FACTORS AFFECTING THE RELEASE OF ASCOSPORES OF ANISOGRAMMA-ANOMALA

Relationships between environmental factors and release of ascospores of Anisogramma anomala, the causal agent of eastern filbert blight, we re examined in four European hazelnut (Corylus avellana) orchards duri ng a 2-year period. In each orchard, Burkhard volumetric spore traps a nd automated weather-monitoring equipment were deployed for 12-week pe riods beginning at budbreak, when hazelnut becomes susceptible to infe ction. Ascospores of A. anomala were released when stromata on the sur face of hazelnut branches were wet from rain but not from dew. Release of ascospores ceased after branch surfaces dried. The duration of fre e moisture on branch surfaces regulated the initiation and rate of asc ospore release, but no significant effects of temperature, relative hu midity, wind, or light on ascospore release were apparent. Most (>90%) ascospores were captured during precipitation events that exceeded 20 h in duration, which represented about 10% of the total precipitation events each season. Quantitative relationships between the hourly cap ture of A. anomnla ascospores and hours since the beginning of a preci pitation event were developed. With the onset of precipitation, the ho urly rate of ascospore capture increased until the fifth hour of rain, remained relatively constant between the fifth and twelfth hours, and then declined gradually. During the 12-week spore-trapping periods, t he likelihood and rates of ascospore release associated with precipita tion were highest at budbreak and then declined through April and May until early June, when the reserve of ascospores ill the perithecia wa s depleted. targe numbers of ascospores were captured in the volumetri c spore traps, indicating that ascospores may be commonly dispersed lo ng distances on air currents as well as locally by splash dispersal wi thin the canopy, as reported previously. The results indicate that mon itoring seasonal precipitation patterns may be useful for estimating t he quantity and temporal distribution of airborne inoculum during the period that the host is susceptible to infection.