Fungicide resistance in Australian viticulture

Powdery mildew, Botrytis bunch rot and downy mildew are the three most economically important diseases in Australian vineyards. To determine the· incidence and severity of fungicide resistance in Australia, samples of all three diseases were collected from vineyards in the main viticultural regions of Australia and tested against a range of commonly used fungicides. Leaf disc bioassays were used to determine the EC511 values ofErysiphe necator (powdery mildew) for sensitivity to pyraclostrobin, pencona ole, myclobutanil and tetraconazole, and of Plasmopara viticola (downy mildew) to rnetalaxyl M, m,mdipropamid and pyraclastrobin. EC511 values, minimum inhibitory concentra­tions and discriminatory dose values were established for Botrytis cinerea isolates using a microtitre assay system and mycelial growth assay against fenhexamid, iprodione, pyrimethanil, boscalid, tebuconazole, azoxystrobin and jludioxonil. Representative isolates of each pathogen were genotyped for the presence of known mutations conferring resistance. The mutation linked with quinone outside inhibitor (Qol) fungicide resistance, G 143A, was found in isolates of powdery mildew from moist viticultural regions. Vt'hile phenotypic resistance of E. necator to the demethylation inhibitorfungiddes (DMis) was not widespread, the mutation linked with resistance, YI36F, was present in approximately 60% of the powdery mildew isolates. Populations of P. viticola resistant to metalaxyl have been confirmed in all states except South Australia, ,md the allele conferring resistance to Qol has been detected. B. cinerea resistant populations to fenhexamid, iprodione, boscalid and pyrimethanil were detected. While 54% of sites had 110 resistance deteded, two sites had populations resistant to all four fungicides. The results of the testing so far has confirmed the presence of resistant populations of these three pathogens to many fungicides throughout Australia. However, more work is needed to confirm how these results relate to the potential for field failure.