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Rhonda Smith
University of California Cooperative Extension, Sonoma County
rape clusters were damaged
by weather conditions
August 23-25, 2010 that at
first seemed to be a typical
summer heat spike, but turned
out to be quite different. High temperatures
coupled with low relative
humidity provided conditions that
caused more fruit damage then in
“normal” heat spikes.
Not every vineyard had heat-damaged
fruit, and in those vineyards
that did, sunburn and/or desiccation
varied considerably. Fortunately, such
weather conditions are rare in the
North Coast region of California.
There was usually — but not consistently
— a clear association between cluster exposure
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and severity of damage.
For most growers, fruit in blocks
with vine rows oriented north-south
suffered more damage than fruit
in other blocks. In general, clusters
exposed to direct light either in late
morning or afternoon were partially
or completely damaged.
Damage on the exposed surface of
clusters was obvious within 24 hours
as berries collapsed and turned offcolor.
About 10 days later, damage to
the rachis became apparent in clusters
that initially appeared undamaged, at
which time pedicles and portions of
the rachis were dead.
The severity of damage varied
by site but not consistently. Specific
blocks that received ample water
in spite of the cool growing season
were not impacted whereas other
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blocks were. Physical characteristics
of the site including elevation,
aspect, height of fruit above ground,
and variety contributed to the variability
in observed damage.
A few growers turned on sprinklers
during the hottest period of the
day to initiate evaporative cooling
inside the vineyard to reduce or prevent
fruit damage.
Disease pressure and canopy
management
Mild temperatures in spring and
early summer coupled with dense, wet
canopies increased the incidence of
powdery mildew and Botrytis infections.
As a result, in late July and early
August, some growers opened up vine
canopies more than usual to allow
more light (thus heat) into the fruit
zone and reduce disease severity.
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