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Characterization and Genetic Analysis of Laboratory Mutants of Ustilago maydis Resistant to Dicarboximide and Aromatic Hydrocarbon Fungicides. A. B. Orth, Department of Molecular and Cell Biology, The Pennsylvania State University, University Park 16802, Current address: Dow Elanco Discovery Research, Indianapolis, IN 46268; A. Sfarra(2), E. J. Pell(3), and M. Tien(4). (2)(3)Department of Plant Pathology, The Pennsylvania State University, University Park 16802; (4)Department of Molecular and Cell Biology, The Pennsylvania State University, University Park 16802. Phytopathology 84:1210-1214. Accepted for publication 22 July 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-1210.

Mutants of Ustilago maydis resistant to dicarboximide (DCOF) and aromatic hydrocarbon fungicides (AHF) were isolated after chemical mutagenesis or by selection of spontaneous mutants on fungicide-amended agar medium. A wide range of mutants, with low to high levels of resistance, were isolated. The resistance factor compared to the wild type was greater than 150 for three mutants isolated on tolclophos-methyl and greater than 50 for three mutants isolated on vinclozolin. Growth rates for these six most tolerant isolates were similar to that of the wild type. The phenomenon of cross-resistance between these two groups was confirmed. None of the mutants were resistant to the ergosterol biosynthesis inhibitor propiconazole. Two mutants, VR43 and VR44, demonstrated the same osmotic sensitivity when grown in the presence of NaCl (18%) as the wild type under these conditions. One mutant, TR02, showed a small but significant difference in osmotic sensitivity. Genetic analysis revealed that a single chromosomal gene codes for resistance in four of these isolates. Further analysis demonstrated the presence of at least three resistance genes (adr-1, adr-2, and adr-3). These studies have laid the groundwork for further research to elucidate the mechanism of resistance to AHF and DCOF and possibly the mode of action as well.