For Sonoma County wine grape grower Doug McIlroy, this crop year has been one of extremes – too much moisture earlier in the season, which intensified and prolonged the usual powdery mildew threat; and, more recently, too much 100-degree plus heat, increasing the risk of sunburn and desiccation damage to the fruit.

Adding to these challenges is the uncertainty he and other growers face in finding enough workers to tend the vines and harvest the grapes. “The labor shortage is always on our minds,” says McIlroy.

Drawing on more than 30 years of experience growing wine grapes, he directs vineyard operations for Rodney Strong Vineyards Estates. This includes 14 estate vineyards near Healdsburg, Calif., and throughout the eastern hills of Alexander Valley, as well as other vineyards in this area.

The high moisture levels in the fields reflected unusually high rainfall amounts this past winter and spring. For the 12-month period beginning July 1, 2016, many of the vineyards received as much as twice the average rainfall, with rainfall totals ranging from about 40 to 70 inches.

Excess soil moisture and cover crops contributing to high humidity levels created ideal conditions for development of powdery mildew, which thrives in temperatures between 70 and 85 degrees, from early in the season through June, McIlroy notes.

“It took a while, but we finally got our arms around the powdery mildew threat and were able to keep it controlled until the end of July when vines began entering veraison and the risk subsided.”

The extra soil moisture this year also encouraged more vegetative growth than usual in the vineyards. “We had some pretty good-sized canopies to manage this year,” McIlroy said. “Also, because of all the moisture in the ground, we had to makes some additional passes after suckering to remove extra leaves.”

Unlike a typical year, daytime temperatures of 100 degrees or hotter were the rule rather than the exception in his vineyards for much of July. On those days, temperatures commonly reached as high as 111 degrees on one occasion.

“Here in the Alexander Valley, where the weather in July is usually cooler with some foggy conditions, temperatures may reach 100 degree or higher for a day or two, McIlroy says. “But, to have as many of those days as we’ve had this year is very unusual.”

Late in July, the fruit on the first of his red varieties were starting to color up, while the clusters on his whites were beginning to soften.

Meanwhile, near- and longer-term forecasts for his area predict continued above-average temperatures.

“Once we get into cane hardening and the vines change from their growth cycle to veraison, we like to see cooler weather,” McIlroy says. “Generally, that’s when we get better sizing of the berries. But, when vines are stressed for water because of the heat, production tends to be pulled down a little and the grapes are smaller than we’d like, no matter how much water we apply.”

At the first of August, McIlroy was expecting about an average to a little smaller crop from is vineyards. However, several varieties show signs of doing better than that.

“The Sauvignon Blanc on our ranches looks really good, and the Cabernet Sauvignon blocks have a pretty fine crop,” he says.

Like McIlroy and his neighbors in Sonoma County’s Alexander Valley, a number of growers elsewhere in the state have also been dealing with hotter-than-usual weather this summer. That includes the Lodi region in the northern San Joaquin Valley, where breezes from the Sacramento Delta to the west tends to limit, somewhat, the upward rise of summer temperatures.

“Vineyards here experience a little less of the spike in heat than those even just 15 to 20 miles north or south of here,” says Paul Verdegaal, University of California Cooperative Extension Service farm advisor for San Joaquin County. “Still, it’s been pretty warm here.”

Sometimes growers here in Lodi will see one or two really hot days in May, followed by some intense heat in June, he notes. Normally, they can expect about 17 days of 100-degree-or-higher temperatures from June 1 – Oct. 1. This year that number had already been reached by Aug. 1, just half way through that period.

“Up to a point, heat encourages grapes to ripen a little faster,” Verdegaal says. “But, once temperatures exceed 90 to 95 degrees, the stomata shut slightly to limit moisture loss, and the vines direct more of their energy to maintaining respiration and less to producing sugar. The hotter it gets the slower the amount of sugar accumulating in the fruit. Some sunburn damage can also occur when the leaves and fruit suffer from heat and water stress.”

Up until veraison, high temperatures can increase the risk of burning the leaves and fruit from applications of sulfur to combat powdery mildew.

“When temperatures rise quickly from cool to hot, treating vines with sulfur can produce a fair amount of leaf and fruit burn, even by spray residues drying quickly in high temperatures,” Verdegaal says. “Spraying at night, when temperatures cool and the wind is calm, can reduce this risk.”

Once the fruit go into veraison and the berries accumulate enough sugar, growth of the powdery mildew fungi is inhibited. “Some research indicates levels as low as 4º to 5º Brix can reduce the risk of powdery mildew, depending on variety,” Verdegaal says. “Generally, by mid-veraison, when sugar levels reach 8º to 10º Brix, the disease is no longer a threat to the grapes.”

Based on results of UC research, he encourages wine grape growers, especially those using drip irrigation systems, to keep the pumps turned on when weather forecasts predict several days in a row of 100-degree or higher temperatures and to keep the water flowing until the heat wave breaks. That advice also holds for growers following a deficit-irrigation plan to meet the needs of a particular wine program.

“Under sustained high heat conditions, drip systems can’t provide enough water at the rate needed to produce a wetted root zone large enough to prevent stressing the vines,” Verdegaal says. “While most wine makers are hesitant to irrigate the vines later in the season, they also understand the importance of keeping up with the water demands of the vines.”