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Resistance of Fungi to the Photosensitizing Toxin, Cercosporin. Margaret E. Daub, Assistant professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616; Phytopathology 77:1515-1520. Accepted for publication 24 June 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/Phyto-77-1515.

Isolates of 18 different species of fungi were tested for sensitivity to the toxin, cercosporin, which is toxic to plants, mice, and bacteria, but not to Cercospora species which produce it. Oomycetes were very sensitive to cercosporin, whereas yeasts and some of the other mycelial fungi tested were resistant. Analysis of the fatty acid composition of Cercospora nicotianae indicated that the predominant fatty acid produced by the fungus was linoleic acid, which is susceptible to lipid peroxidation caused by cercosporin. C. nicotianae grown in the presence or absence of cercosporin showed no change in fatty acid composition, indicating that toxin resistance is not due to the production of fatty acids that are resistant to peroxidation. Differences in levels of oxidative enzymes that quench oxygen free radicals also do not appear to be important in fungal resistance. Levels of superoxide dismutase did not differ in extracts from C. nicotianae and from the cercosporin-sensitive fungus Phytophthora cinnamomi. Catalase activity was considerably higher in P. cinnamomi, although no peroxidase activity could be detected in this fungus. Levels of lipid and water-soluble antioxidants differed slightly between C. nicotianae and P. cinnamomi, with C. nicotianae having higher levels of antioxidants. None of these factors appear to be responsible for the major level of cercosporin resistance found in Cercospora spp.