The etiology of bacterial soft rot of onions and identification of the causal organisms Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/t435gh47h

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  • Observations of bacterial soft rot of onions in the Lake Labish area in the Willamette Valley of western Oregon showed that two distinct symptom types occur during the growing season. The most prevalent symptom type appears in infected plants as a wilting and chlorosis of two or more lower leaves which is followed closely by appearance of elongated lesions emerging from the neck and extending up the adaxial side of one or more young leaves. Although most infected plants showing these symptoms usually decay quickly, some plants may appear to partially recover if conditions unfavorable for disease development return. A second symptom type is evident in those infected plants in which the collapse of all emerged leaves occurs simultaneously. During leaf collapse, foliar color changes from dark green to grey-green and finally, with desiccation, to light tan. Two kinds of soft rot also occur commonly in stored onions. One kind of decay, commonly called "slippery skin, " is characterized by the progression of soft rot down through one or two of the outer fleshy scales. These scales turn yellowish, soft, and are foul smelling. A second kind of soft rot is characterized by the interior of the bulb being totally macerated and putrid, but only slightly discolored. Only one or two of the outer fleshy scales remain firm. This decay is commonly called "stinking rot" because of its very offensive odor. Initial appearance of bacterial soft rot in the third week of June is directly related to occurrence of rain and cool, cloudy weather in the previous week. Occurrence of such periods of inclement weather are found to be predictable when past records of local climatological conditions are analyzed. Area-wide outbreaks throughout the remainder of the growing season also are related to such periods of inclement weather. The relationship between weather and disease suggests that outbreaks of bacterial soft rot can be forecast by occurrence of such periods of inclement weather. Irrigation of onions with contaminated surface waters significantly increases the amount of bacterial soft rot in the field, at harvest time, and in storage, over that found when uncontaminated well water is used. Varying the frequency of irrigations, the rates of application, and the size of sprinkler nozzles affect the amount of soft rot in the field and in storage somewhat less than does the source of water. Longer and more frequent periods of irrigation increase the incidence of bacterial soft rot. Much of the decay seen at harvest is either soft rot which has totally decayed the bulbs and will not go into storage, or is fungal in origin. Including storage rots which are fungal in origin under the general grouping of "soft rots" inflates the estimation of losses due to bacterial soft rot. Three isolates of seventeen unknown isolates capable of decaying inoculated onion bulbs were identified as Pseudomonas cepacia. Five isolates which lack ability to produce a water-soluble fluorescent pigment and possess a nitrate reductase system were otherwise virtually identical to P. cepacia. Two additional isolates were identified as P. cichorii on the basis of physiological tests. Three other isolates were provisionally identified as members of the genus Erwinia on the basis of cell shape, flagellation, and pathogenicity. The remaining four isolates were not identified.
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