The widely-used grape powdery mildew Risk Assessment Index (RAI) may be revised soon, and it could be a money saver for growers.

University of California Extension Sacramento Area Plant Pathologist Jenny Broome is conducting trials in a Sacramento Delta Chardonnay vineyard for an RAI assessment project directed by UC Davis Extension Plant Pathologist Doug Gubler.

The RAI model uses weather station data to create index numbers which translate into appropriate fungicide spray intervals during the season.

Since its release in 1996, the model has enabled growers to eliminate two to three fungicide sprays each year, while improving control of powdery mildew, Gubler says. The model applies to all grape growing regions in the state.

Temperatures between 70 degrees and 85 degrees are ideal for development and growth of the fungus (Erysiphe necator) that causes the disease. But, the researchers suspect that the original model may underestimate the effect of higher temperature in reducing spore germination, colony expansion and longevity and spore production. As a result, the model may indicate the need to spray a fungicide at higher temperatures, when, in fact, treatment is not needed.

In lab trials, Gubler’s team found that as temperatures increased from 86 degrees to 108 degrees, the fungus grows more slowly. For example, after the fungus was exposed to 98 degrees for four hours, it took 11 days for it to recover.

“The effect of high temperatures in the vineyard is most likely to reduce the rate of disease development, but not to kill the pathogens,” says Broome. “That’s important in managing such an explosive disease as powdery mildew. In addition, reducing pathogen viability can increase fungicide effectiveness.”

High temperatures for long durations do kill the pathogen, but many areas of California don’t see those kinds of temperatures for long durations, Broome says.

“We know, though, that extreme temperatures make the fungus less efficient in infection, germination, and sporulation,” says Gubler. “We also know that fungicides applied at high temperatures are more effective in killing the pathogen.”

Consequently, the researchers are revising the RAI model to make it less conservative.

“Based on our lab results, we’re pretty certain that we can improve the model to enable growers to save at least two or three or more fungicide applications in a season,” Gubler says.

The revised model is now being tested in the Delta vineyard, a long-time site for Gubler’s powdery mildew fungicide trials. The annual powdery mildew field day at Herzog Vineyards near Clarksburg is scheduled for July 29. For more information, e-mail Gubler at wdgubler@ucdavis.edu.