Smartphone app advises on treating for powdery mildew

The next generation of winegrape growers should be able to look at an app on their smartphones for recommendations on where in a vineyard block they should scout for early signs of powdery mildew.

U.S. Department of Agriculture researchers are developing a program called Erath Vineyard Viewer that will combine the elevation, slope, satellite vegetation imagery, temperature, weather, past mildew outbreaks and other information to provide recommendations on where to scout.

"We're working toward a model that will tell you where to put sensors," said Walter Mahaffee, USDA research plant pathologist based in Corvallis, Oregon. "We're focusing on powdery mildew right now, but it should work on any pathogen, and (University of California Cooperative Extension farm advisor) Monica Cooper is working on mealybugs."

Targeting when and where to apply mildew materials is vital, because the disease has developed resistance to many of the most effective fungicides.

"Most of the isolates we look at from Oregon, Washington and California have both DMI and strobilurin resistance," Mahaffee said. "We might be able to make the strobilurins work again by shifting out of them for a few years. We might not have to throw out these materials, but we have to learn when and where we can use them."

Mahaffee discussed progress toward the monitoring app during a discussion of ways to target powdery mildew applications, and slow down the advance of resistance at the Central Coast Vineyard Team's 2018 Sustainable Ag Expo in San Luis Obispo.

Researchers have also developed a test to quickly tell if mildew in a vineyard is resistant to strobilurin fungicides, and a monitoring tool to know when the disease spores are present.

Erath Vineyard Viewer will be one more aid in managing powdery mildew, which is particularly difficult because the disease produces an astronomical number of spores that can be extremely difficult to reach with fungicides.

"We're looking at developing this system, so you can include data from any source, including drones," Mahaffee said.

His research team has discovered the unique genetic makeup of powdery mildew that is resistant to the strobilurins, and developed a test to let growers and pest control advisors quickly learn if samples they submit to the USDA lab in Corvallis are resistant.

"Generally, we try to send you an e-mail within 24 to 48 hours," Mahaffee said. "We try to stay within that time frame because we know you have to make a decision."

Researchers are testing easier ways to take the samples, including cotton swabs and gloves that can be removed and submitted after they contact diseased vines.

Another tool that helps growers target their applications is a spore trap that can be mounted in the vineyard and monitored to learn if the pathogens have arrived.

"We try to use it early in the season to see if we need to apply, because if there are no spores there should be no infection," said Daniel Rodrigues, viticulturist and owner of Vina Quest.

Two-inch rods, attached to a small solar powered motor, are covered with a sticky substance that catches the spores. As the season approaches, these rods are swapped out every seven to 10 days and lab tested to see if the powdery mildew spores have arrived.

The entire unit, which is mounted in the vineyard on a two-inch tube, costs $700 to $800 and lasts at least a few years.

"We're trying to push spraying out as long as possible," Rodrigues said. "Logistics is key; you need to take the sample in time to make a decision about spraying. If you're using a 21-day material, we can spread it out a little longer."

Good application basics are particularly important with powdery mildew as part of a resistance management program.

Resistance management also involves rotating materials. At times, each of the growers in a neighborhood may be properly rotating, but one of them may have strobilurin or DMI fungicides out there continuously.

"You can have regional resistance, and I can guarantee you powdery mildew will cross a fence line," Mahaffee said.

(Bob Johnson is a reporter in Davis. He may be contacted at bjohn11135@aol.com.)