Downy Mildew of Alfalfa


	Agriculture Canada

Downy Mildew of Alfalfa

Downy mildew can be found in temperate alfalfa growing regions during periods of cool, moist or wet weather. The disease is primarily a problem on the first cutting and in fall-seeded stands. The causal organism is Peronospora trifoliorum de Bary. The fungus has been divided into pathogenic races based on cultivar reaction. It is an obligate parasite that is pathogenic on Medicago spp. The fungus overwinters as mycelium in systemically infected plants, residing in crown buds, the cortex of crown branches and in surviving shoots. Researches also suspect that oospores found in crop residue may serve as overwintering structures. The infection cycle is initiated in the spring when conidia are formed from the mycelium in infected shoots and systemically infected spring growth. The spores are produced in the dark when the humidity is near 100%. Spores are wind blown or water splashed to susceptible young leaf tissue. Older tissue is resistant to infection. Spores germinate within in a temperature range from 4^oC to 29^oC, with 18^oC being optimum. Free moisture must be present for germination to occur. Germ tubes form an appressorium and commonly invade the host directly through the epidermis. The fungus may also penetrate through stomata. Conidia are formed on newly infected tissue with infection cycles occurring every 5 days under favorable conditions. Oospores may also be produced in infected tissue. Disease development is arrested by warm dry weather but may resume with the return of cool, wet weather.

Symptoms

The most common symptom of downy mildew is the appearance of chlorotic tissue on infected leaves and stems. Localized pale yellow spots may occur on the leaves of rapidly growing plants. Entire leaves or shoots may look chlorotic when the infection is systemic. Severely infected leaflets roll downward or become twisted. A downy mat composed of conidiophores and conidia develops on the underside of infected leaflets. The mat is gray to pale violet in color. Infected stems have a larger diameter and are shorter than normal due to a reduction in internode elongation. Plants that are stunted may be slow to recover after the first cutting. Fall seeded crops may be destroyed when seedlings are severely infected. Plants that are not killed outright are weakened and do not survive the winter.

Plant Health Management

Resistant Cultivars.

Cultivars with a high degree of resistance are available.

Seed Treatment.

Pythium

Phytophthora

Harvest hay as early as possible.

References

Dickson, J.G.1956. Diseases of field crops, 2nd ed. McGraw-Hill Book Company, Inc. New York, NY. 517 pp.

Integrated pest management for alfalfa hay. 1985 University of California Statewide Integrated Pest Management Project, Division of Agriculture and Natural Resources. Oakland, California. Publication 3312. 97 pp.

Nyvall, R.F. 1989. Field crop diseases handbook, 2nd ed. Van Nostrand Reinhold. New York, NY. 817 pp.

Stuteville, D.L. and Erwin, D.C., eds. 1990. Compendium of alfalfa diseases, 2nd ed. APS Press, St. Paul, MN. 84 pp.

Useful Links

Texas A&M University

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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