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A Model Defining the Relationship Between Temperature and Leaf Wetness Duration, and Infection of Watermelon by Colletotrichum orbiculare

Controlled environment experiments were conducted to determine the relationship between temperature, leaf wetness duration, and infection of watermelon by Colletotrichum orbiculare. Flats of watermelon seedlings were inoculated and exposed to various combinations of temperature (12, 15, 18, 21, 24, 27, and 30°C) and leaf wetness duration (2, 4, 8, 12, 16, and 24 h). The experimental design was a split-plot, with whole units represented by temperature and subunits represented by leaf wetness duration. Anthracnose incidence, defined as the percentage of symptomatic seedlings in each flat 10 days after inoculation, increased with increasing leaf wetness duration at all levels of temperature. The optimum temperature for infection ranged from 21 to 24°C. At most temperatures, as little as 2 h of leaf wetness was required for infection. Analysis of variance with orthogonal polynomial contrasts and multiple regression procedures was used to define the relationship of anthracnose incidence to temperature and leaf wetness duration.