On Aug. 23-25, 2010, clusters were damaged by weather conditions 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.

There was usually – but not consistently – a clear association of cluster exposure with severity of damage. For most growers, fruit in blocks with north-south rows suffered more damage than in other blocks. In general, clusters exposed to direct light either in late morning or afternoon were partially or completely damaged. Damage on exposed faces of clusters was obvious within 24 hours as berries collapsed and turned off-color. 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.

Some exceptions to this scenario were blocks that received ample water because they are in well drained sites or shallow clay soil and managers kept to the normal water application schedule in spite of the cool season. A few growers turned on sprinklers during the warmest periods of the day to initiate evaporative cooling and reduce temperatures inside the vineyard to reduce heat damage.

Disease pressure and canopy management

Mild temperatures 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. Beginning August 23 when temperature climbed, many of these exposed clusters were damaged. Affected vineyards were not limited to those with fully exposed clusters as standard leaf removal on the morning sun side of rows results in cluster surfaces with a range of exposures from direct (full) to indirect (diffuse) light.