Central Coast grape grower Rodolfo Callado is a seasoned veteran at keeping powdery mildew under control — he’s been doing it for more than three decades.
As a vineyard manager for Mesa Vineyard Management at Templeton, Calif., he oversees production of wine grapes in the Santa Maria Valley. Straddling part of the border between San Luis Obispo and Santa Barbara Counties, the cool, coastal valley is known as a favorite hangout for the pathogen that causes the fungal disease.
Fungal spores overwinter in buds or as spore structures, and with warm, moist conditions in winter and spring they reproduce, spreading the infection. The spores, which can cover leaves and berries in a white, powdery mass, will continue to reproduce throughout the season when temperatures range from 70 to 85 degrees.
“Here in the valley, we battle powdery mildew every year,” says Callado.
As a result, he’s acquired some hard-earned insights into specific production challenges and practices — ranging from weather conditions and stage of crop growth to timing of fungicide treatments and operation of spray equipment — that enable him to manage powdery mildew successfully from bud break to grape harvest.
Last year, powdery mildew pressure on the Central Coast was unusually high.
“Some areas that are normally warmer often see much less powdery mildew than we do,” he says. “But, because of cool temperatures last year, even those growers in areas, who hadn’t faced much powdery mildew for years, had problems. It was a really peculiar year — some people had a horrible time controlling it.”
Normally, once the developing berries reach veraison and 12° Brix, the disease is no longer a threat. But not last year. “Even after the grapes had enough sugar in them, we saw mildew developing on the leaves after veraison,” Callado says. He had never seen that before.
He credits an aggressive, relentless approach with preventing powdery mildew problems in his vineyards last season.
“We’re responsible for producing some high-value crops, and powdery mildew isn’t something I back away from,” he says. “We know we’re going to get it, and we can’t wait until we see it to start dealing with it. The program isn’t inexpensive, but it has been successful in preventing problems for us, even under difficult conditions.”
The program starts with a spray of wettable sulfur and copper when new shoots reach 2 to 3 inches in length. He repeats the treatment — which relies on contact with fungal spores to kill them — every 10 to 14 days until the vines bloom.
The spray also includes a surfactant and sticker for better, more durable coverage of the contact treatment. “If we’re on a 14-day schedule and we get rain soon after application, we’ll re-spray within 7 to 10 days,” he says.
Once bloom begins, Callado continues with the wettable sulfur and copper, but adds a synthetic fungicide to the tank mix.
“The fungicides provide some systemic movement through the vines to control powdery mildew where I don’t get coverage with the sulfur and copper,” he says. This treatment gives 18 to 21 days of control before he re-sprays the vineyard.
Usually, he starts the fungicide treatment with a strobilurin product, applied at the higher rate recommended to control Botrytis, as well. He reduces the rate of subsequent fungicide applications to that recommended for controlling powdery mildew. To manage resistance to the chemicals, he alternates his choice of fungicides for later treatments among different classes and types of fungicides.
Once berries reach BB size, he stops the wettable sulfur applications because of concerns by winemakers that sulfur could affect wine fermentation.
Callado uses the powdery mildew Risk Assessment Index. If the index shows a low risk of the disease, he may lengthen the spray interval to a maximum of 21 days. Conversely, when the index indicates a high risk of powdery mildew he may reduce the interval to no longer than about 14 days.
This timing of post-bloom powdery mildew sprays is also affected by his crews’ schedules.
“We have windy conditions during the day, so we spray at night when winds are calm,” Callado says. “It takes us one week to get across all of our acreage, then we go back to working days for two weeks.
“This approach pushes us to the maximum 21-day spray interval limit of the products used. So, on the 17th or 18th day after the previous spray, we’ll go in and start our next fungicide spray. That puts us ahead in the event we have an equipment breakdown or bad weather sets us back a few days.”
Callado typically makes up to five powdery mildew sprays between the start of shoot growth and bloom, and another five to six post-bloom treatments. He uses an air-assist, over-the-row sprayer that treats two rows at once.
Early in the season, he applies the materials with 50 gallons of water per acre. “That may seem like a lot of water to some growers,” he says. “But, I want to get the sulfur and copper down into the bark where the spores overwinter.”
Once the vines have put on about 18 to 24 inches of growth, he bumps water volume up to 75 gallons per acre. When vines have produced a full canopy, he increases the rate again.
He uses tables provided by the spray nozzle manufacturer to determine the number and type of nozzles, air pressure and ground speed needed to apply chemicals at the desired rate per acre.
Callado double checks those calculations. He attaches a hose to a nozzle and runs water through the sprayer at a given pressure and length of time, and captures the water in a jar to determine if the nozzle is working correctly.
“We’ll go into a rectangular field, where we know how many rows represent one acre, and we’ll spray those rows and check the tank to see how much water was used,” he says. “Then, we’ll compare that to the number of gallons we want to apply per acre. That tells us if we need to make any adjustments to the sprayer or to our tractor speed.”