PPFS-FR-S-12

Plant Pathology Fact Sheet

Powdery Mildew of Grape by John Hartman and Julie Beale

Although dry weather can slow the development of many grape diseases in Kentucky, this is not the case with powdery mildew. This disease can be a serious problem even during periods of drought. Disease losses due to fruit infections can be severe, sometimes resulting in complete loss of the crop. If not controlled on susceptible cultivars, the powdery mildew fungus not only affects fruit yield and quality, but it also reduces vine growth and winter hardiness. Figure 1. White fungal growth on the surface of infected fruit is indicative of Powdery Mildew Symptoms and Signs Powdery mildew can infect all green parts of diseased fruit may split open. When berries the grapevine. This disease is most easily of purple or red cultivars become infected recognized by the dusty appearance or during ripening they fail to color properly, white powdery growth occurring in patches resulting in a blotchy appearance at harvest. on fruit (Figure 1), leaves (Figure 2), and Most fruit is susceptible to infection from vines (Figure 3). bloom through a few weeks after bloom. Concord grape berries, on the other hand, When young expanding leaves are infected, are quite resistant within two to three weeks they may become distorted and stunted. after bloom. Severely affected leaves may curl upward during hot dry weather. Dark brown to black Cause and Disease Development blotchy lesions form on diseased vines Powdery mildew of grape is caused by the (Figure 3). fungus Uncinula necator. The pathogen overwinters as fungal fruiting structures Infected berries often are misshapen or (cleistothecia) that have become lodged in have rusty spots on the surface. Severely bark crevices on the grapevine. In the spring,

Agriculture & Natural Resources • Family & Consumer Sciences • 4-H/Youth Development • Community & Economic Development is detrimental to the survival of conidia. This means that powdery mildew can become a serious problem even when it is too dry for other diseases to develop. Low, diffuse light also favors powdery mildew development.

Figure 2. Powdery Mildew symptoms and signs on Grape leaves airborne spores (ascospores) are released Figure 3. White fungal growth and dark lesions on from the cleistothecia and are carried by wind powdery mildew infected vine to susceptible tissues. The spores germinate on any green surface of the developing Disease Management vine, resulting in primary infections. After The primary focus of powdery mildew infection, the fungus colonizes the plant disease management is to be aware of tissue and produces another type of spore cultivar susceptibility, to start fungicide (conidia) on the surface. The conidia and sprays immediately prior to bloom, and to fungal mycelium give a powdery or dusty select appropriate fungicides. appearance to the infected plant parts. Conidia serve as secondary inoculum for • Cultivar selection new infections during the remainder of the Cultivars of Vitis vinifera and its hybrids growing season. Late in the fall cleistothecia (French hybrids) are generally much more form on infected plant surfaces. Many of susceptible to powdery mildew than are these fruiting bodies wash into bark crevices native American cultivars, such as Concord. where they overwinter, thus beginning the next cycle of the disease. Cultivars that are highly susceptible to powdery mildew include: Cabernet Franc, Infection and disease development are Cabernet Sauvignon, Chancellor, Chardonnay, favored by temperatures of 68º to 77ºF, but Chelois, Einsett Seedless, Gewurtztraminer, the fungus may be active from 59º to 90ºF. Jupiter, Leon Millot, Limberger, Merlot, Hotter temperatures inhibit the powdery Moore’s Diamond, Muscat Ottonel, Pinot mildew fungus. A relative humidity of 40% Gris, Pinot Meunier, Pinot Blanc, Pinot Noir, to 100% is conducive to spore germination Riesling, Rosette, Rougeon, Sauvignon Blanc, and infection. Seyval, Vidal Blanc, Vignoles, and Villard Noir.

Unlike most other grape diseases (such as Cultivars that are only slightly susceptible black rot and downy mildew) that require to powdery mildew include: Canadice, free moisture for spore germination and Cayuga White, Chambourcin, Cynthiana/ infection, powdery mildew only requires high Norton, Ives, Marquis, Mars, Melody, humidity. Free moisture, especially rainfall, Steuben, and Traminette. • Site selection and cultural practices the onset of cool weather in the fall. Because Avoid planting grapevines in low or shady leaves covered with mildew do not function areas. Keeping plants well spaced and the normally, the effect on the plant can be canopy thinned out will help to promote good much the same as if the vines had become air circulation and reduce humidity. Remove defoliated. Premature defoliation, whether and destroy diseased plant material at the functional or actual, predisposes the vines end of the growing season to winter injury and reduces fruit set the following season. • Fungicides Sprays should begin immediately prior to Additional Resources bloom. Early control of primary infections, Disease management and grape production especially on susceptible cultivars, is advice can be found in the following important to managing this disease. If publications available at County Extension primary infections can be controlled until all Offices and/or on the Internet. the ascospores have been discharged, the amount of inoculum available for causing • Disease and Insect Control Programs late-season (secondary) infections is greatly for Homegrown Fruit in Kentucky, Including reduced. Failure to adequately control Organic Alternatives, ID-21 (University of powdery mildew early in the growing season Kentucky) can also result in increased levels of other fruit http://www.ca.uky.edu/agc/pubs/id/id21/ rots, such as Botrytis bunch rot and sour rot. id21.pdf • Midwest Small Fruit and Grape Spray Effective fungicides include Abound, Guide, ID-94 (University of Kentucky et al.) Bayleton, Elite, Endura, Flint, JMS Stylet Oil, 2.1 MB file Nova, Pristine, Procure, Quintec, Rubigan, http://www.ca.uky.edu/agcollege/ Sovran, and Sulfur. Follow label directions plantpathology/ext_files/PPFShtml/ID-94. and be sure to alternate fungicides with pdf different modes of action over the course of • Midwest Grape Production Guide (Ohio the season. State University) 5 MB file http://www.ca.uky.edu/agcollege/ Producers with susceptible cultivars should plantpathology/ext_files/PPFShtml/ plan for a full-season fungicide program to MwGrapeGuide.pdf control powdery mildew. While growers • Midwest Small Fruit Pest Management may be tempted to put away their spray Handbook, B-861 (University of Kentucky equipment after grape harvest, it is essential et al.) 73 MB file to maintain at least some protection against http://www.ca.uky.edu/agcollege/ infections through the remainder of the plantpathology/ext_files/PPFShtml/ season. Even though the berries become MwSmFruitPMHandbook.pdf resistant with age, cluster stems (rachis) and leaves remain susceptible throughout the season. In some years powdery mildew (Issued July 2008, cak) causes post-harvest defoliation well before Photos: Julie Beale, University of Kentucky

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