Variability in the Sensitivity of Rhizoctonia solani Anastomosis Groups to Fungicides

Tolclofos-methyl, iprodione and cyproconazole, among the eleven fungicides tested in vitro, gave consistently strong inhibition against all ten anastomosis groups (AGs) of Rhizoctonia solani. Carboxin, furmecyclox, thiabendazole, fenpropimorph and vinclozolin also inhibited all AGs but with wide variations in toxicity levels (EC90 values). Pencycuron showed strong activity against four AGs but was ineffective against the other six AGs. Generally, R. solani AGs were insensitive to fenarimol and imazalil. Tolclofos-methyl strongly inhibited 23 AG2-1 and 20 AG4 rapeseed/canola R. solani isolates from different locations in Saskatchewan, Alberta and Manitoba. The same isolates were also sensitive to iprodione, cyproconazole and carboxin. All AG4 canola isolates were insensitive to pencycuron (EC90 > 500 mg/l) while AG2-1 isolates showed highly variable levels of sensitivity with EC90s ranging from 0.5 to 220 mg/l. Tolclofos-methyl, applied to Brassica napus (canola) cv. Westar seed at 1 g a.i./kg, provided 75—100 % control of seedling damping-off in pots infested with AG2-1 or AG4 isolates. In parallel experiments, pencycuron (1 g a.i./kg seed) failed to control damping-off by AG4 canola isolates and gave variable disease control against AG2-1 isolates.