In vitro sensitivity assessment of late season soybean pathogens to fungicide mixtures

Late season diseases cause yield reductions to soybean grown worldwide. In Argentina, fungicide mixtures composed of quinone outside inhibitors (QoIs) and demethylation inhibitors (DMIs), and the newly introduced succinate dehydrogenase inhibitors (SDHIs), have been effective in managing these diseases. Nevertheless, the risk of selecting strains with resistance to these classes of fungicides is considered to be high. This preliminary study was carried out to determine in vitro sensitivities as determined by the effective concentration that inhibited 50% of radial mycelial growth (EC50 values) of Cercospora kikuchii, Colletotrichum truncatum and Phomopsis phaseoli to selected QoI and DMI fungicide mixtures. The results indicated that EC50 values ranged from 0.0065 to 0.0402 μg/ml for C. kikuchii, from 0.0344 to 0.1744 μg/ml for C. truncatum and from 0.0001 to 0.1974 μg/ml for P. phaseoli. To better study the possible resistance against these pathogens, future tests should consider several isolates for each pathogen from different production areas and different fungicide active ingredients.