VIEW ARTICLE

Ecology and Epidemiology

Biotypes of Stemphylium botryosum on Alfalfa in North America. W. A. Cowling, Graduate research assistant, Department of Plant Pathology, University of California, Davis 95616; D. G. Gilchrist(2), and J. H. Graham(3). (2)Assistant professor, Department of Plant Pathology, University of California, Davis 95616; (3)Plant pathologist, Waterman-Loomis Co., Highland, MD 20777. Phytopathology 71:679-684. Accepted for publication 12 December 1980. Copyright The American Phytopathological Society. DOI: 10.1094/Phyto-71-679.

Field symptoms of Stemphylium leafspot of alfalfa differ in California and in eastern North America (including southern Canada and northern USA). Characteristic symptoms of the two forms of the disease were reproduced when susceptible alfalfa clones were inoculated with representative isolates of Stemphylium botryosum from the two regions and placed in controlled environment chambers located in both Maryland and California. Under nearly identical environmental conditions, eastern isolates formed concentrically ringed, spreading, darkened lesions while the California isolates produced restricted, bleached, elongated lesions with sharply defined dark borders. The two groups of isolates shared the same host range and taxonomic features, but differed in growth characteristics on artificial media and relative ability to cause disease at different temperatures. California isolates caused greater disease severity than eastern isolates at 8–16 C, but less at 16–20 C. At 23–27 C, the eastern isolates produced high disease severity whereas the California isolates failed to produce necrosis on leaves. The perfect state (Pleospora herbarum) of California isolates matured at 18 C, but that of eastern isolates required cold temperatures (3 C) to induce ascospore formation in pseudothecia. It is concluded that the two forms of the disease are caused by cool-temperature (California) and warm-temperature (eastern) biotypes of S. botryosum, which are adapted to the environments of the respective regions.