Environmental factors influencing the dispersal of Venturia inaequalis ascospores in the orchard air

A 6-year study was carried out in an apple-growing region of North Italy by trapping airborne ascospores of Venturia inaequalis with a volumetric spor e trap operated continuously during the ascospore season, with the aim of b etter defining the weather conditions that allow ascospores both to dischar ge and to disperse into the orchard air. A total of more than 60 ascospore trapping events occurred. Rain events were the only occurrences allowing as cospores to become airborne (a rain event is a period with measurable rainf all greater than or equal to0.2 mm/h - lasting one to several hours, uninte rrupted or interrupted by a maximum of two dry hours); on the contrary, dew was always insufficient to allow ascospores to disperse into the air at a measurable rate, in the absence of rain. In some cases, rain events did not cause ascospore dispersal; this occurred when: (i) rain fell within 4-5 h after the beginning of a previous ascospore trapping; (ii) lain fell at nig ht but the leaf litter dried rapidly; (iii) nightly rainfalls were followed by heavy dew deposition that persisted some hours after sunrise. Daytime r ain events caused the instantaneous discharge and dispersal of mature ascos pores so that they became airborne immediately; for night-time rainfall the re was a delay, so that ascospores became airborne during the first 2 h aft er sunrise. This delay did not always occur, and consequently the ascospore trapping began in the dark, when: (i) the cumulative proportion of ascospo res already trapped was greater than 80% of the total season's ascospores; iii) more than one-third of the total season's ascospores was mature inside pseudothecia and ready to be discharged.