Baseline sensitivity to three triazole fungicides in Pyrenophora tritici-repentis

Pyrenophora tritici-repentis (anamorph: Drechslera tritici-repentis) causes yellow spot of wheat. The disease can be responsible for yield losses of 30-50% and, when severe, requires management with fungicide. Forty-five single conidium isolates collected from the northern wheatbelt of Western Australia were tested for sensitivity to three triazole fungicides in culture. At 0.2 mu g/mL epoxiconazole and 0.4 mu g/mL propiconazole and tebuconazole, the mean relative diameter of cultures was close to 50%, indicating that epoxiconazole had about twice the in vitro activity of propiconazole or tebuconazole. Isolates exhibited a significant fungicide x rate interaction such that some isolates differed in response to propiconazole compared with epoxiconazole or tebuconazole. Sensitivity among 32 isolates collected from commercial wheat crops over the period 2001-03 was not significantly different from sensitivity among 13 isolates collected in 1986-87 when fungicide use was minimal. The observed EC50 values (concentration of fungicide required to cause 50% inhibition of growth of isolates) provide a basis to monitor potential changes in sensitivity to azole fungicides.