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What is it?
Grape black rot is a plant disease caused by the fungus guignardia bidwelli. Guignardia bidwelli is one of the most destructive grape diseases, and the fungi often destroy an entire crop of fruit.
What does it look like?
Black rot appears a light brown spots which are surrounded by darker brown lines on the actual fruit of the grape. After appearing, the brown spots quickly turn black as the grape shrivels and dries up. You may find shrivelled grapes attached to the vine looking like raisins instead of grapes if your plant is experiencing black rot. Once the fruit has dried, it remains attached to the stems and the fungi spreads into the leaves which may show brownish or reddish spots on them as the disease infects leaf tissue. Once infected with guignardia bidwelli the leaves will usually wilt and may dicolor to yellow or brown as they too dry and die off. The canes and leaf stems may also be affected and show black rot damage by the presence of elongated lesions. Usually these lesions will be sunken and a deep purple or black color.
How does it manifest?
Grape black rot occurs when spores infect new shoots in the spring. Often the fungi over winters on tendrils, support wires or wooden trellises where the grapes grow. It can also survive the colder temperatures on canes and mummified fruit. Once warm and moist spring conditions occur, the spores then infect new shoots, leaves, and tendrils as well as the developing fruit which is especially susceptible to black rot. The most severe damage from black rot occurs when the fruit is approximately one-half to two-thirds grown, however damage from guignardia bidwelli can occur at any stage of the grape plants development and to any area of the plant. Infected leaves, canes and fruit create more spores which can then spread future infection. The main conditions which affect severity of black rot infection are the amount of diseased material which has managed to over winter, as well as the conditions of moisture and warmth which occur in the spring.
What can you do about it?
Black rot infected regions of the plant should be pruned out, being sure to destroy all infected fruit, canes, leaves and tendrils. Do not just prune the areas and let them drop as that can release more spores to otherwise unaffecte regions of your grape vines. Be sure to carefully clear and destroy all infected plant debris. Unfortunately, once the fruit has begun to shrivel fungicidal treatments are no longer helpful. However, you can be prepared next year when the spores may reinfect new shoots. Once the new shoots are between six and eleven inches long you can treat them with a captan containing chemical application. You should also treat just before and just after the grape blooms. Once you begin treatments, continue at ten day intervals until the fruit is full-sized. However, if weather conditions are very wet this may increase the spread of spores and leaf damage in which case you may need to apply the captan at intervals of seven days rather than ten day intervals.
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