Raisin grape vineyards in Fresno County are showing the effects of a string of 100-degree-plus days that have been stressing the vines since the beginning of the month.

Many brown leaves attested to the outbreak of vine-damaging mites triggered by the heat.

That’s despite efforts to control the pest with miticides, says Jerald Rebensdorf, president of Fresno Cooperative Raisin Growers, Inc., who grows organic Selma Pete and Thompson seedless grapes near Biola, Calif.,

“Many of the products growers have been using to control mites don’t seem to be as effective in some cases as they should be,” he says. “The mites still are coming back into the fields.”

Controlling mites has been challenging.

Oil sprays Rebensdorf and other organic growers use increase the risk of sunburn damage to the leaves. Getting effective coverage with these materials can also be difficult. “Because they work by smothering the eggs or the mites, good control requires getting perfect contact of the material with the leaves,” he says “That’s hard to do.”

Controlling mites in conventionally-grown vineyards has required a second miticide application in many cases, he notes. In a cooler season, one treatment is the norm.

The impact of the heat is also evident where water berries have started drying up, Rebensdorf adds. A few raisin growers have sprayed their vineyards with products that are used by walnuts growers top prevent sunburn damage.

Growers have been irrigating frequently to mitigate the heat.

Despite July’s sizzling temperatures, his Selma Pete grapes are progressing well. “They look real good,” he says. “Most of the vines are young and can take more heat. Also, because there’s a lot of growth on them, the mite damage isn’t as severe. But, some of the hottest spots in our Thompson seedless ranches aren’t doing as well.”

Rebensdorf expects Thompson seedless grapes will be ready to harvest onto trays to dry about the third week of August. It will be earlier starting with Selma Pete.  He’s planning to cut the canes of his dried-on-the-vine Selma Peter Aug. 5-6.

 “If we can get through the heat, the crop should be similar in size to last year and with sugar levels high, the quality of the raisins should be good,” he says. “We’ll have a much better idea of sugar levels and tonnage around Aug. 1 when the harvest of Thompson seedless for sparkling wines begins.”

Shipments of 2012 crop raisins to domestic and export markets are down 12,000 packed tons from year-earlier levels, Rebensdorf reports. That compares to the 2011 crop, when shipments for the entire marketing year fell more than 8,000 tons below the 2010 crop. This follows a much bigger decline for the previous marketing year, when the shipments of the 2010 crop totaled more than 28,000 packed tons less than the 2009 crop. Taken together, shipments of the 2010 and 2011 crops declined a little more than 37,000 packed tons. These falling numbers reflect the large world supply of raisins during that time, he notes.

“It will be interesting to see how the world supply affects the price of California raisins this year,” Rebensdorf says.

If you would like to read more about California grape growing, subscribe to GrapeLine, the exclusive electronic newsletter sponsored twice a month by Chemtura: See here for sign-up. It’s free and e-mailed the second and fourth weeks of each month from March through October.

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