Fresno County’s grape crop quality looks promising, despite greater disease and insect threats

Fresno County’s grape crop quality looks promising, despite greater disease and insect threats

“Growers are telling me the sugar accumulation by Thompson seedless and other raisin grapes looks really good. Of course, that can vary, depending on the vineyard and cultural practices.”

It’s too early to say for sure, but the quality of this year’s raisin-type grape crop in Fresno County, the heart of California’s raisin production, appears to be much improved over last year.

The sugar content of a berry is associated with bigger berries and higher soluble solids (º Brix), which result in a better-quality raisin. This season, both measures are higher, reports George Zhuang, University of California Cooperative Extension viticulture advisor for Fresno County.

“Growers are telling me the sugar accumulation by Thompson seedless and other raisin grapes looks really good,” he says. “Of course, that can vary, depending on the vineyard and cultural practices.”

Last year, many growers were disappointed by the quality of raisins that came out of their vineyards. That may have reflected the stress on the vines from four years of drought and limited supplies of irrigation water, he says.

Then, again, both poor strategies for sampling grapes collected to measure soluble solids just prior to harvest as well as variations in fruit maturity within a vineyard can affect the determination of raisin quality, he adds.

One key to maximizing raisin quality is to maintain a healthy canopy during the growing season.

“This will help the leaves maintain the photosynthetic activity needed to produce the carbohydrates that the vine uses to grow the berries and accumulate sugar,” Zhuang says.

This year’s Fresno County raisin-type and wine grape crops have been ripening at a quick pace, he notes.

Typically, growers begin harvesting Thompson seedless grapes when sugar levels reach about 20º to 21 ºBrix readings.

The Selma Pete variety is popular for DOV (dried-on-the-vine) production, where the canes are cut to allow the grapes to dry for several weeks before the clusters are harvested with machines. This year Fresno growers began cutting canes in late July or early August. That’s similar to last year.

Harvest of Thompson Seedless is expected to begin towards the end of August. That would be similar to last season, too.

The county’s 2016 wine grape harvest also began a little earlier than in 2015 with such white varieties as French Colombard for high acid programs and Pinot Gris.

Unlike the production of most wine grape varieties in Fresno County, which has remained stable or declined in the past few years, acreage planted to Pinot Gris, is on the rise. From 2014 to 2015, the number of county acres planted to this variety increased by 26 percent.

This is in response to increasing demand from wineries for Pinot Gris grapes, Zhuang notes. However, yields of this variety tend to be low – from eight to 10 tons per acre. That compares to French Colombard production which, typically, ranges from 15 to 20 tons per acre.

To explore possible options for boosting Pinot Gris yields and fruit quality, Zhuang is planning to begin field trials to evaluate productivity and fruit characteristics of 12 different Pinot Gris clones from Foundation Plant Service.

This year Fresno County vineyards were under unusually high pressure from powdery mildew and growers had to adopt a very aggressive spray program. Growers should always monitor and base their management plans on the powdery mildew pressure in their own fields. However, combining the use of various tools, such as the UC-developed Powdery Mildew Risk Index, to determine the need for treatment with proper timing of spray applications can help growers control powdery mildew in their vineyards, Zhuang says.

Meanwhile, the vine mealybug threat to Fresno County vineyards continues to grow. In the past, most growers have been able to control this pest with a single application of Movento between early-May and mid-June.

“This year, the vine mealybug was their biggest insect control challenge,” Zhuang says. “Populations were pretty high. In addition to their Movento treatments, many growers sprayed another insecticide, like Admire or Applaud, in April to keep numbers down.”

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