After harvest is a good time to apply a few different foliar nutrients to the trees before the leaves fall. Nutrient uptake occurs through wood and leaf tissues. Applying when leaves are still green and active increases the sprayed surface area, thus increasing the efficiency of the spray and uptake of the nutrients. Applications can be made when leaves are off the tree, but the reduced surface area leads to a reduced nutrient uptake. Below are considerations to make in regards to a few commonly applied foliar nutrients.
Boron. Studies have shown that a foliar spray of boron can increase crop set regardless of boron hull content. If the tree is deficient in boron, the resulting “yield bump” is greater than in sufficient trees, but the “yield bump” is still observed in sufficient orchards. Two pounds of Solubor (20% Boron) applied with 100 gallons of water/acre is commonly used. This article detailing boron foliar sprays highlights the specifics.
Urea/Nitrogen. Fall foliar applications of lo-biuret urea are thought to reduce the incidence of bacterial canker and bud drop within almond. It is thought that this reduction is due to an increase of tissue nitrogen content which may increase overall tissue health making it more resistant to bacterial infection. Fall Urea applied for bacterial canker prevention should be applied later in the season (i.e. first week of November) and at a rate of 100 lbs/acre.
Studies have also shown that fall applied lo-biuret urea can increase yield. Although studies have not shown the yield increase to be significant, it has been observed in two trials. Both yield increases were observed with the lower end of urea use, between 10-25 lbs/acre. Higher applications of urea did not produce as much of an effect and in some cases led to foliage burn and leaf drop. Roger Duncan (UCCE Stanislaus) wrote a brief report on his study, which can be found here.
Zinc. Many orchards are deficient in zinc. This is not always due to zinc deficient soils, but rather to the ability for Nemaguard rootstock to exclude zinc, leading to lower root uptake and foliar deficiency. Zinc can be applied in several forms and methods. If increasing zinc tissue content is desired, low applications of zinc (zinc sulfate, 2-5 lbs/acre) can be made without “dropping” the leaves. If the goal is to defoliate the tree to help reduce overwintering rust inoculum, higher rates of zinc sulfate are needed (25-30 lbs/acre).
It is important to note that applying high rates of zinc sulfate (>15 lbs/acre) will prevent the tree from “picking up” other tank mixed applied nutrients (i.e. nitrogen, boron). This is due to the formation of an abscission layer between the leaf and the stem that occurs rapidly after a high zinc application. If the goal is to increase zinc tissue concentrations, use a low rate of zinc sulfate (2-5 lbs/acre). If planning to tank mix other nutrients, this rate should be used as well. Zinc sulfate does not tank mix with phosphonate products. This previous entry discusses some of the specifics of zinc application.
Phosphonate Products. A variety of foliar products containing phosphonate are recommended to be applied during the fall for Phytophthora control. 1-2 qts of a product containing phosphorous acid should be applied to the leaves. A single fall application has been shown to reduce tree loss from Phytophthora the following spring. More information can be found at the UC IPM web page for Phytophthora. Phosphonate products have varying compatibility with other nutrients. Many do not mix with products containing non-chelated calcium or zinc. Double check this with your PCA before tank mixing and spraying a product.