Powdery mildew (Uncinula necator) can infect all green tissue of the grapevine. Cluster infection at or shortly after bloom can lead to a reduction in set or cause berry damage leading to cracked, damaged fruit at harvest. Infection of the foliage can cause a reduction in vine growth, fruit yield and quality and a reduction in winter hardiness. In eastern U.S. growing regions, the fungus overwinters as cleistothecia on bark, but in California, powdery mildew overwinters as hyphal strands on dormant buds where infection takes place very shortly after budbreak. In Texas, the best evidence is that the fungus overwinters as cleistothecia. Sulfur is an integral part of the powdery mildew control program, but under some environmental conditions can become problematic. Sulfur is ineffective when ambient air temperatures are below 50 ‘F and can be phytotoxic to grapevine foliage when temperatures exceed 95’F. Sulfur residue on fruit can also interfere with fermentation, so use of this product is suggested in late spring and for post-harvest applications. There are several systemic fungicides that can be used during the growing season to prevent fruit yield and quality losses as well as protecting foliage.
See the Fungicide Efficacy Table for comparisons of various labelled fungicides.
Strategies to Control Powdery Mildew
Dr. Wayne Wilcox, Cornell University