|Abstract or Summary
- 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
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.