Grape growers urged to review fungicide strategies

Grape growers should consider adjusting their fungicide strategies this season to effectively protect their crops from powdery mildew, according to an international specialist in grape pathology.

Professor Wayne Wilcox's advice reflects growing concerns that soon strobilurin fungicides may not be able to be relied on to provide effective powdery mildew control when applied by themselves, after resistance was reported on several vineyards across Australia last season.

Crop Care and Nufarm invited Professor Wilcox, from Cornell University, to speak at a series of seminars in Margaret River and Busselton recently in a bid to help growers and advisers adjust to the changing conditions.

Professor Wilcox said powdery and downy mildew originated from North America where wet weather in many grape growing areas provided ideal conditions.

He said fungicide resistance was a growing problem that all growers had to learn to live with by understanding and managing it.

He said there were two types of fungicide resistance in grape vines.

"People who have had experience with DMI resistance need to be aware they cannot use the same strategies for strobilurin resistance," he said.

One control strategy for DMIs is to limit the number of individuals that survive the spray by increasing rates or switching to a more powerful product in the same group.

"In contrast, when a fungal colony is resistant to one strobilurin, it is immune to all rates of all strobilurins," Professor Wilcox said.

"The only way you can control it is with a different type of fungicide."

While strobilurin resistance is only starting to emerge in Australia, it has been around since 2002 in the United States.

Professor Wilcox said resistance management strategies should be based on minimising the initial selection of resistant individuals and severely limiting the reproduction of any that do survive.

"One of the strategies we use in the US is to limit the number of strobilurin sprays to two a year," he said.

"The other is to avoid spraying strobilurin fungicides once you see more than a trace level of the disease active in the vineyard."

He explained that where there were large populations of fungus, even if only one per cent were resistant, it still left a large pool of survivors that could multiply out of control.

Professor Wilcox said in situations where a low level of strobilurin resistance was known or suspected, growers could tank-mix strobilurin fungicides with another chemical that would provide effective control of powdery mildew.

"In such cases, the strobilurin will continue to provide control of the susceptible proportion of the population, while the mixing partner will control those that are insensitive," he said.

"In my experience, growers who routinely tank-mix encounter far fewer resistance problems.

"Resistance is common enough in our region that virtually all growers tank mix now."