Influence of meteorological factors on release of Venturia inaequalis ascospores and occurrence of critical infection periods of apple scabWpływ czynników meteorologicznych na wysiewy askospor grzyba Venturia inaequalis i występowanie okresów krytycznych parcha jabłoni

Summary Field and laboratory experiments were conducted at the Department of Plant Protection (Pomology Division) of Research Institute of Horticulture in Skierniewice. In addition, analysis of the relationship between weather data and sporulation of Venturia inaequalis were conducted at the Faculty of Applied Informatics and Mathematics of Warsaw University of Life Science – SGGW. The aim of the study was to determine influence of weather conditions on ascospore release and occurrence of critical periods of apple scab in 2005–2007 at the Dąbrowice Experimental Orchard (IO) near Skierniewice. Among the analyzed, measurable parameters of moisture, the best was the leaf wetness to forecast the ascospore release . More than 58% of ascospore trapping events occurred during leaf wetness, and another 36% occurred during 6 hours before wetting or within 6 hours after disappearance of leaf wetness. The air temperature in the range of 5°C to 19°C did not significantly effect the size of ascospore trapping event. However, at low temperatures (below 4.2°C) no ascospores were detected, and at the temperatures above 25°C the duration of ascospore trapping events were no longer than two hours. The results indicated that in 2005–2007 on average 55% of discharges of V. inaequalis ascospores were related to favorable conditions for the occurrence of critical periods of apple scab.