Summer Black Stem and Leaf Spot of Alfalfa

Summer Black Stem and Leaf Spot of Alfalfa

Summer black stem and leaf spot occurs during the warmer portion of the growing season as the name implies. The disease occurs in the warmer alfalfa growing regions of the United States, Africa, Asia, Europe, and South America. Plant injury typically occurs after the first cutting when the crop has regrown and conditions within the canopy favor disease development. The disease may affect both second and third cutting. The causal organism is Cercospora medicaginis Ellis & Everh. The host range of the pathogen is a matter of debate. Most alfalfa researches agree that the host range of Cercospora medicaginis is restricted to Medicago spp. and that Cercospora zebrina attacks species of Trifolium (clover) and Cercospora davisii attacks species of Melilotus (sweet clover). However, some cross infection occurs. The confusion has arisen because symptoms are similar on these hosts and the fungal species are morphologically indistinguishable. The fungus overwinters as mycelium in infected stems and undecayed leaves. Cercospora medicaginis is also seedborne in areas where the climate is warm and humid. Sporulation occurs when temperatures range from 24-28oC and relative humidity nears 100%. Favorable conditions usually exist in the lower canopy when plants regrowth after the first cutting has reached 10 cm. Conidia produced on residue are water splashed or windblown to leaves and stems. They germinate and invade tissue on the upper or lower leaf surface through stomata. Necrosis of host tissue can occur within 96 hours and fungal stroma which give rise to secondary conidia are formed within 5 days of infection. The conditions that favor sporulation also favor germination and infection. Conidia arising from initial infections serve as inoculum for secondary spread of the disease. This process continues throughout the summer and fall when conditions are favorable. In Nebraska the disease can be most damaging from July to September. Delays in harvest lead to the most significant crop injury.

Disease Cycle

Symptoms

The leaf spot phase of the disease is the first one to develop. Infected leaves contain large spots (2-6 cm) which are roughly circular in shape. The spots are initially brown but take on a grayish sheen when spore production occurs. Lesions occur on both leaf surfaces and may be surrounded by a diffuse yellow margin. The occurrence of few lesions (2-3) per leaflet is enough to cause defoliation. When defoliation occurs, it progresses from lower to upper leaflets. The presence of the disease and the defoliation it causes leads to a reduction in forage yield and quality. Stem and petiole lesions are characterized by reddish to chocolate brown spots. They are elongate and may enlarge and coalesce, covering much of the stem. Some stems may be killed but infection is limited to the tissue around the vascular bundles so the injury is not as severe as that seen with spring black stem. When the disease occurs in fields grown for seed production, the seed may become infected. It becomes shriveled, discolored and maintains mycelium on the surface which serves as primary inoculum in subsequent plantings.

Plant Health Management

Harvest as early as possible.

Use varieties with some resistance.

Cultivars with moderate resistance are available and should be used in areas were environmental conditions favor disease development.

Fungicide Application.

Cupric hydroxide (Kocide) may be applied 10 to 14 days prior to harvest (or earlier) if conditions are conducive for disease development. Before making an application it is extremely important to make an accurate identification of the disease because alfalfa has many foliar disease problems with similar symptoms.

References

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

University of Nebraska

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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