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

Isolates of R. secalis were collected yearly from the Ruens area of the Wes tern Cape during the 1993-1995 growing seasons. These isolates were evaluat ed in vitro to determine sensitivity to triazole fungicides (triadimenol, t ebuconazole, flusilazole and propiconazole). The sensitivity fluctuated but in 1995 isolates were significantly less sensitive towards triadimenol tha n in the previous two years. In a second experiment, isolates collected fro m two fields with a 5-6 year history of triadimenol seed treatments and teb uconazole applications were evaluated for their fungicide sensitivity. A si gnificant positive correlation was observed between tebuconazole and triadi menol sensitivity among R. secalis isolates from these fields. However, suc h a correlation was not found within the R. secalis population collected du ring 1993-1995 where shorter crop rotation patterns and a range of fungicid es were applied. In a third experiment, the fungicide sensitivity of local R. secalis isolates was evaluated towards two new triazole fungicides, name ly bromuconazole and triticonazole. Correlation coefficients observed betwe en these new triazoles and those previously applied in South Africa were no t significantly positive. The lack of significant cross-resistance has impo rtant practical implications for management of fungicide resistance. (C) 20 01 Elsevier Science Ltd. All rights reserved.