Baseline sensitivity of Rhizoctonia solani to four DMI fungicides

A set of 78 Rhizoctonia solani isolates from Shanghai, China, were examined for their sensitivity in vitro to four demethylase inhibitor (DMI) fungicides, epoxiconazole, difenoconazole, hexaconazole, and tebuconazole, by the mycelial growth rate method. The isolates were very sensitive to the four DMI fungicides. The ranges and means of the EC₅₀ values were 0.004-0.045 and 0.014 ± 0.007 μg ml^-1 for epoxiconazole, 0-0.103 and 0.010 ± 0.016 μg ml^-1 for difenoconazole, 0.001-0.078 and 0.011 ± 0.013 μg ml^-1 for hexaconazole, and 0.001-0.210 and 0.034 ± 0.035 μg ml^-1 for tebuconazole, respectively. The frequency distribution of the EC₅₀ values showed a normal curve for epoxiconazole and an approximately unimodal curve for difenoconazole, hexaconazole, and tebuconazole. There was no correlation between the sensitivities of the R. solani population to any two of the four fungicides (r < 0.3). In field trials, in 2015 and 2016, the control efficacies of hexaconzole, epoxiconazole, and tebuconazole were >90.0% at 75 g.a.i. ha^-1 , 80.0%-90.0% at 75 g.a.i. ha^-1 and 80.0%-90.0% at 97 g.a.i. ha^-1 , respectively; these values are significantly higher (p < 0.05) than those obtained in this study for difenoconazole (65.0%-70.0% at 80 g.a.i. ha^-1 ) and jinggangmycin (68.0%-71.0% at 150 g.a.i. ha^-1 ). These results show that sensitivity data can be used to monitor and detect any variations in the sensitivity of the R. solani population to these four DMI fungicides, and demonstrate that epoxiconazole, hexaconazole, and tebuconazole are likely to be good alternatives to jinggangmycin for controlling rice sheath blight.