Written by Frank J. Peryea, Orchard Soils & Fruit Tree Mineral Nutrition Research Scientist (emeritus), TFREC, WSU Wenatchee. First appeared in the Good Fruit Grower in February 1995; revised in April 2018.
In springtime, a young man’s mind turns to thoughts of love. If you are an orchardist in Washington, your mind better turn to thoughts of plant nutrition, especially of zinc.
In 1944, Dr. Bob Lindner (USDA, Wenatchee) and Mr. Bill Luce (Washington State College Cooperative Extension, Yakima) reported that “…it is generally recognized in the fruit growing regions of the Pacific Coast that ‘rosette’ or ‘little leaf’ is second only to nitrogen deficiency in limiting growth of trees”. Rosette and little leaf are visual symptoms of zinc (Zn) deficiency. Most Washington growers have seen these symptoms in their own or a neighbor’s orchard, particularly during cool moist springs. Fortunately since 1944 we have developed management practices to minimize the effects of Zn deficiency; however, failure to use Zn fertilizers will almost guarantee the reappearance of rosette and little leaf.
Zinc deficiency symptoms are usually most obvious in the springtime on last year’s growth. It is characterized by small, narrow leaves (little leaf) that appear similar to leaves injured by glyphosate herbicide, but with less cupping and crinkling and usually occurring higher up in the tree. Often there is blind wood in the middle portion of shoots and a cluster of normal leaves at the terminal end of affected limbs (rosette). Interveinal chlorosis may be present on leaves, particularly of stone fruits. Branches will die back in severe cases. In many mild cases, trees will grow out of these symptoms as soils become warmer and drier and air temperatures increase.
The association of little leaf and rosette with Zn deficiency was discovered by pure serendipity. Between 1907 and the late 1920s, little leaf and rosette had devastated many orchards in the Pacific Northwest and California. The only (and somewhat enigmatic) treatment that seemed to prevent the symptoms was to grow alfalfa as a covercrop. Then in 1931, Chandler, Hoagland, and Hibbard (University of California, Berkeley) reported that soil applications of ferrous sulfate from certain sources cured the symptoms. Chemical analyses revealed that the effective materials were contaminated with Zn. Further tests with pure Zn salts confirmed that Zn was the answer to the problem.
Since that time, numerous methods of applying Zn to fruit trees have been evaluated, including soil applications, trunk injection, and foliar sprays. Soil applications are usually ineffective unless very high rates are used, causing risks of tree injury or death and permanently contaminating the soil with phytotoxic levels of Zn. Although Zn is an essential nutrient for animal and plant health, it is also a heavy metal and can become toxic if present in the environment in excessive amounts. Trunk injection of Zn solutions was found to be very effective at reducing Zn deficiency symptoms; however, secondary factors preclude widespread use of this practice. Trunk injection is time-consuming, requires specialized injection equipment, and may cause severe injury to the tree at the point of injection and sometimes death of the whole tree because of phytotoxicity.
There are several Zn compounds that are soluble enough to be used for fertigation. I discourage fertigation with potentially phytotoxic elements such as Zn because of risk of soil contamination and phytotoxicity resulting from broken or leaky irrigation lines and from trunk interception of Zn-containing irrigation water.
The most common method for applying Zn is to spray the trees with Zn solutions. The preferred timing of sprays is late dormant (stone fruits), silver-tip (apples and pears), and postharvest (all tree fruits except for apricot). Apparently, there was considerable injury resulting from postharvest sprays to apricots some time in the past. There is some evidence that dormant sprays are more effective than postharvest sprays. Zinc sprays can be applied at both times if deficiency persists and may actually be required for sweet cherries, which are highly susceptible to Zn malnutrition. Zinc sprays should be avoided during the growing season unless definitive deficiency symptoms appear during the summer. There is always risk of fruit russetting when Zn compounds (even some chelated forms) are applied directly to fruits. Because boron deficiency can also cause a type of rosette, it is worthwhile to get independent confirmation that Zn deficiency is the actual culprit.
Ground sprayer applications (dilute, or to drip) of Zn are more effective than are airplane applications. It is essential to cover all of the tree structure or leaf tissue because of the limited absorption and within-tree mobility of Zn. Ground sprayers give better coverage of the lower and inside portions of trees where weak spurs, lower Zn concentrations, and smaller fruit are more likely to occur.
There are numerous sources of Zn that are suitable for sprays. The two major inorganic forms are Zn sulfate and Zn oxysulfate (sometimes called basic Zn sulfate). These compounds appear in a number of commercial formulations. Zinc sulfate, in particular, can damage buds and fruits, which is why it should only be applied before green tissue appears in the spring or after fruit harvest. Zinc sulfate is also incompatible with dormant oil or lime-sulfur, which complicates optimal timing of late dormant sprays. Late fall applications of Zn sulfate have been associated with bud injury, particularly on high vigor trees. There is a widespread belief that late fall applications of Zn sulfate may predispose apple trees to winter injury. I have not been able to locate any data in the scientific literature to confirm or refute this belief. Zinc oxysulfate is less likely to damage plant tissues and can be used after budbreak at lower rates. Zinc oxysulfate may be compatible with dormant oil if applied at lower rates (check the label); however, some growers report that the Zn rates in Zn oxysulfate-oil mixes are too low to prevent the appearance of deficiency symptoms if no other Zn is applied. Both Zn sulfate and Zn oxysulfate can be used at very low rates on non-bearing trees if growing season sprays are necessary. Because both materials are highly corrosive to equipment, it is essential to wash all machinery carefully with fresh water when you are done spraying. Zinc oxide has also been used as a Zn source; however, it was dropped from WSU recommendations after severe fruit russetting, foliage injury, and leaf and fruit drop were associated with its use in the first cover spray in 1953. Zinc oxide appears to be less effective than Zn sulfate or Zn oxysulfate in dormant and postharvest sprays.
There are numerous chelated forms of Zn that are marketed for foliar sprays. Most studies have shown that chelated Zn products are no more effective than Zn sulfate for controlling Zn deficiency symptoms. The advantage of Zn chelates is that they are much less likely to cause fruit russet and are therefore the preferred form of Zn for growing-season sprays to bearing trees. Not all Zn chelates are safe to use in foliar sprays. For example, Stiles and Reid (Cornell University, Ithaca, NY) reported that sprays of NTA-Zn chelate caused severe defoliation of fruit trees. Summer sprays of Zn chelates should not be used without reasonable justification. In 1988, I induced Zn phytotoxicity by applying a Zn chelate at the labeled deficiency rate to apple trees that lacked Zn deficiency symptoms.
The rates suggested by Washington State University (WSU) vary depending on the presence or absence of Zn deficiency symptoms. If little leaf and/or rosette are actually present, use deficiency rates. The deficiency rates for Zn sulfate or Zn oxysulfate are 14 pounds actual Zn per acre (dormant timing sprays) and 9 pounds actual Zn per acre (postharvest). Use the label rate if applying Zn chelates late dormant or postharvest; however, the label rate often will not supply enough Zn for Washington orchard needs. If sprays are applied during the growing season, use only 2 to 4 lb actual Zn as Zn sulfate or Zn oxysulfate per acre (non-bearing trees only) or the label rate of an appropriate Zn chelate (bearing or non-bearing trees).
Because Zn deficiency is so widespread in Washington orchards, WSU recommends that Zn be applied every year in the form of Zn maintenance sprays even if little leaf or rosette are absent. The maintenance rates for Zn sulfate or Zn oxysulfate are 2 to 4 lb actual Zn per acre for both late dormant and postharvest sprays. Use the label rate if Zn chelates are applied. There is little justification for applying a maintenance Zn spray during the growing season.
Tree Zn status is commonly assessed using leaf analysis. Although WSU has published guidelines for interpreting leaf Zn concentrations: deficiency (less than 15 ppm) and adequacy (25 to 60 ppm), these criteria are difficult to interpret without accessory information. In regions where Zn sprays are not used, it is often possible to accurately determine tree Zn status on the basis of leaf analysis alone. In areas where widespread use of Zn sprays occurs, such as the western US states, leaves become contaminated with Zn in sprays and wind-blown dust. Because Zn on the surface of leaves is very difficult to remove, analysis of leaves from these areas usually generates a value that includes Zn within the leaf (and may, therefore, be physiologically active) and Zn that is simply sitting on the leaf surface and is effectively inert. This sampling artifact causes the range of leaf Zn concentration found in trees exhibiting little leaf and rosette to overlap with that found in trees without symptoms. To further complicate matters, vigorously growing trees in perfect health often have lower concentrations of Zn (and other nutrients) in leaves because the Zn is distributed within a greater mass of new plant tissue, a phenomenon called biomass dilution. Finally, leaves that have been recently sprayed with Zn can show Zn concentrations in excess of 100 ppm. Although such high concentrations would normally raise concern about phytotoxicity, under this circumstance only a fraction of the measured Zn actually resides within the leaf and most of the Zn will eventually be washed off the leaf surface without harm by rain or irrigation water.
It is even more difficult to interpret soil Zn test results for fruit trees. The most commonly used method for soil Zn is called the DTPA-TEA-CaCl2 extraction. This method was developed for use on high pH soils and was validated for wheat in central Washington. The resulting 0.8 ppm Zn critical value appears to have been adopted without change for orchard cover crops and sods. Unfortunately, the performance and Zn requirements of field-grown fruit trees are poorly correlated with soil Zn tests. I would therefore not base Zn fertilizer requirements on soil Zn test results. The best use of soil Zn test values appears to be as semi-quantitative indicators of long-term trends in soil fertility.
So, given all this information and qualifiers, how is a grower to manage Zn nutrition in deciduous tree fruit orchards? First, in central Washington orchards at least, assume that your trees will eventually suffer from Zn deficiency if Zn fertilizers are not used. Second, inspect the trees for the Zn deficiency symptoms of little leaf and rosetting. If the symptoms are absent, apply Zn sprays using the maintenance rates and the timing that best fits your fruit tree species and cultivars and work schedule. Apply the Zn maintenance sprays every year and use leaf analysis to fine-tune the Zn maintenance rate. If the Zn maintenance program is being used and leaf Zn concentrations are consistently below 15 ppm, increase the Zn rate by a small amount each year until leaf Zn consistently ranges between 25 and 50 ppm. If the tree inspection shows that Zn deficiency symptoms are present, apply Zn sprays using the deficiency rates and the timing(s) that best fits your fruit tree species and cultivars and work schedule. Don’t apply summer sprays unless you are sure that the symptoms are not going to disappear with warmer weather. Continue the Zn deficiency program until the symptoms disappear, then switch to the Zn maintenance program.
It’s important to remember that fruit trees contain large internal reserves of nutrients that make them less responsive than annual crops to changes in fertilizer practices. This actually gives us some flexibility in rates and timing; however, always keep in mind that application of Zn products directly to fruit always creates risk of fruit injury, and that fertilizer companies may not guarantee their products if use does not conform to the label.
Because every orchard circumstance is different, I encourage you to conduct your own trials in a small portion of your orchard to verify the effectiveness of a new management practice before you adopt it wholesale. Application of the WSU guidelines, coupled with your knowledge of and experience with your own orchard, will permit you to develop a Zn nutrition management program that is environmentally sound, horticulturally effective, and financially rewarding.