Reports on Plant Diseases

Black rot, caused by the fungus Guignardia bidwellii, is probably the most serious disease of cultivated and wild grapes in Illinois. The disease is most destructive in warm, wet seasons. The fungus attacks all green parts of the vine – the leaves, shoots, leaf and fruit stems, tendrils, and fruit. The most damaging effect is to the fruit. Infections early in the growing season destroy blossom clusters or cause developing berries to "shell off" the cluster and fall to the ground. Later infection periods can destroy a high percentage of the berries, turning them into hard, black, shriveled "mummies." When warm, muggy weather in the spring and summer is prolonged, unsprayed fruit on very susceptible varieties may become almost completely rotted by harvest time.

Click on image for larger version Figure 1. Black rot on a grape leaf

Symptoms

Leaves

Reddish brown and circular-to-angular spots appear on the upper surface of the leaves starting in the late spring (Figure 1). As spots merge, they form irregular blotches that are reddish brown. The number of spots or lesions per leaf varies from 1 to more than 100, depending on the severity of the disease.

The center of the leaf spot turns tannish brown and is surrounded by a black margin. Fungus fruiting bodies (pycnidia) that are speck sized and black are arranged in a definite ring just inside the margin of the spot. Only young, rapidly growing leaves are susceptible.

Fruit

Fruit infections can take place shortly after the calyx (flower petal) falls, but most infections occur when the fruit is half to almost full size. A small spot appears that is circular and whitish tan, often surrounded by a brown ring. Such spots first appear on the berry – usually while it is still green.

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Disease Cycle

The black rot fungus overwinters in canes, tendrils, and leaves on the grape vine and on the ground. Mummified berries on the ground or those that are still clinging to the vines become the major infection source the following spring. During rain, microscopic spores (ascospores) are shot out of numerous, black fruiting bodies (perithecia) and are carried by air currents to young, expanding leaves. In the presence of moisture, these ascospores slowly germinate, often taking 36 to 48 hours, but eventually penetrate the young leaves and fruit stems (pedicels). The infections become visible after 8 to 25 days. Usually, spots appear first on the lower leaves. When the weather is moist, ascospores are produced and released throughout the entire spring and summer, providing continuous primary infection. The black rot fungus requires warm weather for optimal growth; cool weather slows its growth. A period of 2 to 3 days of rain, drizzle, or fog is also required for infection.

Each older leaf spot contains a number of pycnidia, each of which produces hundreds of summer spores (conidia) that ooze out in winding tendrils during wet weather. The splash of raindrops spreads these spores to other leaves and to young fruit. If water is present, the conidia germinate in 10 to 15 hours and penetrate young tissue. New black rot infections continue into late spring and summer during prolonged periods of warm, rainy weather. The conidia are capable of germinating and causing infection several months after being formed.

During August, the pycnidia are transformed into an overwintering stage (pycnosclerotia) that, in turn, gives rise to perithecia within which the spring spores (ascospores) are produced. This completes the disease cycle.

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Control

Black rot is NOT difficult to control if the cultural and chemical practices outlined below are followed.

1. Space vines properly and choose a planting site where the vines will be exposed to full sun and good air circulation. Keep the vines off of the ground and make sure they are properly tied. This practice reduces the time that vines remain wet from dew and rain and thus limits the amount of infection.

2. Prune the vines each year during the dormant period. Select only a few strong, healthy canes from the previous year's growth to produce the following season's crop. Follow the suggestions outlined in Illinois Extension Circular 935, "Growing Small Fruits in the Home Garden". Remove the prunings, excess growth, diseased and overwintering berries, leaves, and tendrils from the vineyard, and burn or otherwise destroy them. This practice reduces inoculum of the fungus, thus limiting disease.

3. Keep the fruit planting and surrounding areas free of weeds and tall grass. This practice will promote rapid drying of vines, and thereby limit infection by the fungus.

4. Where feasible, cultivate the vineyard before bud-break to bury the mummified berries. Diseased berries covered with soil do not produce spores that will reach the developing vines.

5. Grape cultivars differ in their susceptibility to black rot. The reactions of many grape cultivars to black rot and four other important diseases are given in the Table below.

   Cultivars with large, juicy berries are the most susceptible ones. In general, grapes that ripen late in the season are affected the least. All commercial cultivars now grown in Illinois are sufficiently resistant if adequately protected during prolonged rainy periods with a fungicide spray program.
   
   

6. Use protective fungicide sprays, which are needed in wet seasons, to protect the developing new growth. Follow the grape spray schedule outlined in either Circular 1145, "Home Fruit Pest Control" or for the commercial grower, the "Grape Spray Guide" in Circular MD-1 Illinois Commercial Small Fruit and Grape Spray Guide. These publications are updated frequently and can be purchased from:

   
   Ag Communications
   67-C4 Mumford Hall
   1301 W. Gregory Drive
   Urbana, IL 61801.

   Another useful publication, the yearly Proceedings of the Illinois Small Fruit and Strawberry Schools can be purchased from:

   
   Jeff Kindhart
   Dixon Springs Agricultural Center
   Simpson, IL 62985.

   Thorough coverage of all the plant parts above ground with each application is essential for control and for successful fruit production. The important sprays to control black rot are:

   1. as new shoots merge when they are 2 to 4 inches long, and again when they are 10 to 15 inches long;
   2. just before bloom; and
   3. just after bloom, when the fruit has set.

   
   After these crucial sprays, applications should continue at about 10-day intervals as long as the weather is rainy and muggy. The sprays can be discontinued when the weather turns dry.

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The Relative Susceptibility of Grape Cultivars to Five Diseases

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Author:

Stephen M. Ries (s-ries@uiuc.edu]