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When to start Powdery Mildew control programs?

Ed Hellman
North Willamette Research & Extension Center
Oregon State University


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 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.