Cross‐resistance of <i>Clarireedia jacksonii</i> to demethylation‐inhibiting fungicides

Abstract Dollar spot, caused by Clarireedia jacksonii Salgado, is the most common turf disease in Eastern North America. It is often managed by repeat applications of demethylation‐inhibiting (DMI) fungicides, and decreased sensitivity to the DMI, propiconazole, has been reported. Cross‐resistance, where an organism shows resistance to multiple pesticides, was evaluated in lab studies for C. jacksonii starting almost 30 years ago with four DMI fungicides available at that time. The purpose of this study was to compare sensitivity among 20 isolates of C. jacksonii (low‐to‐high resistance) to 12 DMI fungicides. EC 50 values (effective concentration for 50% inhibition of hyphal growth) were generated for each fungicide by isolate combination, and log 10 transformations of these values were subjected to pairwise correlation analysis, giving R ‐values ranging from 0.60 to 0.99 (all p ≤ 0.05). This suggested that there is significant cross‐resistance in C. jacksonii among the 12 DMI fungicides tested, ranging from partial (mefentrifluconazole and epoxiconazole) to almost complete.