Chapter 20

Managing Vineyard Diseases

Fungal Pathogens

Black Rot

Black rot is one of the most serious fungal diseases of grapes in the eastern United States. It is caused by the fungus Guignardia bidwellii that can attack all new succulent growth, including leaves, petioles, shoots, tendrils, and berries.

Symptoms

Vegetative: Young leaves are susceptible to infection as they unfold, but become resistant once they mature. Lesions become evident on leaves about 1 to 2 weeks after infection, appearing as tan, circular- to irregular-shaped spots on the upper leaf surfaces (See Figure 20.1).

Fruit: Although the fungus can infect all young green parts of the vine the most damaging effect is on the fruit. Berries are susceptible to infection immediately prior to bloom and for about the next 4 weeks after bloom.

Disease Cycle

The black rot fungus overwinters primarily in mummies within the vine and on the ground, and can also overwinter for at least 2 years within lesions of infected shoots that are retained as canes or spurs.  Spring rain or dew triggers the release of ascospores and these can be dispersed moderate distances by wind and water. Ascospores cause primary leaf and blossom infections.

Disease Management

Cultural Practices: The most efficient way to control black rot is the use of good cultural practices. Sanitation is a critical component to controlling black rot. To prevent new infections, remove mummified clusters and canes form the vine in the fall or before budbreak in the spring.

Applying Control Materials: The most critical period to control black rot with fungicide begins from early shoot development up through véraison. Sprays should be reapplied after periods of rain that wash off the protective coating of fungicide. An exception to this is when systemic fungicides are used as preventative sprays.

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