Sensitivity of Sclerotinia homoeocarpa to demethylation- inhibiting fungicides in Ontario, Canada, after a decade of use

In late 2003, nine populations of Sclerotinia homoeocarpa in Ontario Canada (seven of which had been previously sampled in early 1994, prior to the registration of sterol demethylation-inhibiting (DMI) fungicides for turf disease control in Canada) were sampled and tested for sensitivity to propiconazole. Four of the nine populations had not been treated with DMI fungicides during the intervening years, and isolates from these locations were sensitive to propiconazole (geometric mean EC 50 values of 0·005–0·012 μ g mL − 1 , compared with 0·005–0·008 μ g mL − 1 for the original 1994 populations). Among the five populations from 2003 that had been exposed to DMI fungicides, mean EC 50 values were significantly greater, ranging from 0·020 to 0·048 μ g mL − 1 . A significant correlation of determination was found between estimated number of fungicide applications and log EC 50 ( R 2 = 0·832, P = 0·0001), and the equation predicted that 42·3 applications of propiconazole would be needed to bring a sensitive population (EC 50 < 0·01 μ g mL − 1 ) to a resistant level (EC 50 > 0·10 μ g mL − 1 ). Fungicide sensitivity vs. duration of fungicide efficacy was also tested, and it was found that isolates with decreased sensitivity were able to more quickly overcome the inhibitory effects of fungicide application, reducing the duration of control from 3 weeks to 2 weeks.