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Histopathology of Colletotrichum trifolii on Alfalfa. Miguel D. M. Porto, Associate professor of Fac. Agronomia–UFRGS, CP776, 90000 Porto Alegre, RS, Brazil; Craig R. Grau, Gustaaf A. de Zoeten, and Gary Gaard. Professors and Research specialist, respectively, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 78:345-349. Accepted for publication 10 August 1987. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/Phyto-78-345.

The invasion of alfalfa stem tissue by Colletotrichum trifolii was studied using light, electron transmission, and scanning electron microscopy. Conidia were found in depressions on the stem. Appressoria were formed in these depressions but were not associated with stomata. Penetration was accomplished by appressoria and also by hyphal strands without the formation of appressoria. Some epidermal cells were killed before penetration. Chloroplasts were highly sensitive and were usually destroyed in advance of the hyphae. Prior to penetration, epidermal cells showed pronounced swelling, and some were killed. After penetration, the fungus progressed inter-and intracellularly towards the xylem vessels and, in 4 to 6 days, destroyed the pith and endodermis. Acervuli were produced 5 to 8 days after inoculation. Genotypic differences in alfalfa did not affect spore germination and the production of appressoria, but marked differences were observed in the number and distribution of acervuli associated with diseased stem tissues.