This year’s warm and wet conditions created an ideal environment for powdery mildew, which farm advisors say is the most significant disease faced by grape growers worldwide in terms of control of costs and losses in quality and yield.

For example, the aggressive downy mildew took advantage of the weather aberrations to show up in vineyards in Santa Barbara and San Luis Obispo counties.

Officials at the Robert Mondavi Institute Center for Wine Economics at University of California, Davis report the management of Erysiphe necator accounts for nearly three quarters of the pesticide applications by California grape growers. Grower cost to purchase and apply fungicides generally accounts for three percent to seven percent of the gross value of production where mildew pressure is significant.

University of California Cooperative Extension (UCCE) wants to be part of the battle to combat the scourge and toward that end is seeking grape growers statewide to be a part of studying the powdery mildew population in order to better understand how and where resistance is developing — and ultimately to establish an annual rotation plan to help mitigate that development.

The $4.75 million research effort called FRAME [Fungicide Resistance, Assessment, Mitigation, and Extension], funded by USDA-National Institute for Food and Agriculture under a specialty crop research initiative, wants to paint a clear picture of fungicide resistance on a national scale, understanding how mildew spreads and develops resistance.

“We have major classes of fungicides used on powdery mildew and several of those modes of action have shown chemical resistance,” says Viticulture Farm Advisor Gabriel Torres, who represents Kings and Tulare counties, taking swab samples for analysis.  “We’d like growers to help us sample, so we can map out where resistance is developing.”

Powdery mildew is different from other fungi because it is an obligated parasite that needs a live host to survive, making fungicide resistance evaluation more challenging.  Control measures in the San Joaquin Valley generally take six to 12 sprays depending on the grower. The season and repeated use of a single mode of action can result in resistance.

A large effort

This is a large and well-funded effort according to its project director, Michelle Moyer, horticulture professor and statewide Extension specialist at Washington State University.

“For decades growers have combatted powdery mildew by opening canopies for sunlight and increased air flow and applying affordable fungicides, but pathogens adapt and become chemical resistant, leaving growers in a potentially untenable situation when the treatment they’ve relied on for generations to prevent infection no longer works,” Moyer says. “When it comes to fighting mildew, not all fungicides are the same and they become more target-specific — there’s an increased risk of mildew developing resistance to those treatments. Put simply, when you over-expose something, nature fights back.

“We want to tackle resistance before it becomes a larger problem” as occurrences of fungicide resistance have been seen in numerous locations throughout parts of California, Washington, and Oregon over the last two to three years,” Moyer says.

“We have a chance to tackle resistance before it becomes an even larger problem,” Moyer says.  “The end result will mean more effective use of the tools we have for disease management, sustainable grape production, and more affordable grapes for wine, juice, table, and raisins.”

The FRAME Network web page notes the following with a question: “In specialty crops, like grapes, product quality is king. What happens when routine production tactics, such as disease management, stop working?  Many factors can influence the effectiveness of a disease management program, but fungicide resistance is the most feared (because) there is currently no effective system to monitor or predict that resistance which is generally identified only after a management failure.  We need data and predictive tools for growers, coupled with improved approaches to manage and mitigate resistance development.”

Adds Moyer, “Our initial focus is on FRAC 11 (Strobilurian), probably the most-used class of fungicides for powdery mildew, because when FRAC 11 stops working, it’s as if you sprayed nothing at all.”

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