Disease on grape leaves can reduce the fruit yield of plants. Proper care of vines, as well as maintaining dry conditions, will discourage disease. Symptoms of disease often take the form of spots.
White spots on grape leaves are typically caused by Uncinula necator, a fungus more commonly known as powdery mildew. The disease is airborne, and the spores can be spread through water and wind.
White spots on grape leaves will expand until the white fungus covers the entire leaf if allowed to flourish. Leaves can dry out and often fall from the vines prematurely. Blossoms that become infected may wither and drop without producing any fruit.
Site selection is important in reducing powdery mildew. Put grapevines where they will have access to full sunlight and ensure that they have enough room to allow for good air circulation -- both measures will prevent damp leaves, which allow the fungus to thrive. Remove all leaves with symptoms as soon as possible. Use an organic sulphur fungicide or biofungicide soap to treat the fungus if necessary.