Differential sensitivity among Puccinia striiformis f. sp. tritici isolates to propiconazole and pyraclostrobin fungicides

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a destructive disease of wheat in the USA and fungicides have been used to manage the disease. However, it is not clear whether Pst has developed resistance to the commonly used fungicides. In this study, historical and recent Pst isolates were tested for sensitivity to Tilt, a propiconazole demethylation inhibitor fungicide, and Headline, a pyraclostrobin quinone outside inhibitor fungicide. Both Tilt and Headline at their labelled full concentration completely prevented urediniospore germination of all tested isolates. However, differences in germination rate among Pst isolates were observed at reduced fungicide concentrations. In the infection tests of both seedlings and adult plants, the full labelled concentration of either fungicide applied at the same day as inoculation with Pst completely prevented sporulation, but differences in rust sporulation or disease severity were observed among isolates at some reduced fungicide concentrations, or at the same full concentration applied either one day before or 3 or 7 days after inoculation. Thus, the fungicides are still effective for managing stripe rust at the full concentrations, but differences in sensitivity exist among Pst isolates. The results provide the technique and baseline for continually monitoring Pst populations for changes in fungicide sensitivity.