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Genetics

Genetic Analysis of Mutations to Increased Virulence in Magnaporthe grisea. G. W. Lau, Department of Bacteriology, University of Wisconsin, 1630 Linden Dr, Madison 53706, Current address: Department of Biological Sciences, Lily Hall of Life Sciences, Purdue University, West Lafayette, IN 47907; A. H. Ellingboe, Departments of Plant Pathology and Genetics, University of Wisconsin, 1630 Linden Dr, Madison 53706. Phytopathology 83:1093-1096. Accepted for publication 29 June 1993. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/Phyto-83-1093.

An isolate of Magnaporthe grisea, 70-14, virulent on 10 cultivars of rice, but avirulent on rice cultivar Katy, was mutagenized with nitrosoguanidine. Four mutants of independent origin that showed an increase in virulence on Katy were recovered. The virulence on the other 10 rice cultivars was not affected. The lesion size caused by these mutants was intermediate between the parent avirulent isolate, 70-14 (infection type 1), and a virulent isolate, 70-6 (infection type 4). The numbers of lesions also decreased at least 10-fold when compared to numbers of lesions caused by a virulent wild-type isolate. A range of lesion types (0–4) was common in these mutant isolates, with both small and large lesions. In contrast, wild-type virulent isolates always gave large lesions. The lesions of the mutants had a wide, brownish lesion edge compared to the watersoaked appearance of lesions of a virulent wild-type isolate. Genetic analysis showed that in two of the mutants, one of the two avirulence genes in 70-14, P12, was mutated. A new locus, M, was mutated in the other two isolates and identified as being required for the expression of avirulence genes. In addition, a mutation of the suppressor, S11, of the fourth mutant also may have been recovered.