INFLUENCE OF TEMPERATURE, LEAF WETNESS, AND HIGH RELATIVE-HUMIDITY DURATION ON SPORULATION OF CERCOSPORA-CAROTAE ON CARROT LEAVES

The influence of temperature (16-32 C) and duration of moist period (6-96 h) on sporulation of Cercospora carotae was quantified on carrot plants under three types of moisture conditions (leaf wetness, 96% relative humidity [RH], and 96% RH with an initial 12 h of leaf wetness). Sporulation was quantified as the number of spores per lesion and then transformed to proportion of maximum number of spores (PMS). The highest PMS (1.78 X 10(3) spores/lesion) was obtained at 28 C after 96 h of leaf wetness. The effect of leaf wetness and 96% RH duration on sporulation was similar for most of the temperatures, except that no sporulation was observed at any time at 16 and 32 C for 96% RH. Under each moisture condition, PMS increased with increasing temperature up to the optimum (28 C) and then declined. The presence of an initial 12 h of leaf wetness enhanced sporulation and accelerated the beginning of sporulation as compared to continuous 96% RH. The PMS was modeled as a nonlinear logistic function of time for the leaf wetness (R2 = 0.98) and 96% RH moisture conditions (R2 = 0.97). A polynomial model was used to describe sporulation as a function of temperature and duration of 96% RH with initial 12 h of leaf wetness (R2 = 0.95).