Powdery Mildew of Grapevines
General Information | Symptoms | Epidemiology | Management | References
Causal Organism: Uncinula necator
General Information. Powdery mildew, which is caused by the fungus Uncinula necator, is the most serious disease of wine grapes in Washington State. Several severe epidemics, the most of recent of which occurred during 1995, have resulted in devastating yield and quality losses in our wine industry. Powdery mildew is circumglobal, can be severe anywhere that wine grapes are produced, and is particularly problematic in the Pacific Northwest and California. Powdery mildew can have adverse affects vine growth, yield, and winter hardiness. Wine quality can be affected when only 3% of berries are infected. Wine grape varieties vary in their susceptibility to powdery mildew: Chardonnay, Chenin blanc, and Cabernet Sauvignon are highly susceptible to powdery mildew while White Riesling and Merlot are moderately susceptible. The disease is most problematic on vinifera grape varieties
Symptoms. he earliest symptoms on foliage are red cholorotic flecks or spots on the upper leaf surface. Later, an obvious white mealy coating develops on the foliage and may also be evident on berry surfaces and rachises. Severely diseased leaves may become puckered and distorted. Infected berries may have a netlike pattern of scar tissue. Once the mildew is visually evident, the fungus in producing thousands of single-celled asexual spores known as conidia. Infection of shoots results in a red blotches once canes become dormant. Small brown-black fungal fruiting bodies may be evident late in the season on older mildew colonies. These fructifications are called cleistothecia and represent the sexual phase of the mildew fungus life cycle.
Epidemiology and Forecasting. Plant disease epidemics develop under certain environmental conditions and occur in phases. These phases are include but are not limited to survival, dispersal, primary infection, incubation, dispersal, secondary infection, and dispersal. Each phase serves to increase the number of plants or plant organs infected. All of the phases are linked together like the links of a chain. Approach disease control as the purpose were to sever the links. In temperature climates most pathogenic fungi have a survival phase that perennates them over the winter. The survival phase usually produces some type of spore that initiate new epidemics in the spring or early summer. The spores that originate from the overwintering inoculum and initiate epidemics are collectively called primary inoculum. If primary infections are not prevented, a mildew colony develops on infected plant tissue. The spores (conidia) produced on primary mildew colonies travel in the wind to leaves where they produce more infections called secondary infections, which produce more colonies and therefore more conidia. When the mildew is actually visible, the fungus is producing perhaps millions of conidia. Conidia are carried in air currents to leaves and fruit where they germinate to form additional colonies. It is important to understand the once you see mildew colonies, the sequence of events that gave rise to them occurred several days or weeks earlier. Secondary inoculum production and secondary infection are the phases responsible for the spread and increase in intensification of the epidemic. The secondary or repeating cycle repeats until there is no longer any healthy host tissue available for infection, until the onset of adverse weather conditions, or host dormancy.
Management.(also available in mp3 audio and Quicktime or mp4 video formats Because the fungus is favored by high humidity and inhibited by sunlight, cultural practices that promote good air circulation and light penetration should be followed. This promotes drying and lowers the humidity, which in turn should lessen disease pressure. The fungus is inhibited by sunlight and because of this, excess shading should be discouraged. The producer or field consultant should always keep in mind that excessive amounts of nitrogen will promote the production of succulent and susceptible tissue, thicken the canopy, and increase disease pressure. Always allow for good chemical penetration. In vineyards with histories of powdery mildew, the use of over-the-vine impact sprinklers may promote primary infection and intensify the problems with powdery mildew.
Washington Links
Fungicide Recommendations for Eastern Washington
Washington Fungicide Efficacy Trials (Morell, WSU Prosser)
Off Campus Links
El Dorado County (California) Cooperative Extension
Northwest Berry & Grape Infonet
Ohio State University Disease Factsheets
Midwest Small Fruit Pest Management Handbook
Pacific Northwest Disease Control Handbook
Penn State University Fruit Pathology Web Page
Uniroyal Chemical Company
UCDavis Powdery Mildew Risk Index for Control of Grape Powdery Mildew
University of California Statewide Integrated Pest Management
Articles in The Popular Press/Online Publications
Gadoury, D.M. 1998. Ampelomyces quisqualis.(Deuteromycetes).
Grove, G.G., and Watson, J.D. 1997. Washington researchers study grape powdery mildew. Good Fruit Grower Magazine, May 1997.
Wilcox, W.F., Gadoury, D.M., and Seem, R. 1997. Recent Findings on Control of Grapevine Powdery Mildew. New York Wine and Grape Research Foundation Grape Research News, Winter 1997.
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