Lepto Leaf Spot of Alfalfa

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Lepto Leaf Spot of Alfalfa

This disease has several pseudonyms. The list includes Lepdosphaerulina leaf spot, halo spot, pepper spot, brown leaf spot and Pseudoplea leaf spot. Lepto leaf spot can be found in the United States, Canada and in parts of Europe and Asia. Occurrence is favored by cool to moderate temperatures and wet weather. It is particularly damaging in areas with cool, wet summers. Young foliage is most susceptible to infection, especially that which develops after harvest. The causal organism is Leptosphaerulina trifolii (Rostr.) Petr. No asexual stage of the fungus has been identified. Alfalfa and other annual species of Medicago serve as hosts of the pathogen. Soybean was also identified as a host in 1986 when a leaf spotting organism of that host was identified as L. trifolii. The fungus overwinters as mycelium or pseudothecia on infected leaves on the soil surface. Ascocarps are formed on these leaves and ascospores are produced. Ascospore production is strongly influenced by light with few spores produced in the dark. These spores are released and wind blown to susceptible tissue when environmental conditions are favorable. The disease typically occurs in the spring, early summer and fall in northern regions of alfalfa production and in midwinter in southern regions. Spore germination occurs within an optimum temperature range of 22^oC-25^oC ,when free moisture is present on the leaf surface. Host tissue is invaded directly through the cuticle and outer wall of epidermal cells. The tissue ahead of invading hyphae is also affected which indicates that the fungus produces phytotoxic metabolites. An optimum temperature of 20^oC and moist conditions favor further disease development.

Symptoms

Symptoms associated with this disease are variable depending on the age of the plant, its growth stage and environmental conditions, especially light intensity. Young foliage is the most severely affected but older foliage, petioles and stems may also become infected. As the name implies, leaf spotting is the most common symptom. However there are variations in the type of spot produced. Leaf and petiole lesions typically begin as small reddish brown to black "pepper spots". The pepper spots eventually enlarge and form oval or round spots that are 1-3 mm in diameter. They have light brown or tan centers and a darker brown border. These lesions are referred to as eyespots and are commonly surrounded by a chlorotic halo. The degree of chlorosis in the halo is reduced or it may not occur when the intensity is less than 400 ft-c. When conditions are optimum for both plant growth and disease development, large light tan spots develop. These spots coalesce and appear as large bleached areas. Leaflets infected in this manner are killed. Dead leaves and their petioles remain attached to the plant for a while but are eventually shed. The younger foliage on plants that contain older growth may also develop symptoms but leaves are not usually killed. In addition to defoliation, infection by L. trifolii may also result in stunting if young plants are infected early in the spring.

Plant Health Management

Use cultivars showing some degree of resistance.

Moderately resistant cultivars are available. Agate, 520, 530, and Ramsey have demonstrated resistance to this disease.

Harvest as early as possible.

If the disease is established in the field yield loss due to defoliation becomes worse when harvest is delayed.

Fungicide Application.

Cupric hydroxide (Kocide 101 or 404) may be applied 10 to 14 days prior to harvest or earlier if the disease is present. This treatment is not cost effective if the pathogen has not been correctly identified.

References

Nyvall, R.F. 1989. Field crop diseases handbook, 2nd ed. Van Nostand 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.

Sundheim, L., and Wilcoxson, R.D. 1965. Leptosphaerulina briosiana on alfalfa: Infection and disease development, host-parasite relationships, ascospore germination and dissemination. Phytopathology 55:546-553.

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.

Useful Links

Ohio State 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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