FACTORS AFFECTING MATURATION AND RELEASE OF ASCOSPORES OF VENTURIA-PIRINA IN OREGON

The first mature ascospores of Venturia pirina, the causal agent of pe ar scab, were observed at bud swell in late February to late March. Th e percentage of asci that contained mature ascospores within a pseudot hecium reached a maximum during the bloom period. Ascospore maturity d uring three seasons was described with linear regression. The accumula ted degree-days with a base temperature of 0 C was the independent var iable, and ln(1/1 - proportion of mature asci) was the dependent varia ble. The maturity model was validated at two diverse locations in 1993 . Ascospore release usually was associated with rain (nine of 19 event s) or dew (eight of 19 events). Light stimulated the discharge of asco spores, but night release of ascospores occurred in all 3 yr of the st udy. In only three release events did the percentage of ascospores tra pped at night exceed the percentage of the wetness duration that occur red at night.