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Challenges of controlling powdery mildew

SALINAS, Calif. — Veteran coastal vineyard consultant Corky Roche marvels at the continuing inflow of new fungicides for control of powdery mildew in California grapes.

“With so many materials on the market, competition is keen and price is also competitive,” said Roche of Salinas.

“More importantly for me, though, as a pest control adviser, it is great because I have so many choices for resistance management,” said Roche. “These products are all different and that makes it great for resistance management. I can go through an entire season and not repeat using a material.”

Roche has a textbook attack plan for the 1,000 acres of vineyards he consults with, but plans can change. There is a bit of artistry in a season-long coastal powdery mildew management plan. There is also no calendar.

Once growers treated by the calendar. That is no longer the case. “We have the Risk Assessment Index and the Adcon System from Western Farm Service to aid us not only when to treat, but at what intervals,” said Roche. These weather-driven powdery mildew models predict the onset of mildew and the severity expected based on the weather.

These models developed by University of California plant pathologist Doug Gubler and others provide consultants and growers with information to base their powdery mildew control measures.

Rain is one of the factors that can disrupt the models’ predictions.

“That is the biggest risk we face in the spring. If we get as little as a tenth of an inch of rain we may be forced to come back and retreat,” said Roche. That is not only costly, but can explode powdery mildew if not done properly.

All traditional powdery mildew control measures from sulfur to the newest fungicide are preventive measures. When rain washes them off the vine, the leaves and fruit are left unprotected.

Weather forecast

“I am not comfortable with anything less than six hours for the systemic materials to be rain fast,” said Roche.

Roche prefers to wait when rain is in the forecast and the models say there is low mildew pressure. “When we begin to reach treatment thresholds, I check what the temperature forecast is going to be and the risk assessment index for the past few days,” said Roche. Low mildew index numbers of 10 to perhaps 20 and a forecast of rain and Roche will wait.

“If you do not have a high mildew risk, you are gambling a lot more by treating with rain in the forecast than by waiting,” said Roche, explaining that waiting early in the season is not as risky as not knowing if a certain block had wash off from rain. Growth is small early and coverage with less than a full canopy is always good. Therefore, if a grower waits a day or so, he can still gain control.

Roche has a pair of formulas he has in his powdery mildew management book before the season starts.

For areas not susceptible to frost, the first three treatments are sulfur and spreader starting about at one-inch of growth. Then he’ll start the systemic fungicide treatments for one or two treatments, rotating among the active ingredients. For young, non-bearing vines, if the daytime temperature remains at 70 degrees or above, he’ll switch back to sulfur from post bloom to berry sizing. He has seen problems with sulfur’s lack of mildew control in the cool Salinas Valley when temperatures are below 70 for several days.

Wineries demand that grape growers stop using wettable sulfur at bloom and stop dusting sulfur by June 30. Any powdery mildew treatments after that would be with fungicides other than sulfur.

Roche maintains an active powdery mildew control program longer than many, not stopping oftentimes until grapes reach 17 Brix. Some quit as early as 12 Brix, certainly late enough to protect the grapes.

Protects longer

“I protect a little longer because in the cooler, coastal climates on some later varieties like Merlot and Cabernet, where a late season powdery mildew infestation on the leaves can slow down canopy development. That could delay ripening,” said Roche.

He cut off powdery mildew control a bit early last year on several blocks and it became worrisome. “We still had high risk index numbers and I cut it off when the grapes were no longer susceptible (veraison). I became concerned about ripening the fruit, however, with some mildew on the leaves. However, we made it. I was inclined to go a little later this year so I did not find myself in that situation again,” he said.

His program changes a bit for frost prone areas. He mixes in copper with sulfur early from bud break to mid April. The copper kills ice nucleating bacterial and can give a vineyard one to two degrees of added frost protection.

“Timing is everything using copper. It has to be on 24 hours before a frost for it to do any good. Frost usually follows a rain,” he said. The Catch 22 dilemma is “probably the most difficult timing decision we make all year...put the copper on too early and the rain will wash it off, negating any frost control benefit,” said Roche.

“Frost can be significant on the coast ever three or fours years...maybe not in entire vineyards but in pockets. You can get a frost that burns the tips or one where you lose the entire crop. Frost can be costly on the coast,” he said.

The use of fungicides have made a dramatic difference in powdery mildew control, but with rapid buildup of resistance to the first synthetic fungicide on the market it was an early wakeup call that it could happen again. Growers and PCAs do not want a repeat.

“Growers are aware of the importance of rotation and the importance of adding sulfur to any powdery mildew program to avoid resistance,” added the veteran consultant.

‘Biggest challenge’

Powdery mildew is the most devastating disease in grapes and probably the No. 1 grape disease in California. “It is the biggest challenge we face every year. If you drop your guard, you may get lucky and escape serious damage. But if you are not lucky and you have the kind of damage powdery can do, it is something you will never forget,” said Roche.

Powdery mildew is ever-present in a vineyard at low, even undetectable levels. If it gets out of control it is almost impossible to bring down to minimal damage levels.

“The worst thing you can do if mildew pressure gets high and a serious infection becomes established is to use a fungicide to try to eradicate it,” said Roche. “Dr. Doug Gubler is adamant about not doing that. All it does is build resistance to whatever class of fungicide you are using. When you take that approach, there are spores that are resistant and they will escape, and resistance will build.”

Roche has played powdery mildew fireman in the past. He relies on heavy volumes of water—300 gallons of water per acre—mixed with microthiol or Thiolux on non-bearing vines. On vines with fruit, 2 percent JMS Stylet oil by volume is used -- expensive, but very effective, said Roche.

In that situation, Roche also recommends leafing to open up the canopy to allow full penetration of the water. “If you have a full canopy and mildew in the clusters, you have a pretty slim chance of getting even 300 gallons of water into the vines without leafing.”

Coverage is the name of the game in powdery mildew control as with any pest management scheme. Roche said there are plenty of good sprayers on the market to do the job.

“I would like to use electrostatic sprayers more. When properly set up, they do a great job with low gallonages of water. We ran fluorescent dye and black light dye tests in a very heavy canopy and the coverage was very impressive. There were droplets covering every berry, including deep inside the canopy,” he said.

Unfortunately, many of the fungicides registered for powdery mildew on grapes require a minimum of 50 gallons of water per acre. Electrostatic sprayers use only about 20 gallons of water per acre because of improved coverage with the charged droplets.

Longer control

The newer fungicides reaching the market also are promoting longer control periods. “You hear a lot about 21-day control, but growers and I have a tendency to get nervous when it gets to day 19 or 20.

“It will take a few seasons for growers and me to get comfortable with 21 days or longer. However, as the intervals lengthen it will drive the cost of powdery mildew control down,” said Roche.

“Even if growers can consistently save even a couple of days between treatments over a season, it would save a lot of money over the course of a season,” said Roche.

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