California’s Serr walnut growers received an added bonus from this season’s wet start.

SPRING RAINS and cooler temperatures benefit many California walnut groves including this operation in Kings County.

The rain lowered pollen levels, reducing pistillate flower abortion (PFA), according to University of California Cooperative Extension (UCCE) Kings County Farm Advisor Bob Beede.

PFA is a major problem for Serrs. It is caused by excessive pollen load on female flowers which leads to flower abscissions.

The wet weather also filled the soil profile and irrigation reservoirs and put the 2010 crop in good shape.

“The rain certainly has taken growers off the hospital intensive care list, but they remain in fair condition in a normal hospital bed,” Beede said.

“Observations suggest we could have an average-sized walnut crop in the southern San Joaquin Valley (SJV),” Beede said. “Crop quality in mid-June appears excellent. Nut size appears very good. The crop load varies by cultivar.”

The southern SJV Serr crop is large for another reason. Valley growers also applied ReTain to mitigate PFA. Use of the plant growth regulator perfected by Beede and other UCCE farm advisors blocks the ethylene production in the flower to help avert PFA.

“ReTain use and the rain combined to produce a limb-breaking Serr crop in the southern SJV,” Beede said.

Southern SJV growers report a moderate crop on the Tulare variety and a moderate to light crop on Chandlers, Beede said. The Chandler variety represents a third of California walnut production. Overall, the crop load varies by orchards.

The 2010 California walnut crop could total 350-400 million tons, Beede said. Many factors from July to harvest could impact the actual crop size.

Spring temperatures were ideal for vegetative growth and nut development in the southern SJV. The weather delayed the need for codling moth and navel orangeworm control by seven to 10 days early in the growing season.

“Early-season planted orchards developing the primary trunk are growing vigorously,” Beede said. “Later-planted trees had good vegetative growth due to the cool spring weather. Mite problems were low in all commodities.”

Grower Jack Mariani is cautiously optimistic regarding his 2010 walnut crop.

Mariani is a partner in Mariani Nut Co. in Winters, Calif., (Yolo County). The company grows and processes walnuts and almonds.

“The walnut set looks pretty good,” Mariani said. “We have not had any crop sunburn yet (mid-June). Walnut blight appears under control.”

Despite Mariani’s optimism on the phone, his voice sounded hesitant as a strong windstorm was blowing through his orchards.

“I look out there and see nuts on the ground. Every time a farmer sees nuts on the ground they get worried,” Mariani said.

“This year’s above average rainfall was really good for the trees; not just replenishing groundwater supplies, but helping the trees build more vigorous root systems. The trees look healthier now than they have for the last several years.”

Good winter chilling complemented the above average rainfall.

“Now it’s a matter of maintaining the crop on the tree. Besides windstorm management, heat is one of our major concerns from this point forward – making sure we don’t get a lot of scorching days and a lot of sunburn on the nuts.”

The top-five varieties Mariani grows include Chandler, Tulare, Vina, Hartley, and Howard. The Tulare and Vina varieties looked the best in mid-June.

As California’s 4,000 walnut growers nurture another crop, Mariani is optimistic about market demand. China is the world’s largest walnut producer, yet imports California inshell and shelled walnuts to feed its rapidly-growing population.

Mariani spoke at a recent walnut event in China. China is California’s top walnut export customer.

“We need a big crop this year,” Mariani said. “We’re going into this season with one of the smallest carry-ins we’ve had in years. We’ve had two bumper crops back-to-back. We have our fingers crossed that we’ll come in with another record crop. The market needs it.”

Mariani says California’s 2009 walnut crop was a record at about 436,000 tons. He is optimistic this year’s crop could be larger. The National Agricultural Statistics Service’s 2010 walnut crop estimate should be released in early September.

Walnuts are an alternate bearing crop. 2010 is an on-year. Mariani pegs statewide walnut acreage at about 438,000 acres, about 420,000 bearing and about 18,000 non-bearing.

The walnut crop in Sutter and Yuba counties could be a good size, but not the limb busters of the last two years, according to Janine Hasey, UCCE tree nut farm advisor. Among the better looking late-season variety yielders include Chandler and Howard.

Hasey echoes Bob Beede’s findings on reduced pistillate flower abortion this spring.

“Some orchards may have pistillate flower abortion issues, but overall we’re not seeing it,” Hasey said.

Rain and cooler temperatures tend to cause more walnut blight, but Hasey has not seen the disease. “Growers typically apply protective sprays — some blight still occurs. We’re not seeing blight,” Hasey said. “It’s something we’ll look at retroactively.”

From July to harvest, Hasey encourages growers to irrigate properly. Measure moisture levels using soil moisture monitoring devices, including watermarks or pressure chambers, or by measuring evapotranspiration. Hasey says monitoring for pests including the codling moth and husk fly is important. Using the crop protectant Surround can prevent sunburn and help improve overall walnut quality.

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