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 spore trap operated continuously during the ascospore season, with the aim of better de®ning the weather conditions that allow ascospores both to discharge and to disperse into the orchard air. A total of more than 60 ascospore trapping events occurred. Rain events were the only occurrences allowing ascospores to become airborne (a rain event is a period with measurable rainfall P0.2 mm/h ± lasting one to several hours, uninter- rupted or interrupted by a maximum of two dry hours); on the contrary, dew was always insucient 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) rain fell at night but the leaf lit- ter dried rapidly; (iii) nightly rainfalls were followed by heavy dew deposition that persisted some hours after sunrise. Daytime rain events caused the instantaneous discharge and dispersal of mature ascospores so that they became airborne immediately; for night-time rain- fall there was a delay, so that ascospores became air- borne during the ®rst 2 h after sunrise. This delay did not always occur, and consequently the ascospore trap- ping began in the dark, when: (i) the cumulative propor- tion of ascospores already trapped was greater than 80% of the total seasonʹs ascospores; (ii) more than one-third of the total seasonʹs ascospores was mature inside pseudothecia and ready to be discharged.