Spring Black Stem and Leaf Spot of Alfalfa

Spring Black Stem and Leaf Spot of Alfalfa

This disease can be found in almost every alfalfa field. Fortunately it is only considered a problem in areas with cool, wet climates. Significant plant injury generally occurs before the first cutting. However a second infection period may occur in the fall if environmental conditions are favorable. Spring black stem also takes on greater significance in seed production fields where it can infect the seed and reduce yield. As the name implies, symptoms occur on stems and leaves. However, the causal organism may also invade crown and root tissue, contributing to stand decline. Spring black stem and leaf spot is caused by Phoma medicaginis Malbr. & Roum. In Roum. Alfalfa is the primary host but the fungus is also able to infect other species of Medicago, Melilotus, Trifolium, Pisum, and Phaseolus under greenhouse conditions. Isolates of Phoma medicaginis vary in the type of symptoms they produce but they have not been divided into races. The fungus overwinters as pycnidia on overwintering stems and shed leaves. It may also survive as mycelium in crowns and roots. Conidia (pycnidiospores) are exuded from pycnidia in ropy gelatinous masses during periods of wet cool (18-24^oC) weather. Spores are primarily disseminated by splashed water but may also be wind blown or carried by insects to susceptible tissue. Infection occurs when free moisture (dew or rain) is present. New shoots are also exposed to inoculum when they grow through infected stubble or residue. Crowns may become infected from inoculum present in dead stubble. Root infection occurs vie natural openings and wounds. Insects, especially the clover root curculio feed on roots and the fungus has been isolated from its feeding sites. Seeds become infected when the fungus colonizes pods. The pathogen grows from the pod to the seed and becomes established on the seed coat. It can survive here and thus be seed borne to new fields.

Disease Cycle

Symptoms

The first visible symptoms on infected plants are numerous small brown or black dots on leaves, petioles, and stems. Leaf spots enlarge and coalesce forming irregular blotches that cover a large portion of the leaflet. Infected leaves turn yellow, wither and are shed. Lesions on stems and petioles turn from brown to black and enlarge. Large areas of black tissue appear near the base of stems and they may be girdled and die. Dark fungal fruiting bodies (pycnidia) may be visible in lesions on dead tissue. Crown and root rot occurs when these tissues are invaded. Brown to black spots may also be seen on crown buds and they may be killed. Infected seed pods are discolored and shriveled and contain shrunken seeds.

Plant Health Management

Harvest forage

as early as possible when the disease has been detected. Premature defoliation is a sign that forage yield loss is occurring. However it is important to note that the crop should not cut before the bud stage.

Use certified seed produced in arid regions.

Use moderately resistant cultivars.

Highly resistant cultivars are not readily available but using cultivars with a moderate degree of resistance can attain some degree of disease management.

References

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

Gray, F.A., Fernandez, J.A., and Horton, J.L. 1990. Variation among isolates of Phoma medicaginis var. medicaginis in spore production in vitro and symptoms expression on excised leaves of alfalfa. Plant Dis. 74:668-670.

Martens, J.W., Seaman, W.L. and Atkinson, T.G., eds. 1984. Diseases of field crops in Canada, an illustrated compendium. The Canadian Phytopathological Society, Harrow, Ontario, Canada. 160 pp.

Stuteville, D.L. and Erwin, D.C., eds. 1990. Compendium of alfalfa diseases, 2^nd 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 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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