Nick Dokoozlian, University of California viticulturist at the Kearney Agricultural Center, Parlier, said he saw a 20 to 25 percent reduction in berry weight and diameter in his table grape research plots farmed the same as in 2000.
High temperatures in the southern San Joaquin Valley were prominent at the critical first three weeks after fruit set. Weather records show temperatures 100 degrees or higher, with duration from three to seven hours, occurred eight times during that period.
In essence, high temperatures cause vines to consume carbon supplies for survival rather than for developing new cells, he said.
Berry color was also affected by temperature. Although the processes are known, the causes ultimately rest with Mother Nature.
Plant growth regulators can make differences in berry size and color, but they really don’t mitigate the effects of temperature extremes, Dokoozlian said during the recent SJV Table Grape Seminar in Visalia.
After reviewing his data from the hot spells in 2001, he said, "if you have 100 degrees for five hours or more, you will get severe reductions in berry growth. And any time the temperature is above 95 degrees, you can expect some inhibition of berry growth. The warmer the temperature, the less time is required for the exposure to take effect."
Table grape growers also saw shortcomings in color in 2001, and Dokoozlian said response to heat and light is variety specific. A group of pigments govern berry color, and the proportions differ from one variety to the next.
That accounts for the ability of Tokay, once a principal table variety, to develop its intense, flame color in the cooler nights of the Lodi district, while Tokay grown in the warmer Delano district fails to develop color.
Dokoozlian said the improved color on second-crop grapes is because the later conditions are more conducive to pigment development.
An isolated problem with "pink berry" on white table grapes, including Thompson Seedless, Italia, and newer varieties with a Muscat heritage, was also blamed on heat stress in 2001.
An imbalance in pigments can cause red or black berries to lack color, but in this case, the imbalance brings on color where it is not wanted.
Dokoozlian said the genes for producing color are carried in the white varieties but typically are not expressed. However, heat stress after veraison can activate the color mechanism in the plant. The effects usually occur after several weeks of warmer-than-average temperatures.
Accumulations of pigments triggered by heat stress also produced surface browning seen on some Thompson Seedless in the 2001 season. Some growers reported pitting on berry surfaces, and Dokoozlian attributed that to build-ups of phenolics, the same enzymes that cause raisining inside and on the surface of the fruit during hot weather.