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 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.
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.
The black rot fungus overwinters in mummified fruit on the vineyard floor or in old fruit clusters that hang in the vines. The fungus can also overwinter within cane lesions. Spores of the fungus are produced within the diseased fruit and infect leaves, blossoms, and young fruit during spring rains. Fruit infections occur from mid-bloom until the berries begins to color. Mature leaves and ripe fruit are not susceptible. Very few fruit or leaves are infected after late July, and none are infected after the end of August. Black rot infections depend on the temperature and the length of time the leaves are wet. Infections occur if susceptible tissue remains wet for a sufficient length of time, depending on temperature (see table below).
Hours of leaf wetness required for a black rot infection period at various temperatures following a rain.
|Temperature (°F)*||Hours of Leaf Wetness**|
*Average temperature over the wetting period.
**Begin counting when the leaves first become wet; stop counting when the leaves have dried off.
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 resistant varieties is strongly suggested.
Below are two tables with information: one on the effectiveness of different fungicides and one showing a standard spray schedule for disease control. Note that this includes sprays very early in the season--even before the blooms are open.
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.
- P = Phomopsis
- PM = powdery mildew
- BR = black rot
- BO = Botrytis rot
- DM = downy mildew
Pesticide recommendations for grapes
The sprays listed below will not provide adequate control of black rot. Where black rot is a problem, apply a fungicide every 14 days after the "New Shoot" spray up to and including the "Before Ripening" spray. During long rainy periods, shorten the interval to 7 to 10 days between sprays. Spray in the rain, if necessary, to maintain the schedule of applications. 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. Due to a wide array of various products containing the same active ingredient,for insecticide recommendations, when appropriate, the active ingredient is listed instead of the name of formulated product. Follow all instructions and application rates listed on pesticide labels.
|Time to Spray||Suggested Materials||Pests to Be Controlled|
|New Shoot--when new shoot growth averages 4 inches||Captan plus Mycobutanil or Mancozeb + Mycobutanil||Phomopsis-black rot|
|Before Bloom--just before blossoms open||Captan plus Mycobutanil or Mancozeb + Mycobutanil or Carbaryl or Imidcloprid||Black rot-downy mildew-rose chafer-leafhopper|
|Post-Bloom--immediately after Bloom||Mancozeb + Mycobutanil or Bacillus thuringiensis or Imidacloprid or Azadirachtin||Black rot-downy and powdery mildew-botrytis rot-berry moth-leafhopper|
|First Cover--apply 10 days after Post-Bloom||Captan + Sulfur Same as Post-Bloom||Black rot-downy and powdery mildew-botrytis rot-Japanese beetle|
|Second Cover--apply 3 weeks after First Cover||Captan + Sulfur||Black rot-downy and powdery mildew-Fruit rots|
|Third Cover--late July or early August||Captan + Sulfur||Fruit rots-powdery mildew|
|Before Ripening--10 days before picking||Captan + Sulfur||Fruit rots-powdery mildew|
What to do if you have black rot
While you do have the option to start spraying a fungicide now to try to slow down disease progression, at this point in the season, it probably won't help much in salvaging the rest of the fruit for this year. However, taking the time to remove as much of the infected material this year as you can will make a big difference. Then start in with a good spray program early next year and you should be able to get good disease control.
Tables and information from the Fruit Production for the Home Gardener guide.