to reduce sour skin, caused by Burkholderia cepacia. Mode of action of pearl millet reducing sour skin severity is not understood, but tissue analysis of onion bulbs indicated a iron-copper ratio in onions double cropped with pearl millet that was similar to tobacco displaying resistance to tomato spotted wilt virus. A hypothesis was formed that the iron-copper concentrations up-regulate and down-regulate the enzymes Fe superoxide dismutase and Cu/Zn superoxide dismutase. These enzymes are important in detoxifying reactive oxygen species which in turn regulate the accumulation of salicylic acid, a precursor to systemic acquired resistance pathways.

Situation

Recently the Vidalia onion industry has been pressed with a number of different issues. Among other problems, the industry is facing greater competition during their traditional time period or "window" for sales. Much of that competition is from foreign sources, but there also has been increased interest in producing sweet onions elsewhere in the U.S. Any issue which increases production costs while also decreasing quality, will have a negative impact on the onion industry as it struggles to remain competitive in the global economy. One area of concern is quality of the onion product. Postharvest diseases of onions traditionally have been very difficult to manage and have consistently been responsible for major economic losses in stored onions over the years. There is also the consideration of the costs of storing onions for a considerable period of time then to lose all potential profit because they spoiled in storage. Although the pathogens responsible for diseases in storage can spread and increase the level of loss, the vast majority of initial inoculum responsible for the postharvest rots resides in the field. Two old adages are very applicable to this situation. The first is: "Bad onions in, bad onions out", meaning if onions are infected prior to storage then disease severity will only increase over time. The second saying is "One bad apple spoils the whole barrel". In this case, the infected onions coming from the field can result in the pathogen spreading to other onions in storage even though they were healthy at the time of going in to storage.

Response

A team of researchers from the University of Georgia, Auburn University, Colorado State University, Mississippi State University, and University of Idaho, representing multi-disciplines that include Plant Pathology, Agric. & Biol. Engineering, Agric. Economics, Entomology, Food Science, Horticulture, and Rural Sociology secured a SCRI grant to address the situation outlined above. This impact statement represents a progress report in the final year of that project, which received an extension. Specifically the response from this program was multi-faceted. First experiments were conducted in the field using crop rotation and double-cropping methods to develop a crop rotation system that would utilize three successive plantings of "suppressive" crops to reduce sour skin. Second, this program identified current and new threats to onions in the field and storage and took the approach that management in the field was the first line of defense to reduce losses in storage. Third, this program worked closely with entomologists in attempts to better understand how center rot of onion develops and the importance of the vector relationship with thrips. Finally, this program worked closely with horticulturists to evaluate postharvest treatments, specifically pesticide drenches, as a means to reduce rots in storage.

Impact

Any reduction in postharvest losses will be beneficial to onion growers. The drench studies could eventually lead to recommendations to manage Botrytis neck rot, the number one postharvest disease of onions in Georgia. In addition, better disease control will have a positive effect on the product's image. This study also identified several potential "suppressive" crops, e.g. pearl millet and carrot, and some crops, e.g. peanut and corn, which can exacerbate disease development. Knowledge of the best choice for crop rotation in the winter or double-cropping in the summer would have an impact on sour skin management, which probably is the second most important postharvest disease of onion. Furthermore, if the mode of action is due to the iron-copper uptake and ratio of those two elements in plant tissues, cultural practices with prescribed fertilizer applications could be developed to increase systemic acquired resistance in field-grown onions. Finally, a better understanding of the epidemiology of center rot complex, the third most important postharvest disease, could lead to management strategies in the field, specifically ways to reduce transmission by thrips.

State Issue

Agricultural Profitability and Sustainability

Details

• Year: 2012
• Geographic Scope: State
• County: Tift
• Program Areas:
  • Agriculture & Natural Resources