Common Leaf Spot of Alfalfa

Common Leaf Spot of Alfalfa

Alfalfa has many leaf spot diseases and common leaf spot is considered one of the most destructive. The first recorded observation of the disease occurred in 1832. It is now known to occur worldwide. Common leaf spot is favored by cool to moderate, wet, weather and is generally a problem on the first and second cuttings and on fall regrowth. In irrigated fields it may persist throughout the growing season when temperatures are favorable. The causal organism is Pseudopeziza medicaginis (Lib.) Sacc. The host range of the pathogen is limited primarily alfalfa, with two form-species identified other Medicago species. The fungus overwinters as apothecia or mycelium in undecayed residue and does not survive well in the most northern alfalfa growing areas. New apothecia form on this residue in the spring. Several apothecia may arise from a single stroma and be clustered together. Ascospores are forcibly discharged from apothecia formed on overwintering tissue during periods of high moisture and a temperature range of 15-24oC. Spores are windblown to lower leaves and germinate when free moisture is present. The fungus invades the host directly through the cuticle. Once established, a single apothecium is produced per infection site. Ascospores are again released and wind blown. This cycle continues until the weather turns hot and dry. Infected leaves that have been shed but remain in the field and do not decompose provide inoculum for fall infections.

Disease Cycle

Symptoms

Symptoms of the disease are seen primarily on the leaflets. Numerous, small brown to black spots develop on leaflets. The spots are 1-3 mm in size and do not coalesce. The margin of the spot may be smooth or dentate (toothed). A single apothecium is formed in each spot and appears as a light brown raised disc, visible with the aid of a hand lens. The vast majority of the apothecia form on the upper leaf surface. The area around the spots initially stays green. As the disease progresses infected leaves eventually turn yellow and are shed. Defoliation may be extensive affecting both yield and quality of the hay. Plants are seldom killed but may suffer reduced vigor. The exception to this rule occurs when plants become infected during the seedling stage. In this case, excessive plant loss can occur especially if young plants are growing under a thick cover crop. On rare occasions symptoms may develop on petioles or succulent stems. These spots are elliptical and seldom form apothecia.

Plant Health Management

Harvest as early as possible.

This is especially important if defoliation is beginning to occur.

Use Resistant Cultivars.

Moderate and highly resistant cultivars have been developed.

Fungicide Application.

Cupric hydroxide (Kocide 101 and 404) may be applied 10 to 14 days prior to harvest or earlier if conditions are conducive for disease development.

References

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

Heald, F.D. 1933. Manual of plant diseases, 2nd ed. McGraw-Hill Book Company, Inc. New York, NY. 953 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.

Watkins, J.E. and Kehr, W.R. 1980. Leaf spot and black stem diseases of alfalfa. Cooperative Extension Service, Institute of Agriculture and Natural Resources, University of Nebraska - Lincoln. NebGuide G80-488.

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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 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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