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Ultrastructure of Lesions Produced in Leaves of Beta vulgaris by Cercosporin, a Toxin from Cercospora beticola. M. P. Steinkamp, Beet Sugar Development Foundation, Crops Research Laboratory, Colorado State University, Fort Collins 80523; S. S. Martin(2), L. L. Hoefert(3), and E. G. Ruppel(4). (2)(4)ARS, USDA, Crops Research Laboratory, Colorado State University, Fort Collins 80523; (3)ARS, USDA, Salinas, CA 93915. Phytopathology 71:1272-1281. Accepted for publication 23 March 1981. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1981. DOI: 10.1094/Phyto-71-1272.

Lesions induced in leaves of Beta vulgaris 5 days after application of cercosporin, one of two or more toxic metabolites produced by the fungus Cercospora beticola, were examined by electron microscopy and compared with a published report of lesions incited by the fungus. Both types of lesions showed large amounts of granular, electron-dense intercellular material, especially in lesion margins; cells with electron-dense cytoplasmic ground substance and cell-wall appositions; and the loss of cell membranes, especially the chloroplast-bounding membranes and tonoplast. Both lesion types contained necrotic cells that had collapsed during the degenerative sequence. Necrotic cytoplasm within these cells contained starch grains, remnants of the chloroplast lamellar membranes, and sometimes areas that had once held crystalline material. Unlike fungus-induced lesions, however, cercosporin-induced lesions did not have electron-dense bodies in vacuoles and necrotic remnants, lacked a well-defined boundary zone with generalized wall thickenings, and usually had no increases in the size or number of plastoglobuli.

Additional keywords: Cercospora beticola toxin (CBT), nonhost-specific toxin.