Stemphylium Leaf Spot of Alfalfa

Stemphylium Leaf Spot of Alfalfa

Stemphylium leaf spot has been identified in the United States, Australia, New Zealand, Africa and Europe. This leaf spot is unique among all the other foliar disease problems of alfalfa. Disease development is favored by wet weather like most of the leaf diseases but two distinct types of symptoms are formed depending on the temperature and several pathogen species are credited as incitants of the disease. Up to 1986 one fungus was listed as the causal organism of this disease, Stemphylium botryosum and its teleomorph Pleospora tarda. Five species of this pathogen and its teleomorph are now recognized as the causal organism of the disease. The holomophs (asexual and sexual pairs) are: 1) Stemphylium alfalfae E. Simmons (teleomorph : Pleospora alfalfae E. Sommons), 2) Stemphylium botryosum Wallr. (teleomorph: Pleospora tarda E. Simmons), 3) Stemphylium globuliferum (Vestergr.) E. Simmons, (teleomorph Pleospora. sp. undetermined) 4) Stemphylium herbarum E. Simmons (teleomorph: Pleospora herbarum (Pers.: Fr.) Rabenh.), and 5) Stemphylium vesicarium species complex (teleomorph: Pleospora spp.). Three of the holomorphs have been found in the U.S. They are numbers 2, 4, and 5 listed above. The fungi are known to produce phytotoxins which influence the infection process. The host range of these fungi includes alfalfa and other Medicago spp. as well as red and sweet clover. Exact host range is dependent on the species. To complicate matters more, Stemphylium botryosum is divided into two distinct isolates or biotypes that grow best at different temperatures and produce different symptoms on their hosts. Cool temperatures (-18 - 20^oC) favor biotype C-T. This isolate was recovered in California where the disease is primarily a problem in the spring and fall. The C-T type symptoms have also been observed with infections of S. vesicarium in California. Warm temperatures (23-27oC) favor biotype W-T. This isolate was recovered from the eastern U.S. where the disease is most severe in the summer and early fall. The W-T biotype is the one present in Nebraska. All species of the fungus overwinter as mycelia on dead stems, leaves and on seed. In areas with warmer climates, such as California, the fungus survives the winter without going dormant and conidia are produced throughout the year. They serve as the main source of primary inoculum while the role of the sexual stage may be diminished. In colder climates, ascospores produced in pseudothecia are assumed to be the main source of primary inoculum although the fruiting structures are difficult to find in the spring. Both types of primary inoculum are wind blown or water splashed to susceptible tissue. Spores germinate when free moisture is present on the leaf surface. The pathogen invades host tissue primarily through stomata but some penetration may occur directly through the cuticle between epidermal cells. Once established, conidia are produced on hyphae that extend out of stomata. These spores serve as inoculum for secondary infection. Repeated cycles of infection occur as long as temperature and moisture requirements are prevail. The infection period may be prolonged in irrigated fields.

Disease Cycle

Symptoms

Leaflets, petioles, stems, peduncles, flowers, pods, and seeds may all become infected and exhibit symptoms. Leaf symptoms produced by the W-T biotype are initially small, oval, and dark brown. Spots enlarge and appear slightly sunken with a light brown center and dark brown border surrounded by a pale yellow halo. The spots exhibit concentric rings as they expand and may eventually cover a large portion of the leaflet. Affected leaflets become yellow and are shed prematurely. One large lesion may be all it takes for the leaf to be shed. Defoliation is common and reduces the quality and quantity of hay. Stem, petiole, and pod lesions are characterized as sunken brown or black spots that are restricted in size. Leaf symptoms produced by the C-T biotype consist of irregular shaped tan lesions with a brown margin. Spots do not increase in size once they are formed and seldom exceed 4mm in length. Forage quality may be adversely affected but defoliation is not as common as it is with W-T infection. The infection by C-T must be severe for defoliation to occur.

Plant Health Management

Harvest as early as possible.

This practice is especially helpful is the disease is present and threatens to become severe.

Resistant Cultivars.

Cultivars that show a moderate degree of resistance are available.

References

Borges, O.L., E.H. Stanford, and Webster, R.K. 1976. The host-pathogen interaction of alfalfa and Stemphylium botryosum. Phytopathology 66: 749-753.

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, CA. Publication 3312. 97 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 80-488.

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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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