Powdery Mildew of Grapes

Powdery Mildew of Grapes

Powdery mildews, also referred to as surface mildews, have plagued mankind for centuries. Grape powdery mildew was first identified in the United States in 1834 although its exact origin has not been determined. In America it was a minor problem, however, the disease reached epidemic proportions in Europe in the mid-1800's. The disease can now be found worldwide. Originally classified as Oidium Tuckeri, the fungus is now cited as Uncinula necator (Schw.) Burr; anamorph Oidium tuckeri. The most susceptible hosts of the pathogen are members of the Vitis species including Vitis vinifera and several Asian types. American grapes are some of the most powdery mildew resistant members of this genus. Other hosts of U. necator include members of the Ampelopsis, Cissus and Parthenosissus genera. The fungus overwinters as fruiting bodies called cleistothecia (perithecia) on the surface of infected plant material and as hyphae in dormant buds. It can also survive as mycelia and conidia in greenhouses and tropical growing areas. Buds carrying overwintering mycelium become infected at bud-break when the dormant mycelium becomes active and invades new shoot tissue. Mycelium is produced on the external portion of infected buds and gives rise to conidia. These spores are wind blown to nearby vines and are responsible for secondary infections. Conidia germinate and produce an appressoria over a plant epidermal cell. A penetration peg forms below the appressorium and enters the host. A specialized structure called a haustorium is formed inside the host cell. The haustorium is responsible for the transfer of nutrients from the host to the external mycelium. When cleistothecia serve as the overwintering structure, the disease cycle begins when these structures break open and release ascospores. The ascospores are wind blown to green plant material. These spores germinate and infect the host in the same manner as conidiospores. Environmental conditions which favor disease development are moderate to low relative humidities and low light. The absence of free moisture is essential since it inhibits germination of conidia and may actually cause these spores to burst. The optimal temperature range is 20^oC - 27^oC. The host dry conditions that favor development of powdery mildew are a sharp contrast to the warm wet conditions necessary for development of downy mildew.

Symptoms

Symptoms and signs of powdery mildew often begin on shaded areas of the vine, due to the low light requirement for conidia germination. A coating of grayish white, dusty fungal growth i s typical of powdery mildew infections and can be found on most aerial parts of the plants. This "coating" is made up of mycelium, conidiophores and conidia. Most affected plant structures exhibit necrosis of the cells which have been invaded by haustoria and adjacent cells. Leaf infections may result in stunting or distortion if the tissues are infected when they are young. When infection is severe, leaves curl upward during periods of hot, dry weather. Infected areas on young shoots appear as dark brown patches covered with mycelium. As these shoots mature the affected areas take on a reddish brown color. Petioles and cluster stems that are infected show the same brown discoloration. When these structures are infected they become brittle and may break. Poor fruit set is a result of early season infection while decreased fruit quality typifies later infection. Infected berries may be misshapen, be covered with a network of brown corky cells, or have a blotchy rather than uniform color. Affected berries may also split and be invades by other fungi, primarily Botrytis cinerea. When affected fruit is used for wine production the resulting product may have an undesirable flavor . One characteristic sign of the disease that occurs late in the growing season the presence of tiny black cleistothecia. These structures may be present on any infected tissues.

Plant Health Management

Fungicide Application.

U. necator

Plant Site Selection.

Vineyards should be established in areas with good airflow and sun exposure.

Training selection.

Vines may be trained on trellises in a number of ways. Those methods that provide good air flow and reduced shading are best for preventing powdery mildew establishment.

References

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

Braun, A.J. 1952. Ills of the American bunch grapes. Pages 754-760 in Plant diseases the yearbook of agriculture 1953. United States Dept. of Agriculture House Document No. 122.

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

Pearson R.C., Riegel, D. G., and Gadoury, D. M. 1994. Control of powdery mildew in vineyards using single-application vapor-action treatments of triazole fungicides. Plant Dis. 78:164-168.

Useful Links

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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

Questions, Comments, Complaints and Complements?

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

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