May 8, 2000

When to start Powdery Mildew control programs?

Jay Pscheidt
Extension Plant Pathology Specialist
Oregon State University

When do we start our powdery mildew spray programs? That is the number one question for me these days. Do we use forecasting programs, plant phenology, weather or just whenever it suits us? Do we start with something easy like sulfur or start slamming it hard right away with things like DMI or strobilurin type products?

Research in the last few years has told us that many of the forecasting programs are rather conservative and start too early. Waiting until shoot growth is about 6 to 8 inches (EL stage 10-12) worked just as well. Starting with sulfur and then adjusting to modern products at prebloom (EL 17) also worked well.

That was for a normal year. But we know that in some years powdery mildew can come thundering in early, even before bloom. Will this be one of those years? There are a few indications that it might be. The weather has been uncharacteristically dry by western Oregon standards. Our early, heavy spring rains are usually detrimental to powdery mildew spores. They like it dryer and humid like what we have been experiencing. I have already seen a lot of powdery mildew in other cropping systems such as apples and hops.

Dr. Walt Mahaffee's program has already detected events where spores of powdery mildew have been released. The later part of March and early April were rather dry but storms moved back in on April 12, 14 and 21. Grape leaves exposed during these times, then incubated in the lab, developed powdery mildew. This was right around grape budbreak. Using this method in past years we did not detect powdery mildew until much later. Cool and rainy weather in the vineyard will slow development of any colonies for quite awhile. But once the weather turns from cool spring rains to dry summer heat, powdery mildew could rapidly become a problem.

What should we do about it all? First, don't overreact. An earlier start to the spray season may be needed. You might focus only on your traditional hot spots for now. Sulfur or something else? The first powdery mildew colonies are usually well protected near the trunk on the underside of small curled leaves. A hard place for sulfur to reach but easier for locally systemic compounds. Base your programs on the history of powdery mildew in your vineyard, amount of disease observed last year and current weather conditions. Lets hope the indications are wrong and that we have a normal year.

This Information provided (on May 8) by Dr. Jay W. Pscheidt, OSU Extension Plant Pathology Specialist, (E-Mail) pscheidj@bcc.orst.edu, (voice) 541-737-3472, (mobile) 541-740-6621 (FAX) 541-737-2412.