DMI sensitivity and cross-resistance patterns of Rhynchosporium secalis isolates from South Africa

Abstract

Isolates of R. secalis were collected yearly from the Rûens area of the Western Cape during the 1993–1995 growing seasons. These isolates were evaluated in vitro to determine sensitivity to triazole fungicides (triadimenol, tebuconazole, flusilazole and propiconazole). The sensitivity fluctuated but in 1995 isolates were significantly less sensitive towards triadimenol than in the previous two years. In a second experiment, isolates collected from two fields with a 5–6 year history of triadimenol seed treatments and tebuconazole applications were evaluated for their fungicide sensitivity. A significant positive correlation was observed between tebuconazole and triadimenol sensitivity among R. secalis isolates from these fields. However, such a correlation was not found within the R. secalis population collected during 1993–1995 where shorter crop rotation patterns and a range of fungicides were applied. In a third experiment, the fungicide sensitivity of local R. secalis isolates was evaluated towards two new triazole fungicides, namely bromuconazole and triticonazole. Correlation coefficients observed between these new triazoles and those previously applied in South Africa were not significantly positive. The lack of significant cross-resistance has important practical implications for management of fungicide resistance.