PPFS-FR-S-13

Plant Pathology Fact Sheet

Downy Mildew of by John R. Hartman, Julie W. Beale, and Cheryl A. Kaiser

Introduction Downy mildew is an important disease of commercial and backyard in Kentucky. This disease causes direct losses when flowers, clusters, and shoots decay and yields are reduced. Indirect losses result when premature defoliation predisposes grapevines to winter injury. It may take a several years to fully recover after severe winter injury.

Symptoms and Signs Figure 1. Downy Mildew lesions on upper leaf surface Early in the season, infected leaves develop yellowish-green lesions on their upper sporulation is abundant on these berries surfaces. As lesions expand, the affected during humid weather (Figure 4). Later in areas turn brown, necrotic, or mottled the season, infected berries turn dull green (Figure 1). Severely infected leaves may to reddish purple, remain firm, and are easily curl and drop from the . The disease distinguished from non-infected ripening also attacks older leaves in late summer and berries in a cluster. Diseased berries are autumn, producing a mosaic of small, angular, easily detached from their pedicels leaving a yellow to red-brown spots on the upper leaf dry stem scar. Fruit are thought to become surface (Figure 2). Lesions commonly form resistant to downy mildew infection about 3 along leaf veins. The pathogen sporulates to 4 weeks after bloom, but cluster stems on the underside of foliar lesions, producing (rachis) and leaves remain susceptible the white, downy fungal-like growth that is throughout the growing season. characteristic of this disease (Figure 3). When young shoots, petioles, tendrils, or Infected green fruit turns light brown to purple, cluster stems are infected, they frequently shrivels, and detaches easily. White, cottony become distorted, thickened, or curled

Agriculture & Natural Resources • Family & Consumer Sciences • 4-H/Youth Development • Community & Economic Development The pathogen can infect all green, actively growing parts of the vine that have mature, functional stomata. Visible lesions develop in about 7 to 12 days after infection, depending on temperature and relative humidity. At Figure 4. The Downy Mildew night, during periods pathogen sporulating on Figure 2. White Downy Mildew fungal-like growth on of high humidity infected berries leaf underside (right) corresponds to yellow lesions and temperatures on upper leaf surface (left) above 55º F, the fungus-like organism produces sporangia capable of initiating (Figure 5). White, downy sporulation can be secondary infections. The sporangia are abundant on the surface of infected areas. produced on numerous branched structures Eventually, severely infected portions of the that protrude out through stomata and give vine wither and die. the leaf its typical downy appearance.

Cause and Disease Development Disease Management Downy mildew is caused by the fungus- Cultivar selection like organism Plasmopora viticola. This Avoid planting highly susceptible grape organism overwinters as thick-walled cultivars. In general, vinifera ( vinifera) oospores in leaf debris on the vineyard floor. cultivars are much more susceptible than During wet periods in American types, with the French hybrids spring the oospores somewhat intermediate in susceptibility. germinate to form Examples of cultivars highly susceptible to sporangia. The downy mildew include , Chancellor, sporangia, which Chardonnay, Delaware, Fredonia, Ives, are disseminated Niagara, White Riesling, and Rougeon. by wind or rain- splash, release Site selection and cultural practices small swimming Grapevines should not be planted in low or zoospores when shady areas. Keeping plants well-spaced free water (e.g., rain and the thinned out will help to or dew) is present. promote good air circulation and reduce Zoospores are humidity. Remove and destroy diseased plant Figure 3. Close-up of disseminated by material at the end of the growing season. pathogen sporulation on rain splash to grape leaf underside tissues where they Fungicides swim to the vicinity of leaf stomata (tiny Producers with susceptible cultivars should natural pores that are mainly located on leaf plan for a full-season fungicide program to undersides). Infection occurs when encysted control downy mildew. Because symptoms zoospores form germ tubes that invade the do not begin until after bloom, downy mildew inner plant tissues via the stomata. has been labeled the “late season disease.” However, it is especially important on highly Homegrown Fruit in Kentucky (ID-21) for susceptible cultivars to begin an early fungicide recommendations. season fungicide program. The period from immediate pre-bloom through 3 to 4 weeks Additional Resources after bloom is critical for controlling fruit or The following publications are available at cluster infections. County Extension offices, as well as on the Internet.

• Disease and Insect Control Programs for Homegrown Fruit in Kentucky, Including Organic Alternatives, ID-21 (University of Kentucky, 2008) http://www.ca.uky.edu/agc/pubs/id/id21/ id21.pdf • Midwest Small Fruit and Grape Spray Guide, ID-94 (University of Kentucky et al.) 2 MB file Figure 5. Downy Mildew-infected shoots become http://www.ca.uky.edu/agcollege/ thickened and distorted. plantpathology/ext_files/PPFShtml/ID-94. pdf Some growers might be tempted to put • Midwest Grape Production Guide (Ohio away their spray equipment after grape State University) 5 MB file . However, in some years downy http://www.ca.uky.edu/agcollege/ mildew causes post-harvest defoliation plantpathology/ext_files/PPFShtml/ well before the onset of cool weather in the MwGrapeGuide.pdf fall. Premature defoliation predisposes the • Midwest Small Fruit Pest Management to winter injury and reduces fruit set Handbook, B-861 (University of Kentucky the following season. Thus, it is essential et al.) 73 MB file to maintain at least some protection against http://www.ca.uky.edu/agcollege/ downy mildew infections through the plantpathology/ext_files/PPFShtml/ remainder of the season. MwSmFruitPMHandbook.pdf

There are many different kinds of fungicides effective against this disease. Commercial Photos: Julie Beale, University of Kentucky (Figures growers can find information on specific 1 & 3); Clemson-USDA Cooperative Extension. fungicides, application rates, and suggested Bugwood.org (Figure 2); Roger Pearson, New York chemical combinations in the Midwest State Agricultural Experiment Station, Generva Commercial Small Fruit and Grape Spray (Figure 4); University of Georgia Plant Pathology Guide (ID-94). Homeowners should consult Archives, Bugwood.org (Figure 5) Disease and Insect Control Programs for (Issued September 2008)

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