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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.

Table grapes (cv. Dauphine) at different phenological stages were dusted in a settling tower with dry conidia of Botrytis cinerea. The berries were incubated for periods of 3 to 96 h at high relative humidity (RH; ±93% RH, moist berries), or were covered with a film of water (wet berries). Germination of the solitary conidia, appressorium formation, stilbene and suberin induction by germlings, and germling viability were examined by fluorescence microscopy after each incubation period. Isolation and freezing studies were conducted to determine surface colonization (berries left unsterile) and penetration (surface-disinfested berries). Symptoms were determined on berries incubated at a specific wetness regime, kept dry for 10 days, and then incubated for 4 days at high RH. Microscopic observations indicated that germination was delayed on immature berries, but proceeded at a high rate on mature berries. Growth was invariably restricted on moist berries. Attempted penetration was always direct. Stilbene and suberin were generally induced early and were intense on berries at the pea-size and bunch closure stages. Dieback of conidia and germlings occurred at a significantly higher rate on wet than moist berries, and was more pronounced on immature than on mature berries. The segment isolation and freezing studies showed that infections in grape berry cheeks established by this infection mode were few. Extended incubation periods did not lead to substantially higher rates of surface colonization and skin penetration. Disease symptoms did not develop during the 14-day period on the berries transferred to dry perspex chambers, irrespective of phenological stage, incubation period, or wetness regime. According to these findings, this mode of infection should not contribute to a gradual build-up of secondary inoculum in the vineyard, and to B. cinerea epiphytotics.

Infection of grapes by different densities of airborne conidia of Botrytis cinerea was investigated on table grapes (cultivar Dauphine) harvested ripe (16°Brix) and inoculated fresh, or after SO2 treatment and 8-week storage at -0.5°C. Berries were detached at each inoculation and dusted with dry conidia in a settling tower. Following inoculation, the fresh berries were incubated for 24 h at high relative humidity (≥93%), or were overlaid with wet sterile paper towels. Cold-stored berries were incubated at high relative humidity. The effect of conidial density on surface colonization, penetration, and lesion formation was determined by surface sterilization, isolation, and freezing studies on fresh berries. Only symptom expression was determined on cold-stored berries. Fluorescence microscopy of skin segments showed that conidia were consistently deposited as single cells, and not in pairs or groups, on berry surfaces. Individual conidia, at all densities tested, readily infected the cold-stored berries and formed separate lesions after 2 days. Although the cold-stored berries were highly susceptible, lesion numbers were not related to conidial density at low inoculum dosages (0.67 to 2.60 conidia per mm2 berry surface). Lesion numbers tended to increase exponentially at higher dosages (3.24 to 3.88 conidia per mm2 berry surface). Individual conidia, however, did not induce any disease symptoms on fresh berries. Removal of the pathogen after 24-h incubation from the surface of fresh berries by ethanol, and subsequent incubation of excised skin segments revealed that, irrespective of the conidial density or the wetness regime, less than 2% of skin segments were penetrated. Furthermore, increasing densities of conidia did not lead to higher rates of surface colonization and skin penetration. The low incidence of disease caused on fresh berries and high disease incidence induced after prolonged cold storage indicated that infection was not governed by conidial density on berry surfaces, but by the level of host resistance.

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