Genetic Control of Resistance to the Piperidine Fungicide Piperalin in Ustilago maydis

Mutants of Ustilago maydis resistant to the piperidine fungicide piperalin were isolated in a mutation frequency of 2.4 × 10^–5 after UV-irradiation and selection on media containing 50gml^–1 piperalin. Genetic analysis with 15 such mutant isolates resulted in the identifications of two unlinked chromosomal loci, the U/ppl-1 locus with two allelic genes (U/ppl-1A and U/ppl-1B) and the U/ppl-2 locus. The U/ppl-2 and U/ppl-1A mutations are responsible for two levels of moderate and high resistance to piperalin (resistance factor, Rf: 54 and 135, respectively, based on effective concentration causing a 50% reduction in the growth rate, EC₅₀), while the U/ppl-1B mutation gives only a small reduction (approximately 8-fold) in piperalin sensitivity. Cross-resistance studies with other SBIs shows that the major gene (U/ppl-2 and U/ppl-1A) mutants are resistant to fenpropidin (Rf: 43 and 68), fenpropimorph (Rf: 261 and 283) and tridemorph (Rf: 9 and 10), but not to the inhibitors of C-14 demethylase (DMIs) and squalene epoxidase. The minor gene mutation U/ppl-1B codes a low-level of resistance (approximately 5—12-fold) to the above morpholine-type fungicides, but in contrast with the major gene mutations it increases 2–10 times the sensitivity to triazoles: triadimefon, triadimenol, propiconazole and flusilazole. Crosses between mutants carrying the U/ppl-genes with compatible isolates carrying the U/fpd, U/fpm or U/tdm mutations, which have been identified in previous genetic works for resistance to morpholine-type fungicides, yielded, with the exception of U/ppl-2 ×U/fpm-2 cross, a large number of recombinants with wild-type sensitivity, indicating that the mutant genes involved in these crosses, were not allelic. An additive gene effect was observed only between nonallelic minor genes U/ppl-1B and U/fpm-1B or U/tdm-1,2. Studies of the fitness of piperalin-resistant isolates showed that the reduced sensitivity of major gene mutants was not associated with changes on the phytopathogenic fitness determining characteristics, such as growth in liquid culture and pathogenicity on young corn plants. Conversely, the minor gene mutation U/ppl-1B appeared to be pleiotropic, having significantly adverse effects on the phytopathogenic fitness.