Project shows extent of fungicide resistance
Botrytis, powdery and downy mildew are the top three diseases for economic impact in Australian viticulture. A high proportion of those costs are spent on control measures, including fungicides, to keep the diseases at bay. Unfortunately all three of these diseases have the ability to develop resistance to the fungicides commonly used to control them.
Testing for fungicide resistance has been the subject of a four year national project led by South Australian Research and Development Institute (SARDI), and funded by Wine Australia, that has recently concluded. Under the project, samples of these three diseases were taken from vineyards across Australia and tested against a range of commonly used registered fungicides. Isolates were tested for both phenotypic resistance (resistance seen in the vineyard) and genotypic resistance (mutations in the DNA that are known to cause resistance).
Results across Australia for powdery mildew revealed genotypic QoI resistance (activity group 11) was found in most regions. Genotypic analysis of powdery isolates revealed a resistant mutation for DMI’s (activity group 3) in 60% of isolates but the phenotypic resistance was not widespread.
For downy mildew the presence of metalaxyl resistance (activity group 4) was found in all states except SA, whilst the resistance mutation to QoI fungicides was also found in some regions. Botrytis isolates were found with resistance to fenhexamid, iprodione, boscalid and pyrimethanil but 54% of sites tested had no resistance detected.
More specifically for WA regions the results were:
The mutation for resistance to DMI fungicides and QoI fungicides were found in a small number of vineyards in Margaret River and the Great Southern.
None of the WA isolates of downy mildew were found to have the genetic mutation for QoI resistance. Metalaxyl resistance was detected for the first time on two sites in Margaret River.
A total of 371 isolates were tested from 47 sites across WA, a site could be different blocks within a vineyard. Of those, 326 isolates from 26 sites remain sensitive to all fungicides based on the thresholds levels for each of the fungicides. Of the remainder, 14 isolates from 11 sites were classified resistant to boscalid, 35 isolates from 15 vineyards to iprodione and 22 isolates from 14 sites to pyrimethanil. No isolates tested resistant to fenhexamid. Of concern, a total of six WA sites were considered resistant to three of the four modes of action tested.
What does this mean for growers?
The results from this study highlight differences in what is found in terms of genotypic resistance and that of phenotypic resistance, particularly for powdery mildew. In some cases the genotypic mutation has been found but the fungicide remains effective in the field. A new project has been developed to try and understand these differences to aid growers in spray selection whilst maintaining effectiveness.
It is concerning that resistance to these three diseases is occurring in WA and it is now an important time to revisit spray programs to determine whether a change is needed prior to the beginning of a new season.
Growers shouldn’t become complacent in regards to their fungicide regime and should routinely consult Croplife Australia’s resistance management information.
Now is an ideal opportunity to perform sprayer maintenance including nozzle replacement to improve the efficiency of your pesticide applications.
A list of registered fungicides permitted for use in Western Australia can be found in the Department of Agriculture and Food's Viticulture spray guide.
The full fungicide resistance final report can be found on the Wine Australia webpage.