Post Harvest Foliar Sprays

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.

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12 thoughts on “Post Harvest Foliar Sprays

  1. I have a young almond orchard in Fresno County planted in January 2011. The leaf analysis indicated Zinc levels to be 12 PPM. Zinc levels below 20 are deficient. What would you recommend?
    A.)Spray a foliar to increase the levels of Zinc for the young trees
    B.)Don’t spray any foliar. Since the orchard is young and there will be no crop for a couple years.
    Thank You.

  2. I would make the spray. Even though the orchard is young, a micronutrient deficiency can slow the new growth of the young tree, reducing yield for the coming years. There are multiple sources of zinc, but the best bang for the buck is zinc sulfate. Making an application at this time of year, I would plan for 5 – 10 pounds of zinc sulfate in 100 gallons of water per acre.

  3. A reader asked a question regarding using aerial application to fly on zinc sulfate in order to drop the leaves. Here is my answer:

    Yes, it is possible to make an aerial zinc application. Talking to a few growers who use aerial applicators, it is just as effective. It is important to make sure that the same rate is applied across the acreage.

    1. @ Anonymous – I spoke with Patrick Brown regarding boron in walnut. He informed me that a pre-bloom spray (before the tree breaks) will increase crop set. I hope you dont receive this information too late.

  4. If deficient, walnut trees will benefit from boron and zinc applications in the spring during the rapid phase of growth.

    A recent study has shown increased yield when sprays with zinc and boron were made in the post harvest and spring, but older studies suggest that the spring timed spray may be more important.

    Since boron can be toxic, it is critical to know if the levels within the tree are adequate before spraying. If deficient with zinc, it is important to work to remedy the situation by increasing soil zinc levels either through zinc applications or soil acidification.

    Zinc deficiency within almond is usually not due to limiting zinc within the soil, but rather the poor uptake of zinc by almond rootstocks. This rootstock uptake problem does not occur within walnut.

  5. You indicate that foliar sprays of boron can increase crop set regardless of hull boron content. Does this apply to fall foliar sprays too? Or just pink bud applications?

    1. Hull boron content will be affected by soil applications (whether by water or fertilizer) and in-season foliar sprays. The boron applied after fruit set will mobilize and move into the hull, a relatively large boron sink. This movement is greatest during periods of rapid hull growth. Since boron is needed within the buds to provide proper pollen tube development at bloom, it must be present in the wood/buds. Spraying it as a foliar in-season does not guarantee that boron will remain within the wood since most (if not all) of it goes into the hull. Once the hulls are removed, boron applied will be able to remain within the wood. I hope that is clear(er)…

        1. For the most part, yes. If the hull concentration is extremely high – over 150 ppm, a foliar spray may not be necessary. In some years, it may be helpful, but the results tend to become inconsistent on a year-to-year basis once the hull B content is over 150. The post harvest timing has been shown to increase boron levels within the wood. In talking with Dr. Patrick Brown, however, he has raised concerns that boron may leach out of the wood if wet, foggy winters are experienced – which is why he prefers delayed dormant through pink bud applications. I would probably make the application based on the farmer’s practice – if planning a delayed dormant spray (i.e. scale, PTB), include it there, if not, get it on during the post harvest.

  6. You said that fall applied of lo –bi- urea reduce bacteriel canker and with bud drop within almond,so how much at rate of lo-biuret urea could to reduce b.c. without bud drop on almond.. Are there any trial involved applied rate of urea for bacteriel canker without damage of shoot,bud on almond orchard and have they yet test this sprays for almond.(I heard that they tested for peach).What do you recommend?

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