Understanding fungicide resistance in powdery and downy mildew

Abstract

Investigations over the last four years into the incidence of fungicide resistance in powdery mildew, botrytis bunch rot and downy mildew have shown that with some fungicides resistance is widespread (e.g. QoI* in powdery mildew), whereas others are less common (e.g. DMI** in powdery mildew) or occur rarely (e.g. mandipropamid in downy mildew). Some populations of Botrytis are now resistant to more than one fungicide group. Further research is vital to relate the phenotypic and genotypic laboratory results to field performance. For effective resistance management it is critical that growers adhere to the recommended strategies and minimise exposure to ‘at risk’ fungicides.

* QoI – Quinone outside inhibitor
** DMI – Demethylation inhibitor

Summary

Botrytis bunch rot, powdery mildew and downy mildew caused by Botrytis cinerea, Erysiphe necator and Plasmopara viticola respectively, are the top three diseases in Australian vineyards, with high economic impact from management costs and reduced income. Fungicide resistant populations add to the cost due to reduced fungicide efficacy and failure of spray programs to manage disease.

This project utilised a network of collaborators in research organisations, agrochemical companies and the viticulture industry to collect samples of all three diseases from vineyards in the main viticultural regions of Australia. The samples were tested against a range of commonly used fungicides to determine the incidence and severity of fungicide resistance in Australian vineyards. The results of the testing so far have confirmed the presence of resistant populations of these three pathogens to many fungicides throughout Australia.

Leaf disc assays were used to phenotypically test isolates of E. necator for sensitivity to pyraclostrobin, penconazole, myclobutanil and tetraconazole, and P. viticola to metalaxyl M, mandipropamid and pyraclostrobin. Botrytis cinerea was tested by mycelial growth assay for sensitivity to fenhexamid, iprodione, pyrimethanil and boscalid. Representative samples were genotyped for the presence of known mutations conferring resistance.

Results showed that QoI resistance in powdery mildew was found in most viticultural regions. While phenotypic resistance of E. necator to the DMIs was not widespread, the Y136F allele that confers resistance to DMIs was present in over 60% of the isolates. Populations of P. viticola resistant to metalaxyl have been confirmed in all states except South Australia, and the allele conferring resistance to QoIs has been detected. B. cinerea resistant populations to fenhexamid, iprodione, boscalid and pyrimethanil were detected. While one sites had populations resistant to all four fungicides, 54% of sites had no resistance detected and the majority were resistant to only one fungicide. This suggests that a field management approach involving fungicides with different modes of action can contribute to the mitigation of resistance.