When Bill Krueger checked Chandler, Hartley and Howard walnut trees in Glenn County in late March, buds on were still tight. But those on other varieties, like Ashley and Vina, were starting to push.
“The walnut trees could be a little behind normal in terms of development, but it’s hard to say,” the University of California Cooperative Extension farm advisor says. “The recent cool weather has really slowed development of other trees, like prunes.”
He hasn’t heard of any damage to walnut trees from several below freezing spring nights, although trees that were unusually dry at that time could have suffered.
Rain has been sparse this winter, although rainfall improved in March, which could results in more walnut blight, a disease spread by spring rains. It’s the main disease concern for growers in Northern California, where spring rainfall tends to be higher than in areas to the south. The disease affects early-leafing varieties more severely than later varieties.
Growers begin to spray fungicides to prevent the bacteria-caused disease from infecting flowers and nuts when 30 percent to 40 percent of the buds reach prayer stage. That’s when terminal leaves of pistillate flower buds first unfold and appear like hands in a prayer position.
A second spray is recommended seven to 10 days later to treat pistillate flowers that weren't open enough when the first spray was applied. Additional treatments are based on weather conditions
During wet years, growers may make as many as eight to 10 applications on early varieties and three or four for those that leaf out later, Krueger says. Last year, these treatments continued in his area until the middle of May
“Walnut blight wasn’t a huge problem last year,” he says. “But, the disease is definitely out there and is something growers should be looking for. Dry weather helps keep it down; however, the last two springs have been wet, so the relatively low pressures last year may be because growers are doing a good job of controlling the disease.”
Walnut growers have also been putting out traps to monitor codling moths.
Krueger isn’t expecting a repeat of last year’s unusually large and widespread drop of Howards, which occurred in late June
“We think it was related to rapid swings in temperatures and humidity, between cool and moist and hot and dry conditions,” he says. “But, it didn’t seem to have much effect on yields at harvest.”