Hydrodynamics of raindrop impact stimulate ascospore discharge of Venturia inaequalis.

Ascospores of Venturia inaequalis are released from leaves following the impact of raindrops during rain events. Atmospheric humidity or moisture from dew does not have an inductive effect on ascospore release. The hydrodynamics of raindrop impact on leaf surfaces was investigated. Raindrops induced dampened periodic vibrations on leaf surfaces. Vibrations, linear, and/or oscillatory flows of water applied to the leaves all induced spore releases at low energy thresholds. The fungal perceptibility of kinetic energy enables selective discharge of spores when environmental conditions are most conducive to survival. Identification of kinetic effects of rain which reveal start, thresholds, and proportional distribution is not only important for disease prognosis research, but may also serve as a model for a common fungal sensing mechanism.