Kerman, Calif.,-based PCA and grape grower Rod Yraceburu and Sonoma, Calif.,-based premium wine grape grower Mike Saini know what it takes to control powdery mildew.
“Managing for powdery mildew comes down to making choices that will allow your control materials to perform at their best,” says Saini.
Here are a few of the strategies behind their success against powdery mildew.
Be proactive.“The most important thing you can do in controlling powdery mildew is to not get behind,” says Saini. Since powdery mildew is nearly impossible to see before a full-blown infection occurs, a preventive spray program helps stop outbreaks before they start. “Eradicating an existing powdery mildew infection is much more costly and time-consuming than preventing it,” adds Saini.
Watch the weather.“Be cognizant of weather activity,” Yraceburu cautions. Know the weather conditions that favor powdery mildew outbreaks and stay ahead of them with appropriate chemical applications.
In Saini’s case, working around weather is especially challenging. “There is no such thing as a typical weather day in the North Coast area,” says Saini. “In a single day, we’ll see everything from coastal fog and 55 degree temperatures to sun and 100 degree temperatures. Combined with the uneven terrain, this means environmental conditions can vary from vineyard to vineyard and valley to valley—making spray and monitoring programs difficult to implement,” he says.
Use weather forecasts to pinpoint the best application times and identify periods with potentially higher disease pressure.
Have a treatment plan.Choosing the best materials for vineyard conditions is just as important as applying them at the right time. Alternating fungicides from different chemical classes delivers effective control while managing resistance.
Saini’s powdery mildew control program starts with a wettable sulfur and copper application at bud break. After 8 to 12 inches of growth have emerged, he alternates different fungicides every 14 days beginning with Viticure® fungicide, a systemic sterol inhibitor (SI) labeled for protective and curative control of powdery mildew. Saini then rotates to quinoline and strobilurin chemistries before making another Viticure application.
Stick to smart spray intervals.“After so many days, the material you applied starts to degrade and can no longer protect you,” says Yraceburu. “In addition, all the new growth in the vine since the last application can become infected.”
Saini sprays his blocks every 14 days to maintain a consistent level of protection and to prevent breakthrough powdery mildew infections that can crop up when spray intervals are stretched beyond label recommendations. Stick to a 14- to 21-day spray interval depending on your vineyard’s specific conditions and always follow label directions.
Labeled rates.Fungicides are only effective when applied at labeled rates. Applying a low rate in high-disease-pressure situations may not result in the desired level of control.
Saini takes a strong stance on labeled rates. “Skimping on fungicide rates is the last thing you want to do when dealing with powdery mildew,” he says. “Using labeled rates prevents disease outbreaks, making it easier to keep controlling the disease in the current and future growing seasons.”
Aim for good spray coverage.“Spray coverage is probably the most critical thing you do no matter what type of material you’re putting on,” says Yraceburu. “You can ruin any material’s effectiveness if you don’t do a good job of applying it.” Making sure equipment is properly calibrated and avoiding rushing down the row can help.
Timely canopy management also improves spray coverage and penetration. Positioning shoots and removing obstructions like leaves and canes clears the way for droplets to reach the bunches. Saini likens a fungicide application to air and an obstructed canopy to a tightly sealed house. “You can’t expect to get air into the house unless you open the windows,” explains Saini.
Remember to rotate.Continually changing chemistries is key in preventing resistance and ensuring long-term efficacy of powdery mildew fungicides.
“A large amount of grapes in the concentrated area of a vineyard coupled with a powerful disease like powdery mildew means greater potential for resistance development,” Saini says. To manage resistance in his vineyards, he sprays only one application of a chemistry before rotating to a different material.
“To prevent fungicide resistance, I never make more than two back-to-back applications of the same class of fungicide,” echoes Yraceburu. As with spray intervals, be sure to follow label instructions.