Environmental factors influencing maturation and release of ascospores of Venturia pirina in Victoria, Australia

The influence of moisture, light, and temperature on Venturia pirina ascosp ore maturation and discharge was studied during 1992-99 in 2 pear-growing r egions in Victoria. In each year and site, mature ascospores were trapped o ver a 3-month period beginning a few days before or at the green-tip stage of pear tree development in early September and ending in late November, wi th the majority of ascospores (>80%) trapped between green-tip and petal-fa ll. Ascospore discharge was associated with rain and dew, with 90-98% of th e season's total number of ascospores trapped during rain events and 2-10% trapped during dew events in the 12 data sets examined. Most ascospores wer e trapped (82.5-99.9%) during daytime (0600-1800 hours). The 0.1-17.5% of a scospores detected during night time (1900-0500 hours) were trapped mainly within 1-3 h of dawn or dusk. There were linear relationships between the l ogit of cumulative percentage of ascospore maturation and temperature accum ulation (above 0 degree-days), calculated both daily and for days with grea ter than or equal to0.2 mm of rainfall. Six linear regression equations wer e formulated with 10 years of field data and using the 2 methods of accumul ating degree-days, to predict the cumulative percentage of matured ascospor es. Predictions were compared with additional field and laboratory observat ions not used in the formulation of the linear equations. The importance of the temperature-based linear equations is discussed in relation to the pre diction of pear scab ascospore maturity for use in a pear scab management p rogram.