Black Rot Disease in Grapevines

Causal Agent

• Black rot is a fungal disease caused by Guignardia bidwellii. The disease is one of the most damaging and common grape disorders that affects both the cultivated and wild varieties of the fruit. All parts of the plant, including the fruit, are likely to be infected. Unless controlled in time, entire crops may be lost.

Favorable Conditions

• The disease is likely to be started with a single fruiting body or conidium of the fungus that overwinters on the vine, on the trellis and on fallen debris. Spore development is triggered during the moist, warm conditions in spring where splashing rain water spreads the spores throughout the vine. Plant surfaces that stay wet for extended periods are highly favorable to disease development. Once infection occurs, symptoms start to appear within one to two weeks.

Symptoms

• Disease starts with the appearance of small, yellow spots on foliage that grow progressively larger and develop a black border and reddish-brown centers. Small, black dots appear when spots are 1/8 to 1/4 inch in diameter. Similar spots with sunken appearance appear on other areas of the vine. Fruit starts to display symptoms with light brown, round spots after it is half grown. Rotted areas become sunken and the entire fruit gradually blackens and shrivels. These rotted fruits are referred to as mummies.

Management

• Sanitation around the plants is a critical step in controlling the disease. Prune out all infected areas and remove the mummies. Leave only healthy canes that have no lesions. Remove even a few infected leaves as the disease spreads rapidly. Plant vines in a sunny, well-ventilated area and keep weeds in control. Start fungicide application as a protective strategy early in the season. Repeat after every seven to 10 days until the fruit starts to change color. Recommended fungicides include captan and myclobutanil.