Botrytis Bunch Rot on Grapes in Home Gardens

Botrytis bunch rot, or gray mold, exists in vineyards worldwide. This disease is caused by the fungus Botrytis cinerea and is commonly associated with the decay of ripe or nearly ripe grapes. Cool temperatures and damp conditions favor disease development.

Symptoms

Buds and young fruit infected in early spring turn brown. Large, reddish-brown patches may 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. Individual berries may soften and decay. The entire cluster eventually becomes moldy. The gray fuzz which is characteristic of this disease may appear on infected plant parts especially when weather is humid and damp, but may not be as apparent if it is rainy.

Disease Cycle

Botrytis cinerea has a wide host range and is already present in the environment. Within the planting, the fungus overwinters in fungal structures on the canes, leaf debris on the ground, or in mummified grapes. In the spring, spores are produced by the fungus and infect leaves and young grape clusters. Spores are moved about mainly by air currents. Water is necessary for germination of the spores, but only a few hours of wetness are required for infections to take place. Any break in the skin of ripening grapes provides an ideal entry point for the Botrytis fungus. From ripening onward, the grapes are infected directly through the skin or through wounds.

Disease Management

Any practice that opens up the canopy and improves air circulation, thereby reducing humidity and facilitating the drying of leaves and grape clusters, will help to reduce botrytis infection. This includes keeping the planting pruned and weeded, controlling growth of vegetation under the plant canopy, and keeping vines trellised up off of the ground and in a way that maximizes sunlight exposure and air movement. Water only the ground, and do not wet the leaves while watering. Remove all grape clusters during harvest, and remove any later that you may have missed. Any practice that reduces skin cracking or skin punctures near harvest helps control ripe fruit rot. When possible, plant grape varieties that are not prone to botrytis. These include the varieties Concord and Niagara, which are commonly grown for purple and white juice production, respectively, with 'Concord' also being popular for jelly-making.