A predictive model developed in 1995 that helps California commercial wine, raisin, and table grape growers assess environmental conditions conducive to disease development will get a facelift.

The University of California, Davis (UC Davis) Powdery Mildew Risk Assessment Index (RAI) measures ascospore release and the subsequent risk of pathogen population increases in about 130,000 acres of commercial California grapes.

The index, also known as the Gubler-Thomas model, was developed by Doug Gubler, UC Davis Extension plant pathologist, and UC staff research associate Carla Thomas.

“The RAI is a very successful tool for growers to determine disease pressure at any given time,” Gubler said. “There’s absolutely no way to fail in disease control programs if the RAI model is followed.”

The model’s national and international appeal includes vineyards in Oregon, Washington, Chile, Argentina, France, Germany, Italy, and Austria.

Powdery mildew is caused by the fungus Erysiphe necator. More dollars are spent on powdery mildew control than on any other grape pest, yet the disease accounts for more crop losses than any other disease. Powdery mildew persists in each California commercial grape production area.

According to estimates, the annual amount spent on mildew control, plus the loss in yield, frequently equals 10 percent of the state’s entire grape crop value. Despite regular control measures, the disease can cause heavier losses in highly susceptible varieties.

According to the UC statewide integrated pest management program, powdery mildew symptoms include red blotchy areas on dormant grape canes. Leaf symptoms first appear as chlorotic spots on the upper leaf and later as a white, web-like mycelium. Infected areas resemble a white, powdery dusty appearance as spores are produced. A white mass can cover the entire leaf or berry surface.

Early fruit infections cause stunted berries, scarring, and, ultimately, poor flavors in wine. Mildew reduces the storage life of table grapes and can affect the rate of photosynthesis which can lower berry sugar content.

All succulent tissue on a grapevine is vulnerable to mildew, including the stem, fruit and leaves, and shows characteristic symptoms as the season progresses.

Once initial infection occurs, the perfect temperature range for fungal growth is 70 to 85 degrees F, Gubler says. Temperatures above 95 degrees for 12 continuous hours can halt fungal growth.

The RAI measures powdery mildew pressure on a scale of 0 to 100 index points, Gubler explains. Low pressure is rated from 0 to 30 points. Spraying water on grapes every two weeks can provide 100 percent disease control under this low pressure.

At 40 to 50 points, the fungus reproduces about every 15 days. Softer chemistry products can provide excellent disease control. At 60 to 100 points, the fungus reproduces every five days. Higher populations make powdery mildew control an uphill battle despite available chemicals.

“Under high pressure we see the expression of resistance to the DMI fungicides as the resistant members of the population increase while the sensitive members are killed by the fungicide application,” Gubler said.

The RAI predicts the powdery mildew population level five to six days before it occurs.

New RAI research

While the index is accurate, Gubler calls it conservative. The model is being tweaked to further define the role that high temperatures play on fungal control.

“It is important that we understand this information to make the model more precise,” Gubler said.

“The current model assumes temperatures up to 95 degrees for even 20 minutes on any day will reduce the index points by 10,” Gubler said. “If six hours of 70 to 85 degrees then occurred that night which sometimes occurs in the Central Valley, the result would be an index jump of 20 points. In reality, we really don’t know what’s happening to the pathogen population under these conditions.”

Gubler’s new research focus is on how high temperature durations impact sporulation (spore development), spore germination, and lesion (colony) expansion (growth). The results indicate higher temperatures for extended periods reduced conidial numbers (sporulation), slowed or stopped lesion expansion, and decreased the germination of conidia.

“Conidia production decreases at 90 degrees in 12 to 14 hours,” Gubler says. “At about 93 degrees, sporulation generally decreases after about eight hours.”

Temperatures from 86 to 93 degrees did not reduce spore germination overall. Spore germination at 96 to 105 degrees for hours terminated spore germination.

High temperatures had an impact on lesion expansion. Lesion growth held steady at 90 to 93 degrees. Reductions began at 96 degrees after about eight hours.

“Apparently lesion expansion is not affected nearly as much as the sporulation,” Gubler said.

Second year trials are examining leaf surface temperatures for correlation with canopy temperatures. Thermocouples were placed on upper and lower leaf surfaces on inside and outside leaves in the canopy.

“The thermocouples will enable us to look at leaf surface temperatures to help document more specific information about the temperatures that the pathogen is exposed to,” Gubler said. “Leaf temperatures should be cooler than ambient temperatures due to leaf transpiration.”

Gubler and other UC researchers emphasize that the impact of powdery mildew on a vineyard greatly depends on timing of the first infection. Early-season control is essential.

Mildew can infect leaves from bud break to leaf fall and fruit from berry set to veraision or approximately 8 Brix. An existing mildew colony can survive until berries reach 12 to 15 Brix.

Gubler shared his findings with pest control advisors and others attending a Bayer CropScience tree and vine seminar in Monterey, Calif., in January.

email: cblake@farmpress.com