This past season has taught grape growers in Fresno County and other areas of the San Joaquin Valley how unusual spring weather can affect disease and insect pressures in vineyards and their ability to control them.

Warmer-than-normal temperatures in May encouraged vines to grow quickly, resulting in large canopies. Then, unusually cool weather in June combined with the big canopies to create an ideal environment for development of diseases, such as powdery mildew, Botrytis bunch rot and sour rot.

This same weather also caused an earlier-than-typical increase in the activity of omnivorous leaf rollers and raisin moths, which fed on the grape bunches as the berries began to mature.

Additionally, the large canopies reduced penetration and effectiveness of fungicides and insecticides that growers sprayed for control.

Wine and raisin grapes suffered more from diseases and insects than table grapes, says Stephen Vasquez, University of California Cooperative Extension viticulture advisor for Fresno County. “Mid-spring shoot thinning, followed by leaf pulling at berry set, helped adjust the crop, reduce canopy congestion and improve spray coverage,” he says.

The severity of the diseases and insects varied from one vineyard to another, depending on production methods and soils, spray frequency and other management factors. For example, Vasquez says, vines growing in heavier soils on rootstock that imparts more vigor and trained to an overhead trellis system tended to have larger canopies, all of which made control of disease and insects more difficult.

The effects of different management practices were apparent in the vineyards of two growers located across the road from each other, which Vazquez observed earlier this month.

The canes in one of the vineyards showed rust-colored scarring, a telltale sign of multiple powdery mildew, but those in the other vineyard had relatively few infection scars.

“The grower with little scarring may have been spraying for the disease every 5 to 7 days with dusting sulfur,” he says. “The other may have treated his vines with a sulfur dust, too, but extended the application intervals to 10 days during optimal growing conditions for the fungus.” Effectiveness of control measures can also be affected by a number other management factors. For example, spraying at a ground speed of 3 mph will produce much better coverage than traveling at 4 when canopies are large. Also, the proper chemical, rate and timing of sprays, as well as acreage to be covered, will impact pest management.

“We get used to a certain type of season and become comfortable with that in our management practices,” Vasquez says. “Then, something a little different comes along and throws us off. That’s where a good knowledge of pest biology and really monitoring disease and insect outbreaks can pay off.

“Pay attention to differences in weather patterns from one week to the next and how they can affect grape production over the next week to 10 days. Then, adjust your management as needed.”

Because powdery mildew and bunch rot fungi overwinter on or near the vine on old plant material, they could prove to be unusually troublesome next season, depending on the severity of the disease this year, he says. Similarly, high insect populations that were left unchecked in 2009 could be a problem in 2010 and growers should prepare management strategies accordingly.

“There’s nothing really new about managing these fungal grape diseases,” Vasquez says. “What will be new next year are the growing conditions — even though some seasons seem to be similar, something different is always thrown into the production mix. As is the case every year, I expect 2010 to pose some challenges for growers.”