INFLUENCE OF INTERRUPTED WET PERIODS, RELATIVE-HUMIDITY, AND TEMPERATURE ON INFECTION OF CARROTS BY CERCOSPORA-CAROTAE

Carrot leaves were inoculated with a conidial suspension (10(4) conidia per milliliter) of Cercospora carotae. Then they were subjected to interrupted and continuous wet periods of various durations and to several combinations of relative humidity(RH, 84-100%) and temperature(16-32 C) with and without an initial wet period of 6 h. Number of lesions per leaf decreased with increasing length of dry period for dry periods greater than 3 h. However, a dry period of 3 h with initial and final wet periods of 24 and 12 h, respectively, resulted in more lesions per plant than the corresponding continuous wet period (39 h). The number of lesions increased with an increase in initial wet period duration for a fixed dry interruption period of 6 h. For all temperatures, very few lesions developed at 84% RH. However, the number of lesions increased rapidly with increase in percent RH greater than 84%. In general, the plants exposed to an initial wet period of 6 h developed more lesions than those exposed to RH only. The number of lesions per plant was transformed to proportion of those at a continuous wet period and to proportion of maximum number of lesions for experiments on interrupted wet period and RH, respectively. Polynomial models were used to describe the effects of dry period durations, initial wet period durations, and combinations of RH and temperature on infection.