A practical field assay to detect demethylation inhibitor fungicide resistance in <i>Clarireedia jacksonii</i>

Abstract Dollar spot, caused by Clarireedia jacksonii , is one of the most common turfgrass diseases, and requires abundant fungicide use for control. Currently, identification resistance to demethylation‐inhibitor fungicides requires lab facilities for isolation and sensitivity assessment. We have developed a selective medium containing 0.5 µg/mL propiconazole, antibiotics, and tartaric acid capable of identifying propiconazole‐resistant isolates from field samples. Tests with leaves from inoculated turfgrass plots showed that from ∼2000 samples, resistant or sensitive isolates could be identified accurately 96% of the time. Field tests by both end users (turf managers) and researchers were conducted at 20 locations across Southern Ontario involving over 200 samples. Clarireedia jacksonii was found at 13 of the sampled locations on non‐fungicidal media, and at 12 of these locations, propiconazole‐resistant isolates were found using amended media. The proportion of resistant isolates found ranged from 0% to 100% at the locations. This field assay can be developed into a field kit for turf managers, and this proof of concept will assist in the development of similar assays for other pathogens and on other crops.