Grape plants can experience many types of disease that will ruin the year's grape crop or kill the vine over a number of seasons. Backyard grape plants suffer in wet spring weather, which exacerbates the spread of many diseases. Grape plants that are not pruned back carefully each year are also more susceptible to disease.
Grape plants experience two types of mildew, both caused by fungus: powdery mildew and downy mildew. Powdery mildew affects mostly European grapes (Vitis vinifera), but can harm American grapes. This disease appears as a white residue on plant leaves and causes fruit to harden and become inedible. Downy mildew also covers plant leaves with white fungus, but the fruit turns red or yellow and fails to ripen. Wet weather exacerbates the spread of these diseases. Control powdery mildew and downy mildew by applying preventative fungicide throughout the season.
Black rot, caused by the Guignardia bidwellii fungus, occurs on grape plants throughout North America and can cause severe crop loss if not managed. Initial symptoms are black spots on the leaves. Infected berries shrivel midseason. American grapes (including Concord) are quite susceptible to black rot; French and European grapes are more resistant. Gardeners can apply preventative fungicide throughout the season to ward off black rot or plant more resistant varieties.
Eutypa dieback causes new shoots from the canes to yellow and curl in due to rot. Over several seasons, Eutypa dieback progresses, causing many shoots and possibly the entire plant to die back. The disease goes unnoticed for two to three years before displaying symptoms. All types of grape plant can experience this dieback. Gardeners can prune off infected limbs as a tentative solution, but this is not always successful in managing the disease.
French grapes can contract tomato or tobacco ringspot disease, though tomato ringspot is more common. Grapevines lose vigor and produce smaller, yellowed leaves. They are more prone to dying over the winter, since ringspot weakens the plant. Ringspot is spread by nematodes that feed on infected weeds and then on grape roots. Gardeners can prevent this disease by fumigating soil, but cannot remedy an infected plant.
Peach Rosette Mosaic Virus Disease
Another disease spread by nematodes feeding on infected weeds, peach rosette mosaic virus infects American grapes like Concord or Niagara. This disease also cannot be controlled once discovered; gardeners should treat it like the ringspot diseases or plant the less susceptible Delaware grape. This virus warps the growth of the grape plant, causing missing spots where canes should grow.