Occurrence of Botrytis cinerea and subsequent disease expression at different positions on leaves and bunches of grape
The occurrence of Botrytis cinerea and subsequent disease expression at different positions on leaves and bunches of grape was determined from 1996 to 2000. Different techniques were used to detect viable inoculum on material obtained from table (cvs. Barlinka and Dauphine) and wine grape (cv. Merlot) vineyards. Isolations were made from berry skins on Kerssies' B. cinerea selective medium or on water agar medium supplemented with paraquat. Leaves and parts of bunches bearing three to seven berries on a short rachis section were used untreated or treated with paraquat, or were frozen for 1 h at -12°C. Paraquat and freezing were used to terminate host resistance and to promote the development of the pathogen from the tissues. The material was used untreated to detect the pathogen on the surface, or were surface-sterilized to detect mycelia (latent infection) in the tissue. B. cinerea occurred in a consistent pattern in leaves and bunches in all vineyards. Based on the combined data for tissues exposed and unexposed to paraquat, B. cinerea occurred predominantly in bunches and was mostly associated with the bases of the berry and the pedicel. Overall, approximately 30% of the berries yielded B. cinerea at these positions. The next prominent positions occupied were leaf blades, rachises, and laterals, of which approximately 20% yielded B. cinerea. The pathogen occupied the petioles less often (10%), and the berry cheek infrequently (5%). The stylar end of the berries, on the other hand, was virtually free (0.02%) of the pathogen. Disease expression in bunches displayed the pattern showed by the inoculum ecology, and symptoms consistently developed first at the berry-pedicel attachment zone. The isolation studies showed that the pathogen seldom occurred on the surface or in the skin tissue near the base, cheek, or stylar end of berries. Latent infections in the berry base were also few at véraison and harvest. Collectively, the findings indicate that conidia dispersed in early season in bunches, and residing superficially at the berry-pedicel attachment zone, are a major factor in B. cinerea bunch rot.