Pistachio growers will be watching for signs for winter juvenile tree dieback (WJTD) as trees begin to come out of dormancy in the spring. Damage is typically found in trees between three and five years old, although there have been instances where older trees in their seventh or eighth year have been affected. Narrowing down the exact cause of the issue also presents its own set of challenges.
“I’m somewhat hesitant to call it freeze damage because we’ve seen it in places where we couldn’t find a freeze. But I think more than anything it has to do with root hypoxia; that’s a lack of oxygen in the root zone,” said Craig Kallsen, UC Farm Advisor in Kern County. “We see it in old lakebeds, low elevation areas where cold air collects but that’s also where the salt is so it’s hard to separate those factors.”
The number of trees affected by WJTD is particularly bad for areas of the San Joaquin Valley, most presumably due to the temperature variation in the fall and early winter. While it is known as winter juvenile tree dieback, the warmer temperatures in fall can cause issues by preventing trees from quickly entering the dormant stage and leaving the window open for potential damage.
“We think the trees aren’t dormant yet, but the closer you get toward December the colder it’s got to get to produce the symptoms. Whereas in October, just a degree below freezing, and you may be in trouble,” Kallsen noted. “If you’re one of the unlucky guys that gets hit with it, you’ll know it. You may lose 20-25 percent of your trees. They may not die, but you’re going to be way set back.”
There are multiple strategies for helping trees to move rapidly into the dormant stage, such as ending irrigation or applying materials such as zinc sulfate. Helping young trees advance into dormancy can mitigate the risk of WJTD, however, there is still much to learn about the issue and the research process is proving to be difficult. “All of our variables are confounded. The low elevation area is also the area where we have the most salt and that’s where the cold is. So, separating the thing is tricky,” Kallsen explained.
Listen to Kallsen’s interview below.
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