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

Abstract

The influence of moisture, light, and temperature on Venturia pirina ascospore maturation and discharge was studied during 1992–99 in 2 pear-growing regions in Victoria. In each year and site, mature ascospores were trapped over 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, with the majority of ascospores ((>80%) trapped between green-tip and petal-fall. Ascospore discharge was associated with rain and dew, with 90–98% of the 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 were trapped (82.5– 99.9%) during daytime (0600–1800 hours). The 0.1–17.5% of ascospores detected during night time (1900–0500 hours) were trapped mainly within 1–3 h of dawn or dusk. There were linear relationships between the logit of cumulative percentage of ascospore maturation and temperature accumulation (above 0 degree-days), calculated both daily and for days with >= 0.2 mm of rainfall. Six linear regression equations were formulated with 10 years of field data and using the 2 methods of accumulating degree-days, to predict the cumulative percentage of matured ascospores. Predictions were compared with additional field and laboratory observations not used in the formulation of the linear equations. The importance of the temperature-based linear equations is discussed in relation to the prediction of pear scab ascospore maturity for use in a pear scab management program.