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Genetic Studies on Tolerance of Carboxin and Benomyl at the Asexual Phase of Ustilago hordei. Y. Ben- yephet, Graduate Assistant, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel; Y. Henis(2), and A. Dinoor(3). (2)(3)Associate Professor, and Senior Lecturer in Plant Pathology and Microbiology, Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot, Israel. Phytopathology 64:51-56. Accepted for publication 13 June 1973. DOI: 10.1094/Phyto-64-51.

Mutants of Ustilago hordei tolerant to 2,000 ppm benomyl [methyl 1-(butylcarbamoyl—2-benzimidazole-carbamate] and to 5, 25, and 50 ppm carboxin (5,6-dihydro-2-methyl-1,4-oxathiin-3-carboxanilide) were induced by ultraviolet (UV) irradiation. Tolerance was stable even after 50 transfers on fungicide-free medium. No tolerant mutants (spontaneous or by training) were recovered from cultures not treated with a mutagen. No cross-tolerance showed up between benomyl and carboxin. Carboxin-tolerant mutants tolerated oxycarboxin. The tolerant mutants were somewhat less competitive than the sensitive strains in mixed cultures, but did not disappear. Tolerance was dominant in forced-dikaryons in agar cultures and in inoculated host plants. The center of forced-dikaryon colonies was slimy and gave rise to slimy and mycelial colonies when transferred. Mycelial colonies produced dikaryotic and (sometimes) diploid sporidia while slimy colonies gave rise to haploid and dikaryotic sporidia. Dikaryotic sporidia reacted with both mating types but the diploid sporidia were neutral. It is assumed that sexual affinity in diploids does not allow for any further matings. Dikaryotic colonies gave rise to somatic recombinants.