Downy Mildew of Grapes

Kansas State University	American Phytopathological Society
Martin Luther University, Wittenberg, Germany	INRA

Downy Mildew of Grapes

This disease earned its fame as part of a mixed blessing sent from America to Europe. During the 1860's the vineyards of western Europe were being ravaged by Phylloxera, a highly destructive condition caused by aphids. American Vitis (grape) species were resistant to the aphid so rootstocks of these species were sent to Europe to help control the problem. Unfortunately, downy mildew is endemic in America and the causal fungus was also sent overseas. Downy mildew was not a major problem in the United States but unfortunately, European grape species were highly susceptible to the disease and a major epidemic occurred in France between 1878-1882. The disease also progressed into grape growing regions of Germany and Italy. It took an army of biologists, chemists, agricultural engineers, blacksmiths and brassworkers to win the battle against downy mildew. When the "war" was over, the formula for the first foliar applied liquid fungicide, Bordeaux Mixture, was perfected, early versions of the backpack sprayer had been invented and the vineyards were saved! The causal organism of downy mildew is Plasmopara viticola (Berk. & Curt.) Berl. & de Toni. The fungus is an obligate parasite (it can not be cultured on artificial media). Downy mildew is found worldwide wherever grapes are grown, occurring primarily where warm, humid conditions exist during the growing season. In the United States, the disease occurs east of the Rocky Mountains. All common cultivated and wild species of grape as well as a few hosts outside the Vitis species are susceptible to this disease. The European grape (Vitis vinifera) shows the highest degree of susceptibility. The fungus over-seasons as oospores in the soil or contained in shed or attached leaves. It can also survive as mycelium in infected twigs. Plasmopara viticola becomes active in the spring when oospores germinate to form a sporangium. Sporangia are formed in the dark and are disseminated by wind or rain splash. They germinate on host tissue when free moisture is present releasing zoospores. These spores swim to stomata on leaves, twigs or berries and encyst. Primary infection occurs when these spores germinate and penetrate the host through stomata. The fungus becomes established as intercellular mycelium and then produces sporangia that exit host tissue through stoma on the undersides of leaves or lenticels on fruit. Zoospores released from these structures causes secondary infections, entering the host through stomata or lenticels. As the season progresses, oospores are formed from mycelium within host tissue completing the life cycle. The most important environmental factor in the development of downy mildew is moisture. Wet winters and springs are particularly important for oospore survival and germination. In addition, these spores must be subjected to a prolonged cold spell before they will germinate. Wet summers favor the survival and germination of sporangia and zoospores. The optimum temperature for fungal development during the crop's growing season is 25^oC.

Symptoms

Symptoms and signs of downy mildew may occur on all succulent tissue. Leaf infections appear as pale yellow spots on the upper surface. These spots appear oily and may be angular or diffuse. Tissue on the corresponding lower leaf surface is covered with white cottony mycelium. The fungal tissue is initially white then turns gray. As the disease progresses, spots become necrotic and leaves are shed. Premature leaf fall results in reduced sugar content in fruit and hardiness of buds. Fungal spores produced on leaves often serve as a source for berry infection. The symptoms of fruit infection vary depending on the time of infection. Infections occurring when clusters are young often result in a condition called gray rot. Clusters are covered with a gray mat of fungal material and berries rot. It is not uncommon for entire clusters to be affected. An obvious layer of fungal growth is not present on berries infected when they are halfway developed or slightly older. The fungus grows internally on these berries causing a condition referred to as brown rot. Infected fruit on white varieties turns dull green and black fruited varieties turn red. Berries eventually turn brown, fail to soften and become slightly wrinkled. Infected shoots are often swollen and may curl forming a shepherd's crook. They also become covered with a mat of white fungal growth, turn brown and die. Other tissues that may become infected include flowers and tendrils.

Plant Health Management

Fungicide Application.

The most common way of controlling downy mildew is through the use of fungicides. Protectant fungicides such as cupric salts (Bordeaux mixture), dithiocarbamates containing Manganese and Zinc ions (Penncozeb) and captan are used as preventatives. The systemic compound Metalaxyl (Ridomil) can be used as a curative treatment. Timing of fungicide application for downy mildew now incorporates the use of disease forecasting and modeling to ensure greater efficacy and cost effectiveness. (1997)

Sanitation.

Removal of infected plant debris and draining soils to remove overwintering oospores may be effective in small plantings but is impractical in large vineyards.

References

Agrios, G.N. 1978. Plant pathology, 2nd ed. Academic Press, Inc. New York, NY, 703 pp.

Anderson, H.W. 1956. Diseases of fruit crops. McGraw-Hill Book Company, Inc. New York, NY. 501 pp.

Large, E.C. 1962. The advance of the fungi. Dover Publications, Inc. New York, NY. 488 pp.

Pearson, R.C. and A.C. Goheen, eds. 1988. Compendium of grape diseases. APS Press, St. Paul, MN. 93pp.

Tran Manh Sung, C., Strizyk, S., and Clerjeau, M. 1990. Simulation of the date of maturity of Plasmopara viticola oospores to predict the severity of primary infectons in grapevine. Plant Dis. 74:120-124.

Useful Links

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Highlighted Area

One of the most important aspects of the downy mildew epidemic of Europe in the 1880's was the development of Bordeaux Mixture. A lot of controversy surrounded the development of the first foliar applied liquid fungicide. Pierre Marie Alexis Millardet is credited as the person who discovered the fungicidal properties of the copper sulfate-lime mixture but there were other investigators who were eager to take this honor. A chance observation by Millardet led to the development of Bordeaux mixture. He noticed that grapes treated with a blue substance, applied to them to keep passersby from pilfering the fruit, appeared healthy while surrounding fruit was succumbing to downy mildew. Once the vineyard manager identified the compound, Millardet began testing its efficacy as a fungicide and perfecting its formulation. He discovered that the copper component was suppressing fungal germination while the lime component acted as a combination buffer against phytotoxicity and as a spreader-sticker. Up until this time fungicides had been applied as dusts to foliage or slurries to seed. Grape growers were now faced with the task of treating millions of acres of vines with a liquid fungicide. Several means of application were tested before the first practical backpack sprayer was invented. An interesting account of downy mildew and the development of Bordeaux mixture is included in the book "The Advance of the Fungi" by E. C. Large.

Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was researched and drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

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This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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