In in vivo experiments the sensitivity of 18 isolates of Phakopsora pachyrhizi from several regions of Brazil to IDM fungicides (cyproconazole, epoxiconazole and tebuconazole and an IQE (pyraclostrobin) were evaluated. The assessments were based on leaflet uredia density. Inhibitory concentration (IC50) and sensitivity reduction factor were determined for all fungicide x strain interactions. Tebuconazole sensitivity reduction was detected for most fungus isolates. In contrast, there was no fungicide shift in sensitivity of the fungus to pyraclostrobin. We conclude that the control failure of soybean rust found in some farms is due to the reduced sensitivity of the fungus to the IDM fungicide and that it remains sensitive to pyraclostrobin.
Soybean rust; fungitoxicity; resistance; triazol and strobilurin fungicides