Prosser, Wash.—With growing degree days tracking ahead of 2011’s abysmal performance, Washington state growers are focusing on disease pressure to limit risks to this year’s crop.

Cool weather has been the norm since bud break, which was assisted by warm weather in early April, and recent rains have handed some areas up to 15% of their annual precipitation in a matter of days.

Washington State University headquarters in Prosser registered an accumulation of 4.6 inches of precipitation as of June 17, versus a long-term annual average of 7.5 inches.

The wet weather this spring, following two years in which growers have grappled with powdery mildew has Milbrandt Vineyards viticulturist Dustin Tobin sticking to a tight application schedule with his fungicides at the site in Mattawa, Wash.

“The inoculum for mildew is still in the plant, and it’s easier to get (reinfected with powdery mildew) the year after you get it,” Tobin said. “It has been more of an issue than it has been in years past, but with tight spray programs, you should easily be able to not see it.”

While a standard year might see spray intervals of 14 to 16 days, Tobin told Wines & Vines that he’s sticking to 10-day intervals to ensure regular coverage. The schedule is in line with advice from WSU Prosser viticulturists, who note that the most critical time to control powdery mildew on grapes is during bloom.

Spring rains trigger the release of spores that have overwintered in the vineyard. The spores are best able to infect vine tissues when damp conditions persist for 12-15 hours and temperatures are between 50° and 60°F. High temperatures and sunlight inhibit progression of the disease, but in lieu of favorable weather growers can stick to a rigorous regiment of fungicides.

Combating botrytis
Developing berries also will be vulnerable to botrytis bunch rot, which WSU extension viticulturist Michelle Moyer noted in a recent crop update is a significant concern this year.

“Rains and cooler nights are leading to high levels of cap retention in clusters, creating an excess of debris, which is an excellent food source for the botrytis bunch rot fungus,” she said.

It’s a similar story in British Columbia’s Okanagan Valley, which has seen the skies deliver more than half the region’s annual precipitation in the space of the past month. Cool weather also has had a moderating influence on vine development, but the vines have nevertheless been tracking ahead of 2011, and recent conversations with growers indicate disease pressure is in check.

Growers across the region are generally anticipating a warm summer that should allow them to overcome weather-related disease pressure and lead the vines to a decent harvest.

“From bloom to veraison it’s all about heat units, so we can easily catch up,” Tobin said. The Cabernet Franc vines he tends on the Wahluke Slope are already showing pea-sized berries, while Mourvedre is just finishing bloom.

A slower season with steadier growth may even yield wines with better balance.

“Vintages like this, those flavors come at lower Brix, and it makes for balanced wines,” Tobin said.