Dynamics of <i>Cyp51</i> mutants in the <i>Puccinia striiformis</i> populations in the United States

Puccinia striiformis f. sp. tritici (Pst) and P. striiformis f. sp. hordei (Psh) cause stripe rust epidemics on wheat and barley, respectively. Demethylation inhibitor (DMI) fungicides have been used for decades in managing stripe rust in the United States, but the pathogen tolerance was not clear. To determine the dynamics of DMI fungicide targeting gene Cyp51 mutants, Pst isolates collected from 1968 to 2021 and Psh isolates from 1993 to 2021 were tested using a Kompetitive Allele Specific PCR (KASP) marker for the Y134F point mutation in the Cyp51 gene. The mutant allele was found in the Pst and Psh populations as early as in 1968 and 1993, respectively, and the mutant frequencies fluctuated from year to year. The KASP test of the Pst-infected leaf samples from fungicide-testing plots in 2024 revealed that the application of DMI fungicide Tilt increases the mutant frequency. The urediniospore germination tests with 22 selected isolates at different concentrations of Tilt showed that the mean EC50 value of the homozygous mutant isolates was 2.7 times higher than that of the wild type isolates, but those of the heterozygous and wild type isolates were not significantly different from each other. The results indicate that the KASP marker is useful in monitoring the DMI fungicide targeting gene mutants and DMI fungicides are likely less effective in controlling the homozygous mutant population than the wild type and heterozygous populations. The information is useful for managing stripe rust using diverse fungicides as well as growing resistant cultivars.