Downy mildew (Plasmopara viticola) has a complicated life cycle, but inoculum comes from previously infected leaves that overwinter on the ground. Extended leaf wetness during dark conditions is needed for primary infection that can result in unnoticeable subtle foliage lesions. Greatest pressure usually comes about with extended rainy periods in late spring or early summer. Again all green portions of the vine are susceptible to infection. Infection in the canopy can lead to premature defoliation and fruit or rachis infection can cause crop loss. When the rachis, or fruit stem becomes infected, the infection can become systemic within the cluster which is virtually uncontrollable. Again, control of downy mildew is focused on the early to mid-season use of preventative fungicides.
Downy mildew is common in cool, humid environments but seldom occurs in hot, dry areas. Disease 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. Fungicide application should begin before bloom and continue at seven day intervals.
See the Fungicide Efficacy Table for comparisons of various labelled fungicides.