Sensitivity variation of Phakopsora pachyrhizipopulations to demethylation inhibitor fungicides in Brazil

During 2008/09, 2009/10 and 2010/11 growing seasons, the sensitivity of the fungus Phakopsora pachyrhizi to the fungicides tebuconazole, cyproconazole metconazole and prothioconazole (demethylation inhibitors, DMIs) was monitored. Detached soybean leaves were treated with those fungicides at concentrations varying from zero to 32 mg L^-1 (tebuconazole, cyproconazole and metconazole) and from zero to 8 mg L^-1(prothioconazole) and were inoculated with P. pachyrhizi spores collected from different Brazilian soybean producing regions. Inoculated leaves were incubated in Petri plates containing moistened paper towel at 23° C ^± 2° C, and the disease severity was estimated at 15 days after inoculation. The effective concentrations to reduce the disease severity by 50% (EC₅₀) were estimated. The fungicide prothioconazole presented the highest intrinsic activity and its EC₅₀ ranged from 0.000001 mg L^-1 to 0.39 mg L^-1. EC₅₀ values ranged from 0.001 mg L^-1 to 1.49 mg L^-1 for tebuconazole, 0.001 mg L^-1 to 3.27 mg L^-1 for cyproconazole and 0.004 mg L^-1 to 3.89 mg L^-1 for metconazole. EC₅₀medians of all tested fungicides were lower than 0.5 mg L^-1 in the three growing seasons. The correlations (r) among the EC₅₀ values of the four fungicides were significant (p<0.05), indicating cross-resistance among the products. The EC₅₀ variation in the monitoring shows coexistence of populations with different levels of sensitivity to DMIs in the field.

Asian soybean rust; tebuconazole; cyproconazole; metconazole; prothioconazole; triazoles