Black Rot on Grapes in Home Gardens

Even if you are following the cultural control recommendations (proper pruning and removing diseased plant material from the vineyard), the disease may be ruining your grape harvest. If so, consider using a fungicide starting early in the spring to protect your fruit.

Black Rot

Black rot is one of the most serious diseases of grapes in the eastern United States. Crop losses can range from 5 to 80 percent, depending on the amount of disease in the vineyard, the weather, and variety susceptibility. The fungus Guignardia bidwelli can infect all green parts of the vine. Most damaging is the effect on fruit. Later fruit infections can destroy many grapes, even the entire crop.

Symptoms

Infected leaves develop reddish-brown, circular spots (lesions) on the upper leaf surface. As the lesions mature, the center becomes brown and small, black, pimple-like fruiting bodies called pycnidia appear in the center. They are usually arranged in a loose ring just inside a dark border. Infected berries become dark brown and are covered with numerous black pycnidia on the surface. The berries eventually shrivel into hard, black mummies. Most serious fruit infections occur when the grape is pea-sized or larger.

Disease Cycle

The black rot fungus overwinters in mummified fruit on the vineyard floor or in old fruit clusters that hang in the vines. Infected, mummified fruit are the most potent source of spores for new infections and eliminating them from the vineyard before new growth in spring is very important to controlling this disease. The fungus can also overwinter within cane lesions, but these are generally of lesser importance. Spores of the fungus are released from these overwintering sources during spring rains to infect new leaves and shoots before bloom. Scout for this disease before bloom: leaf and shoot lesions indicate the presence of overwintering spore sources, and new infections will go on to produce more spores, adding to the inoculum load during bloom and early fruit development. Symptoms of leaf and shoot infection will appear about 10-14 days after a rain period, if infection has occurred. Fruit infections can occur from the beginning of bloom until about 5-8 weeks later, depending on the grape variety (generally, June and July). Fruit of native varieties (for example Vitis labrusca, Concord, and Niagara) are susceptible until about 5-6 weeks after bloom. Fruit of non-native Vitis vinifera, like Chardonnay, can remain susceptible for up to 7-8 weeks after bloom.  Fruit susceptibility of hybrid grape varieties falls somewhere in between and varies widely from one hybrid variety to the next. The period of highest fruit susceptibility for All Varieties, when it is most critical to protect fruit with fungicides, is from the beginning of bloom to 4 weeks after bloom. Mature leaves and ripening fruit are not susceptible. Black rot infections occur if susceptible tissue remains wet for a sufficient length of time, depending on temperature (see table below). For example, if the average temperature during a wetting (rain) period, is 55°F, then susceptible tissue must remain wet for at least 12 hours for infection (penetration into the grape host tissue) to occur. Warmer average temperatures during continuous wetness will reduce the time required for infection: at an average of 75°F during the wetness period, tissue must remain wet for only 7 hours, to satisfy the requirements for infection and penetration of the grape host. Warm, continuous/frequent wetness (from rain) during bloom and early fruit development, is ideal for black rot epidemic development and severe crop loss.

*Average temperature over the wetting period.
**Begin counting when the leaves first become wet; stop counting when the leaves have dried off.

Disease Management

Infected prunings and mummified berries should be removed, burned, and/or buried in the soil before new growth begins in the spring. In vineyards with susceptible varieties or where black rot was a problem the previous year, early season fungicide sprays should be timed to prevent the earliest infections. Should infections become numerous, protecting against fruit rot is very difficult later in the growing season. Planting less sensitive varieties (for example, Cayuga, Elvira, Verona, Vidal) can reduce problems with black rot.

Below are two tables with information regarding fungicide choice and application:  the first table lists the effectiveness of different fungicides (1 = very effective; 5 = not effective) and the second table shows a standard spray schedule for disease control. Note that this includes sprays very early in the season (generally late April through May, depending on location) even before the blooms are open. Starting sprays early is recommended when having had severe problems with this disease in the previous year.

Efficacy of Pesticides for Grape Disease Control

Always consult the label before making pesticide applications. Labels vary greatly among commercial products of the same material. It is important to refer to the label for the best timing and application rates when applying pesticides. Also read the text for information on cultural practices to minimize the application of pesticides.

Diseases

• P = Phomopsis
• PM = powdery mildew
• BR = black rot
• BO = Botrytis rot
• DM = downy mildew

Pesticide Recommendations for Grapes

The sprays listed below can provide adequate control of black rot if applied in a timely manner, and in concert with cultural control (removal of mummies). Where black rot is a problem, apply a fungicide every 10-14 days, commencing when shoots are 3-5 inches in length, and continuing, through 4-6 weeks after bloom. During long rainy periods, shorten the interval to 7 to 10 days between sprays. Always consult the label before making pesticide applications. Labels vary greatly among commercial products of the same material. It is important to refer to the label for the best timing and application rates when applying pesticides. Also, read the text for information on cultural practices to minimize the application of pesticides. Follow all instructions and application rates listed on pesticide labels. Sulfur and copper can cause injury to grape varieties that are sensitive to them (phytotoxicity) and should only be used on varieties that are not injured by them. As a general rule, red/purple grape varieties are more sensitive to sulfur than white varieties (for example, you should not use sulfur on Concord grape).

What To Do If You Have Black Rot

While you do have the option to start spraying a fungicide to try to slow down disease progression, it will not help much once you are 5-6 weeks out from bloom. However, taking the time to remove as much of the infected material this year as you can, will make a big difference in the following year. Then start in with a good spray program early next year to get good disease control.

Tables and information from the Fruit Production for the Home Gardener guide.