An unexpectedly high nut drop was seen by San Joaquin County walnut growers earlier this summer due to walnut blight and an uptick in codling moth numbers, and they remain wary of possible damage from walnut husk fly from now until harvest.
Even so, Joe Grant, University of California Cooperative Extension farm advisor for the county, is optimistic that growers will bring in a decent crop this year. He expects harvest in his area to begin as usual in the first week or two of September.
The walnut blight threat subsided with the arrival of warm, dry summer weather, but not before taking a toll on production.
“Losses from the disease were more than we’d like to see,” he says.
“For one reason or another, walnut blight caught many growers by surprise. Following a good crop set on most varieties, things looked pretty good until about the middle to end of June, when we started seeing a significant amount of disease symptoms, including nut drop. Normally, with a good spray program, we wouldn’t expect to see the amount of blight we’ve had this year, especially so late in the season.”
Cool, wet weather in March and April probably helped contribute to the problem. Vina seems hardest hit, but other early- and mid-leafing varieties, such as Serr and Payne, also suffered significant damage in many orchards. There were fewer problems with the Chandler variety, which leafs out later in the spring and escaped at least some of the rains that fell when earlier varieties were leafing out, Grant says.
Despite recent hot weather, a fairly mild summer has helped to minimize heat stress and sunburn damage to nuts.
Codling moth pressure has been about normal, which is a change from the past two years. “There’s been a bit more flight activity this year, compared to 2010 and 2011,” he says.
Many San Joaquin County growers began capturing walnut husk flies in monitoring traps in the last half of July. Feeding by the maggots of this insect can lower grower returns by causing kernels to shrivel when nuts are infested early in the summer, or if damage occurs later, it can stain shells, reducing the value of walnuts.
At the end of the July, it was too early to determine how much of a threat, if any, the walnut husk fly will pose to this year’s crop. However, the pest has become an increasing problem in recent years, and researchers are seeking more effective ways of monitoring and controlling it.
Meanwhile, new walnut orchards continue springing up in the county. Some are replacing older orchards, while others are being planted on former row crop ground or in low foothill areas where irrigated crops haven’t grown before.
“We’re seeing a lot more walnuts going into places formerly considered suitable for only field crops or dryland pasture,” Grant says. “Growers are getting better at finding pockets where the soils are a little better and, with a good irrigation system and water supply, will support walnuts.”