A military battle plan and a season-long plan to keep botrytis bunch rot out of wine grapes have a lot of similarities. It takes many elements in both to achieve victory.
“The first thing you do not do in battling botrytis is you don't wear a target on your back — you don't try to grow Zinfandel here on the coast,” said Bill Petrovic, vineyard manager at San Bernabe Vineyards in King City, Calif.
Zinfandel is one of the most susceptible to botrytis, but so is Petite Sirah, and it is part of the varietal mix in the 8,000-acre vineyard.
“Bunch rot can be a real problem with Petite Sirah,” said David Rosenthal, one of the key managers at San Bernabe.
“Petite Sirah bunches and berries are big and as they develop some of the berries get squeezed and split, which gives an entrance for botrytis. Combined with the foggy, wet mornings in Monterey County, the botrytis tends to take off.”
Uncontrolled, botrytis can take 10 to 30 percent or more of a wine grape crop.
A botrytis battle plan encompasses several strategies, but it is based on a one premise: Anything that breaks the skin of a wine grape opens its up for bunch rot.
“You begin with a good powdery mildew program,” he said. “If mildew gets started, skins breaks down and that is where botrytis can start.”
San Bernabe relies on a variety of fungicides for powdery mildew control, sterol inhibitors, sulfur and copper. Compounds are rotated to avoid resistance. “We mix wettable sulfur or two compounds together to avoid resistance. And, we have never had resistance doing this,” he said.
Insect pest control also is part of the botrytis battle plant. Orange tortrix is a serious pest on the coast. If left unchecked, it can damage berries, opening the door for bunch rot.
Proactive measure to prevent botrytis include leaf pulling to improve air flow along with bunch thinning and some berm sweeping. These are to reduce the breeding ground for the disease spores.
There are machines to remove leaves, but Petrovic said they take too much crop off. “I have followed them, and there are two to four bunches knocked off each vine,” he said.
That equates to 400 pounds per acre for a varietal like Chardonnay. With a value of $1,000 per ton or more for premium coastal varietals, that is a significant economic loss.
“I can leaf pull by hand for about $140 per acre,” he said. “In areas where the value of the grapes is less, machines may make better sense.
When San Bernabe was planted in the early 1970s, overhead sprinklers were the irrigation method. Sprinklers are still there, but only for frost protection. Drip irrigation now delivers in-season irrigation water, even right up to harvest without fear of starting botrytis.
“When we had only sprinklers, you were sitting between the devil and the deep blue sea if you had to irrigate after berry softening. Free moisture is all that botrytis needs to get started,” he said. “Luckily, we don't have to rely on sprinklers any more. However, you cannot avoid foggy coastal summer mornings.”
Historically, San Bernabe uses two to three fungicides specifically to minimize botrytis.
A new one last season was Elevate from Tomen Agro. “Based on our first season of use, we're pleased with the level of control received,” Rosenthal said.
“It is important in disease management to keep up with new spray technology as well as new compounds for both powdery mildew and botrytis,” said Petrovic.
San Bernabe utilizes one of the newest technologies, the Adcon mildew index system, a weather station that predicts mildew development.
However, nothing beats people in the field and San Bernabe has a trained staff of in-house scouts who are constantly in the vineyards looking for evidence of disease or insects.
“We put five to 10 people in the vineyard every day from bud break to harvest, counting bunches, checking for insects and diseases and being the eyes we need to make sure we deliver the best quality wine grapes we can,” said Petrovic. “We have a very good staff of people who do all our checking. “We do not use outside checkers.”
These in-house scouts are labor foremen or their assistants.
They are trained to detect problems and are armed with University of California pest and disease insect identification material.