Early in the growing season, treat most forms of grapevine leaf disease by applying a sulfur fungicide to the vines every one to two weeks or more often when it rains. If the disease infects ripening fruit, you may be unable to save the vines, notes Wine Maker Magazine.
Begin applying sulfur to grapevines once the shoots have about 3 inches of growth. Continue applying until the sugar content of the grapes reaches 12 to 15 percent. Some common grapevine diseases, such as powdery mildew, stop developing when the sugar content reaches 15 percent. Don't apply sulfur to softened grapes because the taste can transfer to the grapes or to juices and wines made from the grapes, explains Wine Maker Magazine.
Winter temperatures don't kill most grapevine diseases, and disease spores can drift into other parts of a vineyard through rain and wind. Harvesting all grape clusters, even damaged ones that you must destroy, keeps disease from spreading in the off season. Prevent grapevine diseases by adding organic fertilizers that make the vines more resistant to disease, advises Wine Maker Magazine. Humidity and excessive shade encourage the growth of grapevine diseases, so trim leaves once clusters bloom to give grapes more air circulation and sunlight.