Integrated crop management and induced disease resistance in onions

Growing conditions, plant vigor, and crop management factors may increase the susceptibility of onion to pathogens. In Michigan onions are normally direct seeded on flat beds on muck soils. Most onion fields are subject to flooding after heavy rains, which results in reduced stands and/or increased foliar and bulb diseases. Growers apply fungicides to reduce foliar diseases and maintain disease-free bulb through harvest and storage. Induced resistance is an alternative approach to disease control, and is based on the activation of the plant defense system.; Necrosis was induced on onion leaves in the greenhouse with cucumber pathogens Colletotrichum orbiculare or Didymella bryoniae. D. bryoniae caused localized necrosis and induced resistance to Alternaria porri more effectively than C. orbiculare. Purple blotch lesions were significantly smaller on D. bryoniae induced plants compared to untreated control plants. Under field conditions methyl jasmonate (MeJA) was the best inducer of resistance against A. porri in onion in both years. DL-beta-amino-n-butyric acid (BABA) had inconsistent results between years, and acibenzolar-S-methyl treated plants had more lesions than the control. Marketable yield was reduced with MeJA application in 2002. Therefore, onion plants could experience a resistance cost. MeJA did not have any negative effect on onion storage in both years of this study.; From 140 rotted onion bulb samples, 15 were identified as Burkholderia cepacia, 2 were identified as B. gladioli pv. alliicola and 1 was identified as Pantoea ananatis, using a DNA extraction method with slight modification and specific oligonucleotides for bacteria identification. Plants treated with McJA plus minimum rate of fungicides had the least number of bulbs (1) infected with B. cepacia compared to plants treated only with fungicides (6), suggesting that McJA could help to reduce rot of bulbs caused by B. cepacia.; A study comparing onions grown on flat and raised beds at the Muck Soil Research Station indicated that flat beds contributed to lower plant stands than raised beds, 34% and 22% reduction in plant stand in 2001 and 2002, respectively, 40 days after planting (DAP). Raised beds had less purple blotch disease incidence, 55% and 54% of plants with the disease 121 DAP, compared to 60% and 68% for flat beds in 2001 and 2002. Purple blotch severity was 4 for flat beds (scale 0 = no disease; 5 >70% of the third green leaf from the base affected with the disease) compared to 3 for raised beds, 100 DAP in 2001. Onion yield was higher on raised beds in 2001 than on flat beds.; Foliar desiccation of onion tops with herbicides indicated that diquat, paraquat, and carfentrazone desiccated onion leaves in the field very well but caused internal decay of bulb quality in storage. Endothall was partially effective for onion desiccation and did not reduce marketable bulbs in storage. Bromoxynil desiccated onion foliage 70--97%, depending on the year, without inducing rot or reducing the percentage of marketable bulbs in storage. Without good desiccation, onion growers must depend on good soil management practices and drainage, use of reasonable population densities, adequate weed, fertilizer and irrigation management practices to facilitate onion maturation, cure, and storage.