Plant nutrient levels are important, especially when heavier almond crops are produced. Nitrogen should not be added in late fall or winter due to leaching and potential loss. Potassium, foliar-applied zinc, and boron however, should be added if needed. Zinc sulfate can play an important role in breaking the disease cycle of rust and shot hole when applied as a foliar spray in November.

These fungal diseases can remain on older leaves through the winter and be an inoculum source that infects new leaves in the spring. Young vigorous orchards often become zinc deficient due to their excessive growth and development.

Zinc deficiency symptoms often appear in late summer and are referred to as ‘little leaf’ or ‘rosette’ and are characterized by a shortening of the internodes toward the tips of the shoots and small narrow leaves. Often leaves are bent upward on either side of the mid-rib. Zinc foliar sprays are often applied in the fall to correct deficiency, either as zinc sulfate (neutral zinc 52 percent) or in a chelated form. I personally don‘t like waiting so late in the season that the sprayer blows most of the leaves off during the application.

I often make my zinc applications in mid to late October in order to make sure the leaves burn from the zinc. There is environmental concern that continued zinc sulfate sprays (10 pounds to 15 pounds of zinc sulfate in 100 gallons of water) will lead to soil contamination, so you might consider more frequent chelated zinc applications during the season. Chelated zinc sprays can be applied safely at anytime during the season when other applications are being made.

Many growers apply too much nitrogen and not enough potassium or boron, especially if you are irrigating with water from an irrigation district. Boron nutrition often goes unnoticed and should be monitored with hull samples at harvest. If hull samples are less than 80 ppm boron, your trees could be deficient and you may be experiencing a yield loss as a result. Boron can be applied to the soil, in herbicide mixes, or in foliar applications. Soil applications can be made (not in a band like potassium) at rates of 2 pounds to 4 pounds of actual boron per acre (10 pounds to 20 pounds of 21 percent product). Granular boron can be broadcast on the soil while soluble boron formulations can be injected into micro-irrigation systems.