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Resistance

The Induction and Analysis of Two Classes of Mutations Affecting Pathogenicity in an Obligate Parasite. D. W. Gabriel, Graduate research assistant, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824, Present address: Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; N. Lisker(2), and A. H. Ellingboe(3). (2)Senior lecturer, Stored Products Division, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel; (3)Professor, Department of Botany and Plant Pathology, Michigan State University, East Lansing 48824, Present address: International Plant Research Institute, 853 Industrial Rd., San Carlos, CA 94070. Phytopathology 72:1026-1028. Accepted for publication 8 December 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1026.

Both temperature-sensitive and unconditional mutations were used to study separately the effects of genes controlling gene-for-gene relationships and those controlling the ability of the pathogen Erysiphe graminis f. sp. tritici to invade its Triticum aestivum host. Twenty-nine mutations to increased virulence were induced in the pathogen. Selection was made for a change from infection type 0 to infection type 4 on congenic lines of wheat with either of two unlinked R genes, Pm1a or Pm4a. Since no other changes in pathogenic specificity were detected, the mutations were interpreted as affecting the corresponding gene loci P1a and P4a in the pathogen. When the mutants were inoculated onto congenic susceptible plants, the rate of infection and the final infection type were indistinguishable from the wild type. These loci therefore appear to function for avirulence and not for virulence. Nineteen temperature-sensitive mutations were also induced in the pathogen. Growth and development of the mutant isolates could be stopped by raising the temperature from 20 to 25 C. Growth could be resumed by lowering the temperature, even after a week at the high (restrictive) temperature. The infection types on plants with Pm genes were unaffected by the temperature-sensitive mutations. These mutations appear to affect genes that control basic parasitic growth and development.