Influence of air temperature on the release of ascospores of Venturia inaequalis

The influence of air temperature on the release pattern of Venturia inaequalis ascospores was studied by volumetric spore samplers in two spore sampling periods. In the first period (1991-1996; Passo Segni, Ferrara), 15 ascospore dispersal events were considered occurring in daylight, with high spore counts (168-5892 ascospores per m3 air per event), at an average temperature between 8.4 and 20.3°C. Both the length of the ascospore release period and distribution of airborne spores over time were significantly influenced by temperature. A logistic regression model was used to fit the proportion of ascospores trapped from the orchard air as a function of time after the beginning of the discharge event and air temperature. The accuracy of this equation was tested against data collected in the second spore sampling period (1997-2000; Sala Bolognese, Bologna, and Castelfranco, Modena); 16 dispersal events were considered, triggered by rainfall that occurred both in the dark and in daylight, with low to high spore counts (29-458 ascospores per m3 air per event), at an average temperature between 2.8 and 14.3°C. There was a general agreement between the proportion of ascospores trapped from the orchard air during these events and that estimated by using the logistic equation - in most cases, actual and estimated values showed a high coincidence. Statistical comparison showed a significant correlation (r = 0.93, P < 0.01) between observed and estimated data.