Zinc and copper deficiencies aren’t uncommon in pistachio orchards in the southern San Joaquin Valley and other areas with alkaline soils.
Frequently, the conditions occur together, usually in juvenile trees or trees just starting to produce nuts. Young trees may be more susceptible due to vigorous growth and reduced rooting area compared to older trees.
Typically, the best time to correct these deficiencies is to apply foliar nutrient sprays in early May as the new leaf canopy expands, reports Craig Kallsen, University of California Cooperative Extension farm advisor, Kern County.
“By then, half the canopy is fully expanded but the leaves still aren’t very thick,” Kallsen says. “We think nutrient uptake is better before the leaves thicken and harden. This year, late April also would have been a good time to spray.”
Kallsen recommends treating juvenile and bearing trees with zinc and copper foliar sprays annually, or more often, depending upon the specific orchard conditions. In areas of Kern County where soils are low in boron, growers often add this nutrient to their copper and zinc sprays, he notes.
“When treating, avoid spraying the tender newly-opening rachises, unless you’re using fairly dilute materials,” Kallsen says. “Otherwise, you risk burning them off.”
Since zinc and copper aren’t very mobile in the plant, older leaves appear normal, even as the younger leaves show deficiency symptoms.
“Foliar zinc and copper sprays may restore normal leaf growth in new leaves, but will not correct leaves showing deficiency symptoms,” he adds.
Kallsen describes the causes and symptoms of zinc and copper deficiency in the January 2016 issue of his Kern Pistachio Notes newsletter.
Soils with an alkaline pH, and elevated levels of phosphorous, salinity, and organic matter, can reduce the availability of zinc and copper for pistachio trees. Due to their low cation exchange capacity, sandy soils also may not provide adequate amounts of copper and zinc, says Kallsen.
“Shallow hardpans can make this situation worse since they limit root growth,” he says. “San Joaquin sandy loam, a common soil on the east side of the Central Valley, appears to be especially at risk from zinc, copper, and boron deficiency. “
Kallsen adds, “Trees on this soil type are often deficient in potassium, even when treated with potassium fertilizers, due to the soil’s chemical composition which fixes potassium within its structure, making it unavailable to the roots.”
He says zinc deficiencies are likely to appear earlier in the season.
Symptoms of mild zinc deficiency include interveinal chlorosis and reduced internode length in the terminal portion of developing shoots. As symptoms worsen, small branches in the outer canopy will terminate in a rosette of small, yellowish (chlorotic) leaves and the shoots may die back.
In the case of more severe zinc deficiency, vegetative and flower buds push later in the spring and the new shoots die back.
Bearing trees that lack sufficient zinc produce clusters with small reddish, blank nuts similar to chill-deficient trees, Kallsen says.
“High soil manganese levels (300 ppm in the leaf or 40 ppm plant available zinc in the soil) appear to have induced zinc deficiency in sandy soils in the California high desert east of the Sierras and in Arizona,” the farm advisor says.
Typically, copper deficiency symptoms don’t appear until mid-summer. Leaves fail to expand normally and may appear clover shaped and scorched.
If deficiency symptoms are detected early, a foliar copper spray can correct a deficiency in new growth quickly, says Kallsen. However, without early treatment, the terminals of new shoots die back and some may curl downward.
“Shoot symptoms often progress so rapidly in juvenile trees that you can lose a year’s growth in just a few weeks,” he says. “If copper deficiency is excessive, the nuts of bearing trees will shrivel.”