Resistance And Risk Assessment Of Rhizoctonia Cerealis To Thifluzamide And Fludioxonil

Wheat sharp eyespot is a serious soil-borne fungal disease mainly caused by Rhizoctonia cerealis (AG-DI anastomosis group), and occurs in the wheat growing areas worldwide. Since the late 1970s, with the turnover of wheat varieties and the promotion of high yield cultivation (such as sowing, high density, high-fat, etc.), this disease has occurred in winter wheat region in China and has become an important disease of the Yangtze River Basin and the Huang Huai plain area. Currently, the resistance of wheat varieties to wheat sharp eyespot is generally poor, and chemical control has become an important means to control the disease.Our research investigated and compared the inhibitory activity of eight fungicides against Rhizoctonia cerealis. The study revealed that eight fungicides had good inhibitory activity to Rhizoctonia cerealis. Among which fludioxonil had the strongest inhibition activity and the average value of EC50 was 0.0215 μg/mL. The inhibitory activity of validamycin was the weakest, and the average value of EC50 was 0.4238μg/mL. The inhibitory activity of 8 fungicides on Rhizoctonia cerealis was followed by:fludioxonil> penflufen> thifluzamide> tebuconazole> Difenoconazole> Bixafen> boscalid> validamycin.The sensitivity of Rhizoctonia cerealis to thifluzamide was monitored in this study. All pathogens were isolated from Jiangsu Province in 1984,2001,2008,2010 and 2013 years. The resistance risk was assessed by fungicide timing and UV irradiating. The results showed that the EC50 of Rhizoctonia cerealis in 1984-2001 and 2008-2013 to thifluzamide ranged 0.0145-0.0764 μg/mL and 0.0178-0.0889 μg/mL respectively, and the sensitivity of different isolates collected in these years to thifluzamide had no significant difference. Ten strains of Rhizoctonia cerealis were subcultured on sublethal dose (0.6 μg/mL) of thifluzamide, no resistant mutant was obtained after 8 generations. Three strains (R1312, R1322 and R1384) isolated in 2013 was selected to make the UV mutagenes, and 2 mutants resistant to thifluzamide were obtained. The resistance factors of the two strains were 6 and 33. Compared with the parent strain, the mycelial growth rate of resistant mutants did not change, but the capacity of producing sclerotia declined. The study suggests that the risk of resistance of Rhizoctonia cerealis to thifluzamide was low to moderate.The resistance risk of Rhizoctonia cerealis to fludioxonil was also assessed through fungicide timing Fifteen mutants resistant to fludioxonil were obtained from 10 strains. Among which,5 mutants were highly resistant to fludioxonil, the resistance factors were higher than 80; 10 mutants were low resistant, and the resistance factors were 5-12. After 10 generations of successive cultures in the PSA without fungicides contained, the EC50 of high resistant mutants droped, but the resistant levels were still high. Cross resistance of high resistant mutants to iprodione was found. These mutants were slight cross resistance to validamycin. No cross resistance between fludioxonil and thifluzamide, tebuconazole was found compared with the parent strain, the hyphae growth of resistant mutants was lower, and the capacity of producing sclerotia and the virulence decreased. The resistant mutants were more sensitive to high salt and sugar. The study suggests that the resistance risk of Rhizoctonia cerealis to fludioxonil was low to moderate.