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Both the fruit and foliage show mildew symptoms, indicating that the fungicide product, timing or rate was not effective in controlling grape powdery mildew. <b>(Good Fruit Grower file)</b>

Both the fruit and foliage show mildew symptoms, indicating that the fungicide product, timing or rate was not effective in controlling grape powdery mildew. (Good Fruit Grower file)

It took just two years for the powdery mildew that New York grape growers were trying to kill to develop resistance to their sprays.

In 1997, growers began spraying azoxystrobin. They noticed resistance in 1999.

Fortunately for Washington growers, they don’t need to spray as often in their drier climate, and they can learn from New York, said Michelle Moyer, viticulture extension specialist for Washington State University.

All the same, fungicide resistance is a real threat to Washington’s grape industries, but growers can do things to manage it, Moyer told them in November at the Washington State Grape Society annual meeting in Grandview.

Here are some of her tips:

Dr. Michelle Moyer, Washington State University

Michelle Moyer

View FRAC as a friend: The organization, the Fungicide Resistance Action Committee, is a coalition of chemical manufacturers and university scientists that advise how to use products in a way that minimizes resistance pressure.

Companies want their products to work for you. If they don’t, you won’t buy them. FRAC organizes fungicides into groups based on their modes of action to help growers rotate those products.

Read up. Moyer recommends two websites that discuss fungicides and resistance: Pacific Northwest Pest Management Handbooks (pnwhandbooks.org/plantdisease/pesticide-articles) and FRAC (frac.info/resistance-overview).

Keep the canopy open. Use irrigation management, pruning and other techniques to ensure your canopy remains open. Sunlight in particular is a natural killer of powdery mildew.

Spray at the right time: Spray early in the season, before veraison. Avoid using synthetic materials if powdery mildew is already visible in the field.

This includes late-season botryticides that also have activity against powdery mildew, as that timing might select for mildew resistance.

Spray in the right place. Hit the canopy, not above or below it. That wastes chemicals and leads to resistance due to insufficient product hitting the plant.

Spray the right way. Select days with low wind and spray at full rates. Wind scatters the product and reduces the rate, setting you up for resistance.

Tank mix fungicides: Use products with different FRAC groups, or modes of action, in the same spray. Moyer advises mixing products that are at high risk of resistance development with contact products, such as sulfur. “Sulfur’s cheap, and it’s easy to add into your spray rotation.”

Sulfur also doesn’t prompt resistance. Never mix sulfur and oil. Some products come pre-mixed.

Rotate fungicides: Again, rotate chemicals with different FRAC groups. Don’t just switch trade names. Rotate within the season, not between seasons. That is, don’t spray one chemical four times one year, then switch to another the next year.

If you eliminate one mode of action, make sure you have others for replacement.

Having too few options in your product toolbox will put resistance-development pressure on those remaining products. “The key is rotation. Don’t overuse the same product repeatedly. We want to mix these things up.”

Don’t panic. Resistance is not the end of the world. Yes, it has occurred in many grape growing areas, where they spray more often than in Washington. Washington growers have the benefit of learning from them. “But it is something we need to think about and not ignore,” Moyer said.

If these tips aren’t enough, remember the R.U.L.E.S. of fungicide applications, taken straight from the Pacific Northwest Pest Management Handbooks:

R = Rotate or mix fungicides of different groups.
U = Use labeled rates and at times of critical disease control.
L = Limit total number of applications of any fungicide group in a given growing season.
E = Educate yourself about fungicide groups and resistance management tactics.
S = Select fungicides that have multiple sites of activity. •

– by Ross Courtney