Black Rot on grapes is a fungal disease caused by an ascomycetous fungus - Guignardia Bidwellii, that attacks grape vines during warm and humid weather. Black Rot originated in eastern North America, but now occurs in some areas of Europe, South America and Asia. It can cause complete crop loss within vineyards, but is virtually unknown in wine regions with long dry summers.
The name comes from the black fringe that borders growing brown patches on the leaves. The disease also attacks other parts of the vine: including the shoots, leaves, fruit stems, tendrils, and fruit. The most damaging effect is to the vine berries. Unfortunately, grape black rot affects many grape-growers and vineyards throughout the United States.

 

Black Rot on grapes can persist in grapevines for several years if not treated. The earliest signs of disease appear as yellow circular areas on young leaves. As these spread, they turn brown and then sprout black fungal spores (pycnidia). The number of spots or scares per leaf varies from 2 to more than 100 depending on the aggressive nature of the disease. This happens while the berry is still green, before veraison. The black spots grow rapidly and may cover half of the berry within 48 hours.
With advancing disease, the affected area can attack the petiole of individual leaves and killing them. Eventually, the fungus spreads to the shoots, causing large black elliptical scares. Although leaf symptoms are annoying, the real damage from black rot comes from fruit symptoms. In most cases the berries are about half-grown before they start to show signs of the infection - the same small brown marks on leaves will begin to appear on the grapes and fruit stems (pedicels). The infections become visible after 8 to 25 days. These areas soften, sink and then rot in just a few days and what remains of the berry shrivels up into a tiny, hard raisin shell covered in fungal fruiting bodies, now looking for more area to attack.
Rains release the dormant winter spores that form within the mummified berries and can be blown throughout the vineyard by strong winds. A second type of spore (Conidia) can also form within cane scares or berries that have remained within the trellising, and these are dispersed short distances by splashing rain drops.
When the weather is moist, ‘asco-spores’ are produced and released throughout the entire spring and summer, providing continuous primary infection. A period of 2 to 3 days of rain, drizzle, or fog is also required for infection. Conidia spores can also form, within the cane or on shriveled berries that have remained on the vine, and these are dispersed by splashing rain drops. Raindrops transfer these spores by moving the spores to different plant parts, especially susceptible young leaves.
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.
Infection of the fruit is the most serious phase of the disease and may result in substantial quality and financial loss. A managed spray program can be beneficial, with one the most common fungicides that prove to be effective for certain regions of the United States for controlling black rot being Sovran 50WG. Vine management is important to control black rot disease, along with good natural aeration and sunlight exposure.
A managed pruning regime is another control method to limit the disease. Remove excess growth, diseased berries, leaves and tendrils from the vineyard, and burn them. This practice reduces inoculum of the fungus, along with during the growing season, minor pruning to train the canopy growth will help with aeration.