DISCONTINUOUS WETTING AND SURVIVAL OF CONIDIA OF VENTURIA-INAEQUALIS ON APPLE LEAVES

The viability of Venturia inaequalis conidia on apple leaves was determined after exposing them to wet and dry intervals at 10, 15, 20, and 25 C. Relative humidities (RH) during dry intervals were 60% or >90%. Viability of ungerminated conidia was not affected by exposure to dry intervals of greater than or equal to 24 h regardless of temperature or RH and decreased only slightly after dry intervals of 96 h. Viability of germinated conidia (germlings) decreased by 20% during the first 15 min of drying and decreased an additional 10-30% after 96 h. Viability of germlings that had formed appressoria significantly decreased following drying intervals of 24 and 96 h but was greater (10-20%) than viability of germlings without appressoria. After the different dry intervals, either the spore or the germ tube in up to 19% of the germlings had dried. However, after a second 24-h wetting interval, as many as 85% of the viable cells or conidia continued growing and formed appressoria. When RH during the dry interval was >90%, the viability of conidia or germlings was similar to or less than that at 60% RH.