Botrytis cinerea inoculum sources in the vineyard system

Vitis vinifera L., the species grown to produce winegrapes in New Zealand, is susceptible to the necrotrophic and ubiquitous fungus, Botrytis cinerea Per.:Fr, which causes serious damage to crop yield and quality. Control of the pathogen is a major concern of grape growers, who have traditionally relied upon chemical fungicides that have not always been effective. This research programme investigated the vineyard sources of inoculum, to allow for the development of more sustainable disease control strategies. A survey of 10 Marlborough vineyards in December 2000, during grape flowering, investigated the quantity and sporulation capacity of the different vine trash types on the ground. Sporulation capacities of trash types were assessed by counting the conidia produced after incubation under high relative humidity at room temperature for five days. When calculated by area of vineyard floor, the most important were found to be rachides, followed by tendrils, petioles and cane pieces, which had mean sporulation potentials of 1.0 x 10⁴,5.6 x 10³ , 3.3 x 10³ , and 1.7 x 10³ conidia per quadrat (0.25 m²), respectively. The sporulation potential of necrotic tissues was further investigated. For rachides placed in the field, it diminished between flowering and harvest, with mean conidium numbers per ground rachis reducing from 3.9 x 10⁵ to 2.7 x 10³ and per canopy rachis, from 3.5 x 10⁵ to 2.6 x 10⁴. In the laboratory, sporulation from rachides occurred over a temperature range of 10-30°C, but requiring a minimum of 3 h wetness at the onset of the five day incubation period. Increasing amounts of sporulation occurred with increasing length of the wetness period, with 36 h required for optimum sporulation at 10°C, but only 24 h at 25°C. Field observations during the summer of 1999/2000 showed that longer wetness periods were required for visible sporulation from rachides, since sporulation was observed from rachides in only 7-8% of canopy wetness events (defined as lasting at least 6 h). Necrotic leaves, which are produced by trimming and leaf-plucking practices, are abundant in the canopy during the growing season. An in vitro assay determined that B. cinerea could colonise necrotic leaves under a wide range of temperature conditions, with the greatest colonisation occurring at 25°C. Necrotic, infested leaves collected at post-véraison and harvest were found to have high sporulation potential, with 11.7 x 10⁶ and 9.3 x 10⁶ conidia per mm² of leaf tissue, respectively. An investigation into the sporulation potential of necrotic grapevine tissue trapped within bunches found that aborted berries, aborted flowers, damaged berries, leaf fragments, tendrils, stamens and calyptrae produced 198, 152, 2, 996, 986, 2, 011, 0 and 249 conidia, respectively per mm² of tissue. Wide sampling found that a consistent relationship between the presence of aborted berries within bunches and berry infection at harvest, but not for other trash types, indicating that the relationship may not have been one of simple linear cause and effect. Conidium dispersal trials in the field, using a marker strain of B. cinerea, found that within the 3 h period of the trial the conidia dispersed 5 m from a ground source to the vine canopy, the entire trapping distance. This showed that inoculum sources on the ground could act as foci of infection. A field trial was used to determine whether placing infested rachides on the vineyard floor during flowering and infested necrotic leaves into the canopy up to véraison affected berry infection at harvest. Relative to plots cleared of trash, the presence of rachides alone was not positively correlated with increases in berry infection, but presence of necrotic leaves and both debris types together was, with 61 % and 114% increases in berry infection, respectively. Therefore, a synergistic relationship may exist between the two types of debris, with necrotic leaves acting as a direct source of inoculum for the infection of berries. This study has provided valuable information about the potential of grapevine trash to generate B. cinerea inoculum and to contribute to berry infection at harvest. These findings are discussed in relation to providing improved, sustainable strategies for control of bunch rot in grapes