The following information has changed thinking on foliar zinc analysis and recommendations. It is based on the following resources: observations of zinc deficiency in many orchards and consistently high levels of phosphorous in soils; some unexplained disorders on peaches; the importance of zinc in calcium metabolism; conversations with Cornell Professor Emeritus Warren Stiles; and Orchard Nutrition Management (Information Bulletin 219, Cornell Cooperative Extension).
My analysis and recommendations may differ slightly than those listed in the Penn State foliar analysis report. According to Dr. Stiles' publication, many of the peach leaf samples evaluated in 2000 foliar sampling are either low or deficient in zinc. This also is not mentioned in our 2001 Tree Fruit Production Guide for New Jersey.
According to Dr. Stiles and Dr. Reid's publication, interpretation of leaf zinc levels is complicated by zinc-containing materials in foliar applications and by interactions with phosphorous. If no foliar sprays containing zinc have been applied, 35-50 ppm indicate an adequate zinc status, 20-35 ppm a low zinc status; less than 20 ppm a zinc deficiency. Relying strictly on these levels may be misleading for two reasons: 1) growth is reduced as zinc becomes limiting. This limited growth results in an accumulation of zinc to higher concentrations than would occur with normal growth (probably not a problem in peach trees that are growing well); 2) high levels of phosphorous tend to reduce the availability of zinc within the tree as the result of the formation of inactive zinc precipitates. When zinc is limited, the reduced growth tends to result in higher concentrations of phosphorous within the leaf tissue, further accentuating the problem.
Dr. Stiles and Reid suggest an evaluation of the ratio of phosphorous to zinc in the leaf tissue to provide a second means of determining relative zinc status. This ratio is calculated by dividing the ppm of phosphorous by the ppm zinc (since leaf phosphorous is represented by percentage, simply move the percentage point four places to the right to get ppm. For example, if your leaf P were 22 this would equate to 2200 ppm phosphorous). If your zinc level is 22 ppm then divide this into 2200 ppm. Your ratio would be 100 to 1 zinc. A ratio of 150 or more indicates zinc is deficient. A ratio of 100 or less indicates an adequate supply of zinc. Between 100 and 150, zinc is low. On your soil analysis report I will indicate the ratio by assigning an R or ratio number for zinc.
These interpretations cannot be applied directly to samples that have been sprayed with zinc contaminated materials because of the presence of inactive zinc contaminants. Ziram is a zinc salt derivative of dithiocarbamic acid and labeled on all tree fruits. It will provide some zinc to the tree but can also be a leaf surface contaminant when taking leaf samples for analysis. The mancozeb fungicides also contain the zinc ion but in small quantities. They are not labeled for stone fruits.
According to Stiles and Reid, soil surface applications of zinc fertilizers have not been sufficiently effective to be recommended. Availability of zinc is reduced by high soil pH, high levels of soil phosphorous, and low soil temperature.
Foliar application is effective for supplying zinc to established fruit trees. One of the most critical periods when a zinc shortage may seriously impair tree performance is between bud break and fruit set. A zinc shortage at this time often results in poor growth of the leaves and new shoots, as well as abnormal development of pollen tubes, ultimately resulting in poor seed set in apples. Later in the season, the effects of limited zinc are small fruit and/or poor color development. Zinc is not readily mobile within the tree and applications must be thorough and timely for optimal response.
Various methods of applying zinc are available, the most common being late dormant sprays of zinc sulfate, summer applications of zinc chelates or other materials, and post harvest applications of zinc-containing products. Zinc-containing fungicides have been partially effective in established orchards, but have not met total requirements or completely corrected a zinc deficiency.
Application of zinc sulfate (20 or 36 percent zinc) at the late-dormant period for stone fruits, or dormant to silver-tip stage for apples and pears is effective in supplying part of the total zinc requirement (the most common tested in New Jersey). These materials are applied at approximately 3.5-5 pounds of actual zinc per 100 gallons as dilute sprays, alone or with fresh hydrated lime as a safener. These sprays should be diluted (not over a 2X tank-mix concentration) to obtain adequate coverage of the buds and shoot surfaces. Oil sprays following zinc sulfate dormant sprays increase penetration of the zinc sulfate into buds and spur tissues and have resulted in extensive damage. Likewise, freezing weather (frosts) occurring within 2-4 days before or after the dormant spray has increased the uptake of zinc sulfate and resulted in killing of spur systems on apple trees. Caution: Excess zinc and high rates of application can cause severe injury to shoots, buds, leaves and fruits. When using zinc sulfate crystals, be certain all crystals are dissolved before spraying, or injury can occur.
Zinc sulfate is highly corrosive. After use, thoroughly rinse spray tank, pump, lines and nozzles.
Sprays of EDTA-zinc chelates provide a greater degree of safety and can be applied during later stages of tree growth. Two or more sprays of the EDTA-zinc chelates applied at 10 to 14 day intervals beginning 1-2 weeks after petal fall are frequently required in a zinc maintenance program. Several formulations of chelated zinc are available and should be used at rates recommended by the manufacturers (read the label because there are many zinc products with high percentage of zinc that may damage your fruit). A prebloom application may also be needed to stimulate early bud, leaf, and shoot development if the zinc status of the orchard is marginal to deficient. In any case, foliar applications of zinc are necessary on an annual basis and not as a one-time curative treatment.
CAUTION: Several forms of chelated zinc are available, many of which have not been thoroughly evaluated for use on fruit crops. For example, NTA-zinc chelate is used safely in fertilizers applied to the soil for some crops and through trickle irrigation systems for tree fruits, but this material has caused severe defoliation when used as a foliar spray. Likewise not all EDTA zinc chelates are the same, for example, Zinche-10 has caused severe fruit injury to apples apparently because of the non-reacted zinc sulfate in the product.
Zinc oxide has been suggested for foliar application but has not improved tree performance. Leaf samples from trees sprayed with zinc oxide or similar materials may show high levels of zinc, but a high percentage of this zinc is apparently present as physiologically inactive contamination.
Postharvest tree sprays of zinc materials have shown variable results. In some cases, 3-6 pounds of 36 percent zinc sulfate in combination with 5 pounds of urea per 100 gallons as a dilute spray has been effective in mature apple orchards following harvest. In other cases, this and other materials applied in this manner have not justified recommendation.
Some special considerations when spraying foliar applications of zinc according to Stiles and Reid are:
- Generally urea, Solubor, EDTA-zinc chelates and Epsom salts are compatible. They have been used safely together in pre-bloom sprays on apples and pears. Many orchardists prefer not to add all in one tank mix but rather to spread them over three sprays;
- Unless the compatibility of the pesticide with the zinc compound is known, it is better to spray it alone.
Applications of liquid chelated zinc through a trickle irrigation system will effectively supply this element. Research trials suggest that 8-10 weekly applications of EDTA-zinc (6 percent ZN) at rates sufficient to supply 10 to 15 pounds or more of zinc per acre per season may be necessary.
The following web site has some excellent information on nutrient sprays on fruit trees: www.tfrec.wsu.edu/Horticulture/nutspray.html.
Stiles, W.C and W. S. Reid. 1991 Orchard Nutrition Management, Cornell Cooperative Extension Information Bulletin 219, 22 pages.
The article originally appeared in the February 13, 2001 issue of the Plant and Pest Advisory, Fruit Edition.