Warmer minimum temperatures in Washington State and other northern states could translate into less color development for red grape varieties and other crops like Red Delicious apples, said Mercy Olmstead, Washington State University Extension viticulture specialist.
Washington’s cool nights have been conducive to producing colored and elongated fruit varieties. "That’s what Washington State is known for—red, elongated varieties," she said.
Olmstead said one research study showed a reduction in color development of Vitis vinifera wine grape crosses when temperatures rose to 86°F compared to 68°F.
Because Concord juice grapes could be especially impacted by reduced color development, WSU researchers have been studying how the Sunbelt juice grape variety, developed in California, responds to Washington growing conditions. In WSU trials, Sunbelt is highly colored and heat tolerant, with similar bud hardiness to Concord grapes.
The variety has yellow leaves in the fall, compared with Concord’s green, and the crop developed better color compared with Concord, Olmstead said. "But there are some question’s about yields."
The intense red color of wines, produced by anthocyanins in the grape berries, could also be affected by a change to warmer temperatures.
"Warmer climates tend to have problems with high pH and low acid, which affects color," she explained. Winemakers may need to lower the pH in Washington wines, a practice that is not typically needed now.