Practical Winery
65 Mitchell Blvd, San Rafael, CA 94903
phone: 415-453-9700 ext 102
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Summer 2011
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
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
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.