Effect of water on germination of Plasmopara viticola oospores

The effect of moisture in grape leaf litter holding overwintering Plasmopara viticola oospores was investigated. Oospores were incubated under different regimes of water activity ( a_W 0·991 to 0·123) for 2 to 15 days and their ability to germinate and cause infection was determined using a sensitive leaf disk assay. Reduction of a_W caused a significant shift in the infection dynamics, with maximum effect when a_W  ≤ 0·56. Dynamics of a_W in the leaf litter under natural conditions were estimated from moisture data using a Chen-Clayton equation. Daily patterns of leaf litter moisture (M in % weight) were determined in non rainy periods between mid February and mid June, while the Chen-Clayton equation was calculated using data of a_W and M measured in both sorption and desorption conditions, at different temperatures. Water activity was highest at 08·00 hours, decreased progressively until 14·00 hours, and then increased. Water activity was favourable for oospores to develop in about 25% of the measurements, all made between 18·00 hours and 08·00 hours. A close relationship was found between vapour pressure deficit (VPD in hPa) and a_W of the leaf litter, so that when VPD is lower than 2·13 hPa there is sufficient water for oospores to develop. Results showed that leaf litter moisture due to water from the atmosphere makes oospore development possible during non rainy periods.