Mar_Apr_PCC_2017_Web

Photo Credit: Jack Kelly Clark
Continued from Page 14
susceptible,” Jordan said.
The same is true after berry set,
Jordan continued. “That’s new,
young, juvenile tissue and powdery
mildew can attack that.”
There is less concern about pow-
dery mildew at the end of the season,
because of the hot summer days
and a lack of new growth makes it
less conducive to the disease—so
long as the powdery mildew was
successfully controlled early in the
season, Jordan said.
“Obviously, if you get a bad infection
and it sticks around, that’s a
different story,” Jordan said.
Susceptible Varieties
Some varieties, like Chardonnay,
are more susceptible than others,
and they are just naturally more
inclined to powdery mildew. But if
it’s a high pressure year, any variety
could have problems with the disease,
Jordan said.
“I always say if it’s a bad powdery
mildew year, it might be particularly
bad on those very susceptible varieties.
But ironically, those might
be the growers who have a stronger
control program in place,” Jordan
said.
“Even if you’re growing a variety
where it might not always be a problem,
in a bad year it could become
a problem for you very rapidly,”
Jordan said.
“If you know you’re in a bad year,
you do have a series of decisions you
make in addition to, ‘Oh I need to
go spray’,” Jordan said.
Illustration adapted from Grape Pest Management. Oakland: Univ. Calif. Agric. Nat. Res. Publ. 3343.
Powdery mildew on Thompson Seedless grapes.
Powdery mildew life cycle on grape.
Page 16 Progressive Crop Consultant March/April 2017
• What’s your canopy look like?
• What spray timing are you considering?
• What products are you using?
If the rains continue into the
spring this year, it could create
problems with powdery mildew for
all varieties, Jordan said.
Treatment
“Sulfur is still widely and incredibly
used throughout the grape
industry. We’ve known about it for
well over a century, probably closer
to two centuries now. It works,
there is no known resistance to it,
and frankly, it’s affordable,” Jordan
said.
There are issues with sulfur in
terms of restrictions from wineries
as well as environmental conditions,
Jordan said.
“Synthetics (fungicides) offer us
other tools for managing powdery
mildew,” Jordan said.
“It’s always important to consider
resistance management in a
powdery mildew control program
and look at rotating your modes of
action,” Jordan said.
There are documented cases of