Downy Mildew (fungus - Plasmopara viticola): Downy mildew is common in cool, humid environments but seldom occurs in hot, dry areas. Downy mildew is first observed as a pale yellow area on the upper surface of the leaf. The underside of the leaf is marked by a downy appearance. As the disease advances the infected tissue dies and turns brown. Young stems become thickened and are often covered with the white fruiting structures. Fruit that is infected is covered with the white growth or it turns the berry a dull green and then brown. Downy mildew is a particular problem in areas of high humidity. The disease develops in temperatures of 50 to 60 degrees F. Some American grape varieties show resistance, but additional chemical protection is usually needed. Most European varieties are very susceptible. Fungicide application should begin before bloom and continue at seven day intervals.

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