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Cucurbit Genetics Cooperative Report 6:14-15 (article 7) 1983

Ulocladium cucurbitae Leafspot on Cucumber (Cucumis sativus)

David P. Lane and Henry M. Munger

Cornell University, Ithaca, NY 14853

In cucumber breeding plots at Ithaca, cucumber mosaic virus (CMV) resistant progenies with ‘Ashley’ and ‘Poinsett’ in their parentage have repeatedly shown more severe leafspot symptoms and earlier defoliation than other cucumbers with comparable CMV resistance. An unusual race of anthracnose was initially suspected, but in most years we saw few lesions on fruit and in one year when they did appear, a study by plant pathology graduate students showed that the organism was race 1 of Colletotrichum. The consistently late defoliation of ‘Tablegreen 65’ and early defoliation of ‘Poinsett’-related progenies is the opposite of expectation if anthracnose were involved. Cornell ‘SR551’ and ‘PMR551’ have been outstanding in freedom from leafspot symptoms, seldom being defoliated before frost.

When we learned that ‘Poinsett’ is unusually susceptible to target leafspot caused by Corynespora cassiicola and that ‘SR551’ and its hybrids are among the least susceptible to this disease in Florida, we thought the same disease might be present in New York State also. However, several isolations in 1981 and 1982 failed to yield any Corynespora here. Instead, spores of the unknown fungus were consistently isolated, both from fields which had not been inoculated with any organism, and from a field inoculated with a strain of Corynespora obtained from J. M. Strandberg of Leesburg, Florida. In the latter field, Corynespora conidia were also routinely isolated from the plants. The lesions caused by Corynespora appeared as dark, sunken necrotic spots, while those incited by the unknown fungus appeared as pale, bleached spots surrounded by alternating concentric rings of dark and pale necrotic tissue. Examination of lesions of the latter type from the field inoculated with Corynespora and from uninoculated fields revealed only spores of the distinctive type of Alternaria, and never of Corynespora.

In order to insure proper identification of the unknown fungus, an isolate and a leaf sample were sent to the Commonwealth Mycological Institute, Kew, England, where the fungus was subsequently identified as Ulocladium cucurbitae by S. M. Francis. According to the scant literature available, the first report of Ulocladium cucurbitae being pathogenic on cucumber is that of Butler et al. (1) in 1979. The disease is quite common in the United Kingdom. The organism has also been isolated from C. sativus in New Zealand, Canada, India and the U.S.A., although it is apparently not widespread in those countries.

However, the disease may be more widespread than is commonly thought. This is due to the ability of the organism to produce more than one characteristic spore type, depending upon the environment. On the leaf itself, Ulocladium cucurbitae looks more like an Alternaria and has only occasional Ulocladium-type spores. In culture, the Ulocladium-type spores predominate with very few, if any, Alternaria-type spores being present (S. M. Francis, personal communication). Simmons (2) has also prepared an excellent overview of the situation.

Koch’s postulates have been verified repeatedly, although effort is still being directed at discerning optimum conditions for artificial inoculation, with hopes of developing a rapid screening method.

Literature Cited

  1. Butler, D., M. J. Griffin and J. T. Fletcher. 1979. Leaf spot on cucumber caused by Ulocladium atrum. Plant Pathology 28:96–97.
  2. Simmons, E. G. 1982. Alternaria themes and variations (11–13). Mycotaxon 14(1):44–57.
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