Botrytis bunch rot, or gray mold, exists in all vineyards worldwide. This disease is caused by the fungus Botrytis cinerea and is commonly associated with the decay of ripe or nearly ripe grapes. Temperature and damp climates favor disease development. The bunch rot phase of the disease causes the greatest economic losses.
Buds and young fruit infected in early spring turn brown and dry out. Prior to bloom, large, reddish-brown patches appear on the leaves. By the end of bloom, the fungus develops on aborted berries that are attached to or trapped in the fruit clusters. From ripening onward, the grapes are infected directly through the epidermis or through wounds. The entire cluster eventually becomes moldy. When weather is dry, infected berries dry out; in wet weather, they tend to burst and a brownish-gray mold forms on the surface.
Botrytis bunch rot also infects numerous wild hosts and cultivated plants. The fungus can live on these alternate hosts as a saprophyte on dead tissue. The fungus also overwinters in debris on the vineyard floor or on the vine in bark and dormant buds. In the spring, spores are produced by the fungus and infect leaves and young grape clusters. Spores on decaying and dead vegetation are moved about mainly by air currents. Water is necessary for germination, but this requires only 1 to 4 hours, depending on the temperature. High relative humidities allow infection to take place after the spore has germinated. Any break in the skin of ripening grapes provides an ideal entry point for the Botrytis fungus as well as a moist medium in which the spore can germinate.
Management of botrytis is best accomplished through a combination of cultural and chemical procedures. Any practice that opens up the canopy and improves air circulation, thereby reducing humidity and facilitating the drying of leaves, will help reduce botrytis infection. Applying two fungicide sprays on very susceptible varieties is suggested during the bloom period. The sprays will reduce the number of infected flower parts and the incidence of young fruit infection. Any practice that reduces skin cracking or skin punctures near harvest helps control ripe fruit rot. Preharvest fungicide applications are also recommended. When possible, plant botrytis-resistant grape varieties. Variety selections are presented in Table 6.3.