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Monomorphic Nonpathogenic Mutants of Ustilago maydis

We have developed conditions which promote the dimorphic transition of haploid cells of Ustilago maydis in vitro by controlling the pH of the media. At low pH (below 5.0) mycelial growth occurs, whereas at neutral pH yeastlike growth takes place. We screened for mutants unable to form mycelium at low pH and obtained 26 mutants. These mutants have been characterized by their cell and colony morphology in different media. Mutations in 18 strains were found to be recessive when these strains were crossed with the wild type. Other crosses indicated that they were affected in genes other than a and b. Crosses between mutants suggest that the mutations fall in at least two complementation groups. In addition, mutants were characterized by their pathogenicity to corn seedlings. Mutations which were recessive for pathogenicity were also recessive for morphogenesis in vitro.