Although powdery mildew hasn’t been much of a problem yet this season, there are reports that it is increasing in some areas.

“Someone told me that they heard mildew is now increasing in the San Joaquin Valley, but I don't think it is a problem,” says Doug Gubler, UC plant pathologist.

That assessment is confirmed by John Moore, independent PCA at Bakersfield.

“Powdery mildew is perhaps a little more prevalent this year than normal, but it’s not bad. The crop looks good overall — probably a little better than average. We’ve had very little disease or pest pressure this year. One of the fields I consult on got hurt by the freeze earlier, and we lost some canes after 48 hours of 12-degree temperatures, but it only affected a small portion of the crop.”

In other areas, powdery mildew seems to be a bit more of a concern in recent weeks.

“The central coast has pretty high disease pressure and has had for a few weeks now,” Gubler says.

That may come as a surprise to some growers, according to Jim Smoot, viticulturist at Paso Robles.

“The tricky thing about powdery mildew is that you could have been infected weeks ago and not realize it. It can start showing up weeks after the initial infection.”

Temperature is a major factor in the risk index, but even that can be misleading at times.

“The spores are killed at about 90 degrees,” Smoot says. “If you’re going by the ambient temperature, you still don’t know what the temperature is inside the canopy.”