Biotic and Environmental Factors Affecting Infection of Sweet Corn withExserohilum turcicum

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Abstract
Optimal temperatures for conidial germination and appressorial formation of E. turcicum on leaves of the susceptible corn cultivar Jubilee were 25.degree. and 15.degree. C, respectively, whereas that for infection peg formation on corn leaf impressions was 20.degree. C. The minimal dew period required for these processes at the corresponding optimal temperatures was 1, 3, and 4 h, respectively. Conidial germination and infection peg formation were not affected by leaf and plant age, but under laboratory conditions, appressorial formation was favored in younger plants that were also more susceptible to the blight incited by the fungus. Infection occurred at 15-30.degree. C with an optimum at 20.degree. C. A 5-h dew period at 20.degree. C was required for lesion formation. The number of lesions increased with lengthening dew period and increasing inoculum concentration, but under all combinations of dew period and inoculum concentration, the number of lesions was largest at 20.degree. C. Environmental conditions in Israel during the growing season (May-Oct.) are favorable for infection. Epidemic outbreaks of the blight do not occur until late Aug. when dew periods are long enough for fungal sporulation.