With almost two dozen powdery mildew products on the market this year, California grape growers have no shortage of choices for fighting this annual scourge of wine grape vineyards everywhere.

“Compared to botrytis, powdery mildew is by far the most severe disease growers have to plan for year-in and year-out,” says University of California Extension Plant Pathologist Doug Gubler. “Botrytis pressure is cyclic with severe pressure every few years when we have rainfall at bloom and after veraison.”

About half of the powdery mildew products on the market are soft chemistry, according to Gubler. “This is exciting to see, because these soft products do perform well under the right conditions,” he says. “They have a definite fit if you know what your disease pressure is. If you have a long period of high disease pressure you will want to switch to a stronger, systemic chemistry.”

One aspect of these highly effective powdery mildew products that has to be addressed, however, is that most of them have single-site activity and therefore are the most at risk for fungicide resistance in the pathogen population to develop, the plant pathologist warns. Gubler suggests that growers plan strategically to utilize the power of these effective materials while preserving their efficacy for years to come.

Don't stretch time

For the next spray after a single-site-of-action product, Gubler says there is still a huge choice. “But growers must be aware that if they are using a single-site product, they should use no more than two consecutive sprays without switching class of chemistry — not just product.”

He also recommends that growers not stretch intervals under high pressure “because you will have lost control in three days — the time it takes for disease spores to develop. Stretching intervals also puts selection pressure on the fungicide product, contributing to resistance development.”

Another way to alleviate the threat of resistance development similar to alternating is to tank-mix. Studies show that there is no significant difference between the two techniques in terms of resistance management, Gubler says, adding that growers should always use the shorter interval of the two products in a tank mix if the intervals are different. “If you use the longer of the two intervals in a vineyard with resistance, you can lose control and cause resistance to increase,” he warns.

As the season develops and bloom-time approaches, growers need to strategize the bloom sprays. “It is important to come into and out of the bloom period disease-free,” Gubler says. “Bloom sprays and pre-closure sprays are very important in high risk situations, such as susceptible varieties, or where botrytis is a serious problem.

Tank-mix option

“Tank-mixing at bloom is a valuable way to manage resistance to powdery mildew problems,” he says. “For instance, if you're using any of the DMIs or strobilurins for powdery mildew, you can tank-mix with Elevate or another botrytis product with powdery mildew activity and a different mode of action. This program should increase activity against botrytis bunch rot while providing excellent powdery mildew control.

“In our trials on grapes over the past two years, Elevate alone gave about 75 percent control of powdery mildew. This level of control is not economic control and one would not want to use the product for powdery mildew alone. However, with the degree of control achieved in our field trials, I certainly would not hesitate in using the product for bloom applications with another chemistry, i.e., a DMI or a strobilurin, or for a less costly bloom-time application, growers could even tank-mix Elevate with sulfur, assuming temperature was not an issue.”

For post-bloom applications, growers who sell to wineries that prohibit sulfur can bring in some other powdery mildew products, depending on the class of chemistry used at bloom-time. Then at bunch closure and veraison, the same botrytis product used at bloom can be used again, according to Gubler. “We find that unlike powdery mildew materials, resistance is not an issue with botrytis products in grapes,” the plant pathologist says.

Research studies

Market-based research studies conducted by the Crop Production Research Department of John Taylor Fertilizers (a business unit of the Wilbur-Ellis Co.) bear out the value of using different classes of chemistry for effective powdery mildew control. Jim Cook, product development manager, and Vicki Rose, project manager, set up large block trials every year at a Delta test site that is highly prone to powdery mildew pressure.

“We compare all powdery mildew products side by side using 32-vine treatments and following a protocol that is based on grower economics,” Cook says. “We do our own evaluations, and then bring in others for ‘blind’ evaluations so that we can have a high degree of confidence in the results.”

In 2002, a “blowout” year for powdery mildew, the best performance of commercially available materials was in a plot that was treated with Flint at the lower 1.5-ounce labeled rate pre-bloom, followed by two applications 14 days apart of 1 pound of Elevate, and a final application of Flint at 1.5 ounces. The plot was also treated with the grower standard application of sulfur dust approximately every seven days.

“Resistance management is huge,” Cook says, which is why he alternated products with different modes of action. “The Delta region is a tough powdery mildew area, yet this program, based on grower-budget protocols, gave us 90 percent control of the disease.”

Cook also reports that where the Wilbur-Ellis Co. deposition called “In-Place” was used with each treatment, it increased performance of most powdery mildew products by about 5 percent.

Cleanest vineyards

Mike Huffman, a PCA with John Taylor Fertilizers in the Delta, confirms that the cleanest vineyards he saw last year were treated with Elevate fungicide at bloom preceded and followed by the standards, a program that was used on varieties susceptible to botrytis.

“Elevate appears to give us a higher percent of powdery mildew control, while at the same time taking care of botrytis,” he points out. This is important, he adds, because quality has become increasingly important in a wine grape market that has become more competitive. “And it gives us an opportunity to rotate classes of chemistry to prevent resistance.”

Although heavy spring rains predicted in this El Niño year have not materialized, the rainy season isn't over yet, warns plant pathologist Gubler. “April and May can still deliver a lot of rain and we can have severe pressure when there is rainfall at bloom,” he says.