Botrytis Bunch Rot (Gray Mold) of Grapes

Damon Smith, Oklahoma State University

Botrytis on Riesling grapes.

Symptoms

The most striking sign of Botrytis bunch rot is the gray-brown sporulating growth of the fungus on the surface of infected berries. On leaves, symptoms of Botrytis infection manifest as dull green spots, which turn reddish-brown and necrotic (dead). Pedicel and rachis symptoms appear as brown patches that turn black and can cause portions of the cluster to shrivel and drop. As mature berries are colonized, berries of white fruit turn brown, while berries of purple fruit turn red.

Planting Stock Management Options

Research has shown that cultivars with very tight clusters have more severe Botrytis bunch rot symptoms, so choose cultivars with open clusters and avoid tightly clustered ones. Certain cultivars are particularly susceptible to Botrytis and require special management practices such as cluster zone leaf removal and application of fungicides specific for Botrytis.

Cultural Management Options

Because the primary source of spores for new infections results from structures formed in old plant tissue, sanitation is extremely important. Proper dormant pruning and destruction of canes, clusters, and other plant parts can significantly reduce the amount of primary inoculum (spores). Also, canopy management during the season can help to increase airflow, which reduces free moisture and humidity within the canopy. Practices such as shoot positioning and strategic leaf pruning can reduce drying time.

Chemical Management Options

A complete fungicide program targeting black rot will help manage damage by other fungi early in the season and reduce the risk of early infection by Botrytis. A specific category of fungicides is available to control Botrytis. These are applied 3 or 4 times each season at bloom, bunch closing, veraison, and pre-harvest. This more aggressive fungicide program may be necessary to manage Botrytis bunch rot on highly susceptible cultivars, in vineyards with a history of the disease, or when weather is especially conducive for disease. If fungicide use is required near harvest, remember to check the pre-harvest interval (PHI) of that fungicide to be sure it can be used close to harvest. Consult with a county Extension office for current fungicide recommendations for Botrytis bunch rot management.

Causes

Botrytis bunch rot is the most important disease of grape clusters in the world. Caused by the fungus Botrytis cinerea, it can occur anytime during the growing season, but grapes are most likely to be damaged near harvest, and the pathogen can overcome a cluster very quickly. The disease is typically more severe on cultivars with tight clusters and where canopy growth is thick, covers the fruit, and humidity is high. Bird damage, hail damage, and other fungal infections also can encourage infection by the fungus. The disease can cause significant yield loss and reduce quality appreciably.

The fungus overwinters as mycelium on canes, plant debris, buds, and bark, or as hardened survival structures (sclerotia) on canes and old berry mummies. Spores (conidia) are produced in the spring and are windblown to susceptible plant parts where the fungus can directly penetrate the tissue or enter through wounds or natural openings in the plant tissue. Late-bloom infections of the ovary also can occur. When this type of infection occurs, the fungus will remain inactive until veraison. Then, the life cycle of the pathogen resumes and damage to the fruit occurs. Optimal conditions for infection include temperatures between 59°F and 68°F and free water on the plant surface. However, spores can germinate with temperatures from 34°F to 86°F and in the absence of free moisture when humidity is high (at least 90 percent) for approximately 15 hours.