Mar_Apr_PCC_2017_Web

GRAPES
Photo Credit: Jack Kelly Clark
Photo Credit: Jack Kelly Clark
Powdery Mildew Remains an Issue
for Grapes
Kathy Coatney
Editor
Powdery mildew is caused by the
fungus Uncinula necator, and it
has been a problem for California
grapes since commercial production
started over a century ago.
Powdery mildew is the most serious
and widespread disease in California
vineyards in terms of yield
loss and control. Even with consistent
control measures, in some
years, there can still be heavy losses,
especially in susceptible varieties.
Lindsay Jordan, University of
California Cooperative Extension
(UCCE) Viticulture Area Advisor
for Madera, Merced and Mariposa
Counties, said, “It (powdery mildew)
has been a serious and probably
the single largest disease control
Scarring on canes resulting from powdery mildew shoot infection.
issue we have for the whole state.
And that has been the case for a
long time, and it’s going to continue
to be.”
Climate
Powdery mildew thrives in temperatures
between 70 and 85 degrees,
Jordan said.
Vineyards that have high summer
temperatures may have reduced
problems with powdery mildew, but
vineyards with moderate temperatures
for extended periods of time,
even with good control programs,
may continue to have problems with
powdery mildew.
Weather conditions do effect
powdery mildew. “In general, the
coast is going to be a little more
subject to it (powdery mildew) than
the hotter inland regions,” Jordan
said.
“If you’re in a region that’s
cooler, you just have more hours in
that 70-85 degree range in the heat
of summer,” Jordan said, and these
areas are more susceptible because
these are ideal temperatures for the
fungus to thrive.
Powdery Mildew
Powdery mildew does need some
water or moisture at the beginning
of the season to trigger spore release,
Jordan said.
“It doesn’t technically need to
be a rain event,” Jordan continued,
adding it could be overhead irrigation
used during a frost, heavy fog,
or even mist.
Powdery mildew is unique in that
the grape fungus does not need water
to continue to propagate, Jordan
said.
Powdery mildew also prefers new
tissues. “That’s why when vines
have young shoot growth they’re
all very susceptible, because that
young, green, succulent tissue is
Powdery mildew, Erysiphe necator, on grape leaf.
Page 14 Progressive Crop Consultant March/April 2017
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