AN ANALYSIS OF THE EFFECTS OF ENVIRONMENTAL-FACTORS ON CONIDIAL DISPERSAL OF UNCINULA-NECATOR (GRAPE POWDERY MILDEW) IN VINEYARDS

Aerial spore concentration of Uncinula necator (the causal agent of gr ape powdery mildew), weather data and cropping practices were monitore d during two consecutive seasons in two vineyards of the Bordeaux area . During days with no rain, spore dispersal was mainly diurnal and sho wed variations that followed the same pattern as that of wind speed, a nd a reverse pattern to that of relative humidity. Light fails of rain , of approximately 2 mm, coincided with increased spore densities in t he air. Pesticide sprays using high pressure equipment generated high wind speeds at the canopy level. This may trigger high spore dispersal . High conidial stocks were produced under spontaneous conditions in t he canopy. These stocks were released only under particular events, su ch as heavy rains, or pesticide applications with high pressure spraye rs. Other cropping practices causing leaf shaking, such as pruning, ma y enhance spore dispersal. Over the observation period, the onset of s pore dispersal was observed during a period with no rain following a r ainy period, suggesting the detrimental effect of rains on epidemic on set. Epidemiological and disease management implications are discussed .