As a vineyard manager in the Santa Maria Valley of California’s Central Coast, dealing with heavy pressure from powdery mildew in grapes is nothing new for Jim Stollberg — but in his 11 years of growing wine grapes he’s never experienced a season like 2011.
“The number of incidences of the disease and the pressure was, by far, the most I’ve ever seen,” says Stollberg, whose vineyards span both sides of the Santa Barbara-San Luis Obispo county line.
“The disease started sooner, and because the temperatures didn’t warm up later in the season as usual, the pressure continued through harvest. All the growers I’ve talked with spent more money than ever to protect their grapes from powdery mildew. That was especially difficult to do with last year’s yields, only average at best, and the relatively low grape prices.”
The heightened threat from the fungal disease last year reflected both the season’s unusually cool, wet weather and a high-number of powdery mildew spores that overwintered from 2010, when pressure from the disease was also higher than normal.
Growers and Pest Control Advisers could be facing a three-peat, according to Doug Gubler, University of California, Davis, Extension plant pathologist. As was the case from 2010 to 2011, increased prevalence of powdery mildew last season could mean more overwintering spores to infest vineyards this year, he says. Just how much of a threat the disease poses to growers this season depends on early season weather. Any symptoms would show up on the lower leaf surfaces of basal leaves after bud break and within the first 7 to 10 days after a rain.
“If we have two or three rains after bud break and cool to moderate temperatures, then there’s a good chance of powdery mildew developing early in the season,” Gubler says.
However, should this winter’s dry weather persist into the spring, growers might not start seeing the disease until May or June when spores start blowing in from vineyards where the disease wasn’t controlled.
“Even if the weather is hot and dry in the spring, we almost always see a little mildew by June,” Gubler says. “So, there’s always a need to treat for powdery mildew each year.”
“If there were any holes in your powdery mildew control program last year — whether fungicide materials, coverage, application rates or treatment intervals — you saw it in the fields last year,” Stollberg says.
Normally, he gets a break from the disease mid-to-late July, when temperatures reach the mid-80s and higher. But not last year. Temperatures remained in the 65 to 75-degree range throughout much of the summer, an ideal range for growth.
To increase air flow and reduce humidity within the canopies and clusters, Stollberg pulls leaves. “Last year, despite all our fungicide applications and hand work, we struggled to stay get ahead of powdery mildew,” he says. “The amount of fruit lost to the disease in this area was much higher than usual.” He estimates the disease could have accounted for as much as 10 percent to 20 percent of any yield losses in Santa Maria Valley vineyards last year.
Last year he followed his typical program for controlling powdery mildew. He made his first stylet oil application when the vines had produced about 5 to 6 inches of new growth. Then, based on weather conditions, he continued to treat the vines with on a 14 to 21-day interval – going with the shorter intervals in cooler/wetter conditions that favor growth of the disease-causing spores and using the longer intervals with warmer/drier weather. When higher diseases pressures required additional control, he tank-mixed a synthetic fungicide, such as a strobilurin or sterol inhibitor with the stylet oil.
Normally, he continues with this program until veraison in late July, when the softening berries are no long susceptible to powdery mildew. Last year, however, due to the combination of high disease pressure and lack of hot weather to control the disease, he continued spraying into late August.
“Because of their sugar levels, the clusters weren’t being damaged by mildew after veraison,” Stollberg explains. “But, we were still seeing active spores on shoots and leaves. We didn’t want to risk losing photosynthesis activity to the disease, so we continued spraying the vines for another 30 days after veraison, nearly up to harvest.”
As a result of the powdery mildew pressures last season, Stollberg, like other growers in his area, has been doing more dormant spraying with sulfur or stylet oil this past winter than in the past. He sprays vines after pruning and before bud break in fields that had significant powdery mildew pressure the previous year. The idea is to soak the wood to knock out any overwintering inoculum in the vineyards.
“It’s one of the few things we can do at this time of year to help control powdery mildew going into the new season,” he says. “The disease spores are just waiting for warmer temperatures and rainfall to start spreading. The dormant sprays cause those spores to release prematurely and kill them before they can infect green tissue. It’s worth doing as insurance on a yearly basis.”
Last season also highlighted the value of adequate spray coverage for controlling powdery mildew. “Where we were able to hit the powdery mildew directly with our sprays, we got good control,” Stollberg says. “This year we’ll really focus on applying fungicides and managing the canopies to get good coverage with our sprays. If you have a crew coming in to hand pull leaves, it’s worth waiting an extra five days to spray so that you can get the better coverage.”