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

Mutants of Ustilago maydis (DC.) Corda, resistant to the piperidine fungicide fenpropidin, were isolated in a mutation frequency of 3.2 ´ 10±5, after UV-irradiation and selection on media containing 75 lg/ml fenpropidin. Genetic analysis with 15 such mutant isolates resulted in the identi®cation of two unlinked chromosomal loci, U/fpd-1 and U/fpd-2. The U/fpd mutations are responsible for moderate resistance levels to fenpropidin (Rf: 42±56 or 15 based on e€ective concentration causing a 50% reduction in the growth rate (EC50) or minimal inhibitory concentration (MIC) values, respectively). Haploid strains carrying both U/fpd mutations do not exhibit higher levels of resistance to fenpropidin, indicating no additivity of gene e€ect between non-allelic genes. Cross- resistance studies with other Sterol Biosynthesis inhibi- tors (SBIs) showed that the U/fpd-mutant isolates exhibited a positive cross-resistance to the piperidine pip- eralin and to the related morpholine fungicides fenpro- pimorph and tridemorph, but not to the inhibitors of C-14 demethylase and squalene epoxidase. Crosses between mutants carrying the U/fpd-genes with compatible iso- lates carrying the U/fpm or U/tdm mutations, which have been identi®ed in previous genetic studies for resistance to morpholine fungicides fenpropimorph and tridemorph, yielded, with the exception of U/fpd-2 ´ U/fpm-2 crosses, a large number of recombinants with wild-type sensitiv- ity, indicating that the mutant genes involved were not allelic. Analysis of progeny from crosses between U/fpd-2 and U/fpm-2 mutants yielded no recombinants with wild- type sensitivity, but a 1 : 1 progeny segregation was observed at the MIC for the U/fpd-2 isolates, indicating that these genes are alleles of the same locus. A study of the ®tness of fenpropidin-resistant isolates showed that the U/fpd mutations do not a€ect the phytopathogenic ®tness-determining characteristics such as growth in liquid culture and pathogenicity on young corn plants.