Growing grapes is among the most versatile of home gardening projects. Grapes can of course be eaten raw and can also be processed into wine, grape juice, preserves and raisins, offering many diverse uses for grapes you harvest. Disease control is a critical part of bringing your grapes to harvest time, since many of the most common grape diseases can kill the grapes while they are still on the vine.
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Black rot is a common disease of grapevines caused by the fungus Guignardia bidwellii. By the time the grapes themselves begin to wither and die, the disease has already reached an advanced stage, so learn to recognize the earliest symptoms of this disease before the grapes actually die. Check the grapes during mid-season while they are still white on the vine for white, circular spots 1/8 inch in diameter. If the grapes shrivel and die two weeks later, the chances are good that your grapes have been infected with black rot. Control the disease with fungicide.
The fungus that causes downy mildew, Plasmopara viticola, overwinters in the ground beneath the grapevines. Spores are spread form the ground to the plant by wind, rain and other natural factors. Whole clusters of grapes shrivel and die on the vine, particularly after prolonged periods of warm, wet weather. Before the grapes shrivel, you will notice small, pale yellow circular lesions 1/8 to 1/4 inch in diameter on the upper surface of the leaf; the leaf tissue around the lesion quickly dies after. If you notice these lesions just before the grapes shrivel and die, your grapevines have most likely been infected with downy mildew. Control the problem with fungicide.
Of course, there are several reasons grapes die on the vine that are not related to disease. Grapevines require pollination from bees to set fruit healthily. If there is an abnormally low level of bee activity in your area and the grapevines do not pollinate sufficiently, the grapevines will set fruit but fruit yield will be seriously diminished and the few grapes on the vine will quickly shrivel and die. Check with your local university extension office to see if there has been abnormally low bee activity during the growing season. If so, this is the most likely cause for your grapevine's failing to set fruit.
Many other ailments and other factors could potentially cause grapes to shrivel and die on the vine, ranging from simple drought stress and over-fertilization to a laundry list of fungal diseases that can not only threaten future grape harvests but can kill the grapevines outright. If you are unable to confirm a diagnosis, send a tissue sample to a local university extension office and ask for a remote diagnosis.