Factors affecting maturation and release of ascospores of Venturia pirina in Oregon

The first mature ascospores of Venturia pirina, the causal agent of pear scab, were observed at bud swell in late February to late March. The percentage of asci that contained mature ascospores within a pseudothecium reached a maximum during the bloom period. Ascospore maturity during three seasons was described with linear regression. The accumulated degree-days with a base temperature of 0 degrees C was the independent variable, and ln(l/l--proportion of mature asci) was the dependent variable. The maturity model was validated at two diverse locations in 1993. Ascospore release usually was associated with rain nine of 19 events) or dew (eight of 19 events). Light stimulated the discharge of ascospores, but night release of ascospores occurred in all 3 yr of the study. In only three release events did the percentage of ascospores trapped at night exceed the percentage of the wetness duration that occurred at night.