Pistachio trees, particularly young ones, can be severely affected by micronutrient deficiencies.

Common in two- to three-year-old trees, deficiencies limit canopy development and reduce early bearing potential, according to UCCE advisor emeritus Bob Beede. Keys to successful uptake of micronutrients are active root growth, presence of water and oxygen and presence of leaves.

In the Central Valley, where nearly all California pistachios are grown, the most likely micronutrient deficiencies are from zinc, copper and boron. Pistachio demand for boron is highest among all nut tree crops, and, according to Beede, speaking during a pistachio production presentation, “Pistachios are boron pigs!”

Severe boron deficiency in soil has been recognized in the Sierra Nevada foothills of central California and sometimes in Lake and Mendocino Counties. Moderate deficiencies are common in orchards on the east side of the San Joaquin Valley, especially on sandy and moderately acidic soils. Heavy soils on the west side of the valley can have concentrated amounts of boron, which can cause boron toxicity problems. Boron is mobile and soluble in most soils. Knowledge of solubility characteristics of soil is important.

UCCE Merced pomology advisor David Doll said boron deficiency in pistachios could also be driven by high-quality irrigation water that leaches boron below the root zone.

Boron functions in the differentiation of cells and is crucial in function of stem cells in plants. It also regulates carbohydrate metabolism and is important in pollen germination and pollen tube growth. Boron is unique in that it is the only element that varies in mobility in different tree nut crops. It is highly mobile in almonds, but not in pistachios or walnuts.

Pistachio trees deficient in boron “look like they want to grow but can’t,” said Doll. “They look like they don’t have the energy to grow.”

Besides a stunted appearance, the leaves of mature trees will have a crinkly, leathery appearance even though they are normal size and uniformly green. In young trees, there will be weak terminal growth, severe tip dieback and short internodes. Flower clusters that remain green and shatter off the tree could be associated with low boron levels.

With sufficient boron applications, the trees will resume growth. Beede said second leaf orchards could be affected severely by deficiencies in micronutrients because of the tipping growth to create branches. Pistachio rootstocks can also differ significantly in the ability to take up nutrients from the soil. 

Doll said foliar applications of boron are acceptable if leaf tissue samples do not indicate deficiency.

“It is hard to get the level needed with foliar applications,” Doll noted.

Samples in the 120 -150 ppm range indicate acceptable levels. Tissue levels less than 60 ppm in May indicate a critical need for soil-applied boron to ensure adequate uptake by the tree since boron does not move in phloem. The application can be done via fertigation or with an herbicide spray.

Correcting severe deficiencies might require 20 pounds of a boron product to correct, Doll notes. To maintain acceptable levels, applications of five to seven pounds per year may be needed in deficient soils.

Soil treatments must be done by end of August to achieve adequate boron levels in the tree by the next spring. If trees are showing signs of deficiency earlier in the year, treatments should begin earlier. With marginal boron levels, the recommendation is to apply five pounds of Solubor per acre in March.

Doll said the best long-term correction can be expected from a combination of soil and foliar treatments applied as yearly maintenance.

Knowledge of soil and site characteristics, plant requirements and cropping history along with observation of tree health are all components of micronutrient management of pistachios.