When just a fraction of San Joaquin Valley grape farmers use a new UC Powdery Mildew Risk Assessment Index (RAI), they can cut fungicide applications on their vineyards by 1 million pounds per year, according to industry reports.

The dramatic reduction in fungicide sprays and the resultant decrease in pesticide drift and air pollution impacts are prime examples of the benefits available to farmers by employing science-based pest management techniques.

This particular integrated pest management (IPM) tool addresses powdery mildew, the No. 1 disease of grapes. The most common fungicide used to control the disease is sulfur, but many other fungicides are commercially available. Typically, growers apply fungicides on a calendar schedule, usually beginning within two to three weeks after “budbreak,” the moment when green shoots begin growing after the vine has been dormant for the winter, and then every 7, 10, 14 or 21 days, depending on the product.

However, UC Cooperative Extension research has shown that powdery mildew is only a problem under very specific weather conditions.

“After budbreak, you must have three days in a row during each of which the temperature is between 70 and 85 degrees for at least six continuous hours,” said Stephen Vasquez, the Fresno County UC Cooperative Extension viticulture farm advisor. “This year we had two days of optimal weather, then on the third day, there was a cloud cover and the high temperature was below 70, so the RAI did not ‘kick off’ and growers didn't need to spray.”

Weather watch

Keeping track of the weather proved to be a deterrent for busy farmers, so Vasquez and Madera County UCCE viticulture farm advisor George Leavitt began to do it for them. Vasquez maintains a network of nine weather stations in Fresno and Madera counties. The weather stations monitor the temperature, then a computer program developed by UC IPM calculates a score from 0 to 100 for each day for each site and posts it on the UC IPM Web site daily. (The index may be found on the UC IPM Web site at http://www.ipm.ucdavis.edu/TOOLS/.) Farmers check the index online to know immediately whether the conditions are optimal for powdery mildew growth. If they are not, growers can keep their spray rigs parked.

Leavitt and Vasquez have held classes each spring for three years to teach growers biology and control of powdery mildew, laws regarding the use of fungicides, and how to use the RAI model.

“The point I make,” Leavitt said, “is that they're going to feel uncomfortable using the program the first year. I understand how the growers feel. I was uncomfortable the first year. If they are uneasy, I tell them to pick up their telephone and call us. That's what the farm advisors are here for.”

Raisin growers

The Sun-Maid Raisin Growers of California, a cooperative that represents roughly one-third of raisin producers in Fresno, Madera, Tulare, Kern and Kings counties, has been supportive of UC's grape powdery mildew risk index.

“This is the third year we have provided seminars for our growers devoted to controlling powdery mildew with UC's system,” said Sun-Maid technical services representative Joe Kretsch. “It's a proven, scientific way to maximize fungicide efficiency. The whole idea is to get growers to apply fungicide based upon what the disease pressure is, rather than just going by a calendar.”

An industry estimate determined if 25 percent of raisin growers used the UC model in 2003, and if those growers opted for sulfur treatments, then up to 1 million pounds of sulfur would not have been applied to treat powdery mildew during that season.

“The majority of our growers are quite pleased,” Kretsch said. “The more they use it, the more comfortable they become.”

Vasquez said an individual farmer growing 40 acres of grapes can save as much as $667 by eliminating a single fungicide application. (Fungicide application on 40 acres ranges from $128 to $667, depending on the product used.) In 2003, many growers eliminated three applications early in the season and in 2004 most cut two.

“This spring, as of April 1, most vineyards already have at least four inches of growth, but growers shouldn't be applying anything for powdery mildew,” Vasquez said, “The RAI has not yet kicked off, but I suspect that it will after this last series of storms passes. Growers should have their sprayers ready for action.”

Growers monitor

The index is also used in other parts of the state. However, in some of the smaller valleys with widely varying terrain, such as in Lake County, the farm advisors' recommendation is for growers to monitor the weather themselves in their own vineyards to determine the optimal times for fungicide application.

Sun-Maid, the Raisin Bargaining Association and the San Joaquin Valley Viticulture Technical Group support the program by paying for the Fresno and Madera county weather stations to be recalibrated each year.