FRESNO, Calif. (KGPE) — Day after day of extreme heat can have detrimental effects on grapes grown for wine in the Central Valley.

Fresno State Winemaker Tom Montgomery says vineyards at Fresno State are still okay, although the plants are starting to show signs like withering and dry, brown leaves. 

“The vines do shut down. Above about 90 degrees the vines — the metabolism — starts to shut down. And that’s a protective measure for self-preservation. So they’re conserving water and not transpiring and not ripening fruit. Sometimes we see them stall in very hot weather,” Montgomery said.

He pointed to some grapes beginning to wrinkle like raisins. “That berry is all shriveled up. That means less crop, lower acidity, higher sugar, higher PH,”  he said.

All are important factors in making good wine. Although all kinds of grapes are grown in the Central Valley, he says most are varieties that tolerate heat well.

“On campus, we grow some of the Spanish varieties. Albariño. Touriga Nacional. Those come from Spain and Portugal. And those are much better adapted to heat and the fruit from those vines are much better adapted for heat. So, in the case of a very warm summer or fall or harvest season, those vines will be less affected. Vines like Chardonnay or Pinot Noir are grown in cooler climates and the effect on the fruit and the vines themselves can be much greater and pose a greater winemaking challenge.”

Montgomery does not believe crops in Central California are seriously damaged, at least not yet. 

“I would say we might be right on the cusp. If it does go on much longer than we’ll see things like sunburn on the fruit, severe dehydration, the grapevines themselves will start to shut down physiologically. Yes, it does have an effect if it continues much longer.”