Changes in Sensitivity of Sclerotinia homoeocarpa to the Demethylation Inhibitor Fungicide Propiconazole 20 Years After First Use
In 2013, samples of dollar spot disease from turfgrass were collected from 13 locations in southern Ontario. At least 50 Sclerotinia homoeocarpa isolates from each site were tested for fungicide sensitivity and growth inhibition (EC50) was calculated. The 2013 populations which had received many DMI treatments (25-78) over 20 years were much less sensitive (EC50=0.046 µg/mL) than those of the baseline 1994 study (0.008 µg/mL) or the 2003 study (0.030 µg/mL). To assess whether isolates with the most resistance would pose disease control issues, seven isolates of S. homoeocarpa (EC50 of 0.005-0.55 µg/mL) were inoculated onto three plots areas, and then plots were treated with low rate propiconazole. The least sensitive isolates were not inhibited by this low rate. We projected that over 65 DMI fungicide applications are needed to shift population EC50 values to over 0.1 µg/mL, which might result in reduced levels of disease control in the field.