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Host-Parasite Relationships Between Pseudopeziza trifolii f. sp. medicaginis-sativae and Alfalfa. S. L. F. Meyer, Department of Plant Pathology, University of Georgia, Athens 30602, Present address: Nematology Lab., Bldg. 011A, BARC-West, USDA-ARS, Beltsville, MD 20705; E. S. Luttrell, Department of Plant Pathology, University of Georgia, Athens 30602. Phytopathology 77:309-319. Accepted for publication 23 May 1986. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-309.

Pseudopeziza trifolii f. sp. medicaginis-sativae, the causal agent of common leaf spot of alfalfa, produced ascomata in restricted lesions in living alfalfa leaflets. Light and electron microscopy indicated that during early disease stages, intracellular hyphae invaded the host tissues, resulting in breakdown of the protoplasts of many of the infected plant cells. In the area occupied by the developing ascoma, entire parenchyma cells were replaced by fungus tissue. Beneath the mature ascoma, spongy parenchyma cell contents were replaced by hyphae, but many of the host cell walls were not destroyed. At the lesion margins, hyphae extended into host cells that contained intact organelles. The fungus remained within host tissues until the time of ascospore dispersal. Possible reasons for the success of Pseudopeziza as a parasite of alfalfa include the ability of the fungus to penetrate directly into host cells, the relatively short time period between inoculation and production of fresh inoculum, and the sequestration of the fungus inside plant tissues during a major portion of the fungusí life cycle.

Additional keywords: Medicago sativa, fungus-plant relations.