Botrytis bunch rot or gray mold is a disease that exists in all vineyards worldwide. The disease is caused by the fungus Botrytis cinerea and is commonly associated with the decay of ripe or nearly ripe grapes. Temperature and cold, damp climates favor disease development. The bunch rot phase of the disease causes the greatest economic losses, especially on French hybrids and Vitis vinifera cultivars.

Symptoms

Buds and young fruit infected in early spring turn brown and dry out. Prior to bloom, large reddish-brown patches appear on 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 caused by insects, hail, or cracking. Eventually the entire cluster becomes moldy and gray masses of spores are often visible on infected plant parts. When weather is dry, infected berries dry out. In wet weather, they tend to burst and a brownish gray mold forms on the surface.

Symptoms on grapes infected with Botrytis

Disease Cycle

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 pathogen overwinters on bark, in dormant buds, and in debris on the vineyard floor or on the vine. 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. Production of spores and subsequent infection are greatly favored by prolonged periods of wetness and high levels of humidity. The fungus can more easily attack injured or senescing tissue. 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.

Disease Management

A combination of cultural and chemical procedures will control Botrytis. Practices that improve air circulation and thereby reduce humidity in the canopy are beneficial. These include site selection to avoid fog pockets and heavily wooded areas, management of canopy densities through pruning, avoid excessive nitrogen use, and use of selective leaf removal procedures during the growing season. Well-timed sprays of an effective fungicide are important in many years, particularly if weather is relatively wet between veraison and harvest. The sprays will reduce the number of infected flower parts and the amount 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. Plant resistant grape cultivars when possible.