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.
on image for larger version
Figure 1. Black rot
on a grape leaf
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 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.
Black rot on
The spots rapidly enlarge, darken, and may cover half or more of the berry within 48 hours. The center of the spot rapidly becomes sunken, wrinkled, and dark. Within a few days, the entire berry becomes coal black, hard, and mummified (Figure 2). Most of the diseased fruit 'shell' or shatter and drop early. The surface of the withered fruit is soon covered with minute, black pimple-like pycnidia that are often arranged in circular zones.
Shoots, Leaf and Fruit Stems, and Tendrils
The lesions on these parts are dark purple to black, oval to elongated, and somewhat sunken. The speck-sized black pycnidia are scattered over the surface of the lesions. As the canes grow, the bark tends to split along the length of the lesion. If the berry stem is infected early, the flow of sap is shut off, and the berry shrivels and fails to develop.
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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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Black rot is NOT difficult to control if the cultural and chemical practices outlined below are followed.
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.
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.
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.
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.
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.
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:
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:
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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|HS=highly susceptible; MS=moderately susceptible; MR=moderately resistant; R=resistant. Not known is indicated by two dashes|
|CULTIVAR||Black Rot||Downy Mildew||Powdery Mildew||Botrytis Bunch Rot||Phomopsis Cane & Leaf Spot|
|ABerries not susceptible.
Note: The disease reactions of these cultivars may differ from one location to another because of the presence of physiologic races of causal fungi.
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Stephen M. Ries (email@example.com][an error occurred while processing this directive]