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Micronutrients

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Managing Micronutrients in Pacific Northwest Orchards

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 March 1994; revised in February 2019.

Introduction

A plant micronutrient is an essential mineral element that is required in relatively small amounts by plants, typically less than 100 mg/kg (ppm, parts per million) in plant shoot dry matter. An essential mineral element by definition must: (a) be present for a plant to complete its life cycle; (b) have a function that cannot be fulfilled by another mineral element; and (c) be directly involved in plant metabolism or be required for a distinct metabolic step.

The biogeochemistries of micronutrients in orchard environments are complex. Plant roots only take up nutrients that are dissolved in the soil solution (water portion of soil). Soil minerals and organic matter serve as micronutrient reservoirs that replenish soil solution micronutrients absorbed by plant roots or lost because of leaching. Micronutrients in minerals tend to be released very slowly. The forms of soil micronutrients that are likely to influence plant nutrition, called available micronutrients, are therefore those that are dissolved in the soil solution, adsorbed (bound) to mineral surfaces, and organic matter. Mineral surfaces and organic matter also act as sinks – they can remove micronutrients from the soil solution when soil solution levels increase because of micronutrients added to the soil in fertilizer or irrigation water. This property can be either deleterious or beneficial, depending on the circumstances. For example, Zn added to soil can be so tightly bound by adsorption that it is not available to plant roots. On the other hand, incorporation of added B into organic matter helps to buffer against B toxicity, reduces loss of B by leaching, and provides a slow steady release of B to the soil solution.

Micronutrients are introduced to orchards in fertilizers, applied either in foliar sprays or to the soil surface, or as natural constituents of irrigation water. Micronutrients are recycled within orchards by the breakdown of orchard plant residues, such as tree leaves, dropped fruits, and pruning waste, and cover crop clippings. Permanent losses of micronutrients include the amounts removed in fruits and pruning waste carried out of the orchard, and by leaching.

Although the amounts of micronutrients in plant tissue are small, they are as critical as macronutrients for plant growth and development. Most micronutrients function principally as constituents or activators of enzyme molecules. The absence or reduced activity of these enzymes severely restrict the ability of plants to carry out necessary biochemical processes.

Individual Micronutrients

The following sections describe individual micronutrients, their function in plants, typical visual symptoms associated with their deficiency or toxicity in deciduous tree fruit crops, and general guidelines for managing microelement nutrition in deciduous tree fruit orchards in the Pacific Northwest (PNW) region. These guidelines are based on my personal experience and my review of published and unpublished reports by scientists who have conducted mineral nutrition research in deciduous tree fruit orchards. Because soils, plants, and environmental conditions vary considerably in the PNW, using management practices based on the guidelines may not always produce the desired results. Furthermore, there undoubtedly are some practices not listed that may work as well or better but just need testing and publicity. I strongly recommend that growers conduct their own trials in a small portion of their orchards to verify the effectiveness of a new management practice before it is generally adopted.

Multiple micronutrient deficiencies commonly occur simultaneously; for example, Zn and Fe deficiency often appear together on trees grown on calcareous soils. Deficiency of one nutrient can also mask that of another; for example, eliminating B deficiency by foliar sprays may reveal a hidden Zn deficiency. External factors may strongly influence uptake, transport, and accumulation of all nutrients within a tree. Improper water management, root injury, winter injury, rootstock incompatibilities, diseases, and insect damage can all produce nutrient deficiencies that may not be eliminated by simply applying more nutrients. Complex nutritional problems will likely require carefully considered, site-specific solutions.

Iron (Fe)

Iron is a component of many plant proteins and enzyme systems. It is required for nitrate and sulfate reduction, N2 assimilation, and energy production. Iron functions as a catalyst or part of an enzyme system associated with chlorophyll production (hence, chlorosis as a deficiency symptom). It is possibly involved with protein synthesis and root tip meristem growth.

Fe deficiency symptoms

Iron deficiency is common in the inland PNW, and is usually associated with: (a) calcareous soils (pH 8 or higher); and/or (b) excessive soil moisture caused by over-irrigation, high water table, or restricted drainage because of tight subsoil; and/or (c) high bicarbonate irrigation water.

Symptoms include yellow leaves with a fine network of green veins; the entire leaf may be yellow and marginal necrosis may be present in severe cases. Chlorosis often appears first on developing shoot tip leaves early in the season, particularly during cold wet weather. All or only part of a tree may be affected.

Correcting Fe deficiency

Soil and water management
  1. improve drainage by soil profile modification or installing tile drains; and/or
  2. optimize irrigation water amounts and scheduling; and/or
  3. subsurface banding of Fe-containing chemical amendments.
Airblast foliar sprays (effect usually only temporary)
  1. apply multiple sprays of Fe-chelates at labeled rates during the growing season.
Trunk injection with Fe-compounds
  1. can be effective if done properly but usually is time-consuming and requires special equipment.  It can cause severe tree injury and death if done improperly.  We are testing new procedures for trunk injection that may be safer.

Manganese (Mn)

Manganese is essential in photosystem II for splitting water and generating molecular O2, the first step of the electron transport chain of the photosynthesis process (hence, chlorosis as a deficiency symptom).  It is a component of enzymes that protect the photosynthetic apparatus from injury by superoxides.  Manganese replaces magnesium in activating several enzyme systems and activates indoleacetic acid (IAA) oxidases.

Mn deficiency symptoms

Manganese deficiency is rare in Washington but common in other arid areas. This may reflect a lack of research on Mn nutrition in Washington. Manganese deficiency is usually associated with high soil pH and often with Fe deficiency.  The principal symptom is leaf chlorosis, usually starting near the leaf margin and moving inward towards the midrib. Veins and adjacent tissue stay green; other leaf areas are yellow. Symptoms usually occur on older leaves but may affect newer leaves in severe cases.

Correcting Mn deficiency

Soil maintenance program
  1. maintain slightly acid to neutral soil pH (pH 6.5 to 7.0).
Foliar sprays (only if Mn deficiency is confirmed)
  1. apply 2 lb Mn as MnSO4 per acre in the early growing season; or
  2. apply label rate of Mn-chelate in the early growing season.

Mn toxicity symptoms

Manganese toxicity can occur on deciduous fruit trees, principally on young, spur-type ‘Delicious’ apple trees. It is most likely to occur at soil pH values below 5 but can occur at higher pH in poorly drained or compacted soils where inadequate aeration promotes Mn2+ formation.

The principal symptom is bark measles, also called internal bark necrosis. Bark measles is typified by smooth, raised pimples in young bark, underlain by small dark-brown spots. The surrounding tissue has a water-soaked appearance. Over time, the pimples develop sunken patches, and the bark begins to crack and scale. Boron deficiency can also produce a similar-appearing form of bark measles.

Correcting Mn toxicity

Soil management
  1. reduce causes of soil acidification; eg., lower nitrogen rates, change nitrogen source; and
  2. lime soil to pH 6.0 or greater.

Boron (B)

On a molecule-for-molecule basis, the plant requirement for B is higher than that for any other micronutrient. Boron is required for synthesis of one of the bases (uracil) required for forming RNA and energy-rich phosphate compounds. It is important in cellular activities (division, differentiation, maturation, respiration, growth, etc.). Boron is associated with pollen tube germination and growth, and it improves pollen tube stability. Because B deficiency is so common in Washington orchards, routine maintenance applications of B are recommended. Multiple applications of B may be required in orchards on very sandy, low-organic-matter soils. Caution is required – B can become toxic to plants at relatively low concentrations.

Boron deficiency symptoms

Apple and pear vegetation
  1. bark measles
  2. dead terminal buds and shoot dieback
  3. shortened internodes, leading to witches-broom effect
  4. dwarfed, stiff, thick brittle leaves, often with smooth margins.
Apple blossoms and fruit
  1. blossom blast
  2. small, flattened, or misshapen fruit
  3. drought spot
  4. internal cork
  5. cracking and russet
  6. premature ripening
  7. increased fruit drop
  8. seed count may be low
Pear blossoms and fruit
  1. blossom blast
  2. reduced fruit set
  3. external and internal corking of fruit
  4. fruit cracking.
Stone fruit vegetation
  1. dead terminal buds and twig dieback
  2. retarded shoot growth
  3. dwarfed, narrow leaves with upturned edges, often with thickened midribs; the leaves may blacken and fall off.
Stone fruit blossoms and fruits
  1. buds fail to break, or break and fail to develop normally
  2. blossom blast
  3. cracking, deformation, shriveling of fruit
  4. internal and external browning of fruit
  5. cork formation around pit and in flesh
  6. differential ripening within a single fruit
  7. increased fruit drop.

Correcting B deficiency

Soil applications (only if soil test value is low; sample to 3 feet)
  1. 3 to 5 lb B per acre uniformly broadcast or sprayed over the soil surface (timing does not appear to be critical except for aircraft application, which should be made only during the dormant season to ensure good uniformity of distribution.).
Airblast foliar sprays
  1. apply a single spray of 1.0 lb B per acre, postharvest, prebloom, or when B deficiency symptoms appear during the growing season.

Preventing B deficiency

 Soil applications (only if soil test value is low; sample to 3 feet)
  1. 3 lb B per acre once every three years, uniformly broadcast or sprayed over the soil surface (timing does not appear to be critical except for aircraft application); or
  2. on coarse-textured soils, 1.0 lb B per acre annually, broadcast or sprayed over soil surface (timing does not appear to be critical except for aircraft application).
  3. Aircraft applications of B should be made only during the dormant season to ensure good uniformity of distribution.
Airblast foliar sprays, irrigated areas
  1. (apples and stone fruits) apply a single annual spray of 0.5 lb B per acre, at prebloom, first cover, or postharvest;
  2. (pears) apply a single annual spray of 0.5 lb B per acre, postharvest or at first to full white.
Airblast foliar sprays, non-irrigated areas
  1. (apples and stone fruits) apply a single annual spray of 1.0 lb B per acre, at prebloom, first cover, or postharvest;
  2. (pears) annually, apply one spray of 0.5 lb B per acre, postharvest or at first to full white, followed by a second spray of 0.5 lb B per acre in the first or second cover.

B toxicity symptoms

Excessive amounts of B can cause plant injury and death. Boron toxicity is rare in Washington orchards but can be induced by over-fertilization. The cases on record resulted from accidentally applying B fertilizer in place of nitrogen fertilizer, and from overlapping aircraft applications of B. Boron toxicity is usually alleviated by leaching the soil with large amounts of water. Liming may help in acid soils.

Apple and pear vegetation
  1. dead terminal buds and shoot dieback
  2. marginal leaf chlorosis and necrosis
  3. defoliation.
Apple fruit
  1. reduced or no yield
  2. increased internal breakdown after harvest
  3. increased watercore after harvest
  4. premature ripening.
Pear fruit
  1. reduced or no yield.
Stone fruit vegetation
  1. tips of new shoots wither and dieback
  2. cankers and gummosis develop along stems
  3. brittle, partially deformed leaves; may have small necrotic spots along the midrib that drop out, creating a shot-holed effect
  4. small cankers on the underside of midribs and petioles
  5. enlarged nodes at bases of buds.
Stone fruits
  1. reduced or no yield
  2. fruit malformation
  3. no pit development
  4. early maturation
  5. poor flavor.

Zinc (Zn)

Zinc is a component of at least four plant enzyme systems; it is specific for carbonic anhydrase. It activates various types of enzymes that influence carbohydrate metabolism (not the principal reason for Zn deficiency symptoms) and protein synthesis. Zinc influences auxin metabolism, particularly IAA (reason for stunted growth and “little leaf” disorder).

Zn deficiency symptoms

Zinc deficiency is common in the PNW, particularly after winter damage or cool wet springs. Usually, only part of a tree is affected. The principal symptom is “little leaf” and “rosette”, typified by small narrow leaves, blind wood on last year’s growth, and clusters of normal leaves at the terminal end of affected limbs. Interveinal leaf chlorosis may also occur, particularly on stone fruits. In mild cases, symptoms may disappear as the growing season progresses. Because Zn deficiency is so common in Washington orchards, routine maintenance applications of Zn are recommended.

Correcting Zn deficiency

Soil applications of Zn fertilizer are usually ineffective because the Zn is tightly bound by the soil.

Foliar sprays
  1. apply 14 lb Zn per acre, late dormant; or
  2. apply 9 lb Zn per acre, postharvest (not on apricots); or
  3. apply label rate of Zn-chelate, early growing season; or
  4. (non-bearing trees only) apply 2 to 4 lb Zn as ZnSO4 per acre, early growing season.

Multiple spray applications (eg., dormant and postharvest) may be required to eliminate deficiency symptoms.

Preventing Zn deficiency

Foliar sprays (maintenance sprays)
  1. apply a single annual spray of 2 to 4 lb Zn per acre, late dormant or postharvest (consult label for compatibility with oil).

Zn toxicity

Excessive application of inorganic Zn-salts or Zn-containing sewage sludges can induce Zn phytotoxicity. Symptoms include leaf chlorosis and retarded growth, attributed to Zn-induced Fe deficiency. Remediation of excessive Zn levels in soil is difficult; seek professional help.

Copper (Cu)

Copper is a constituent of chloroplast proteins required for electron transport in the photosynthetic process (hence, chlorosis as a deficiency symptom). It participates in protein and carbohydrate metabolism and N2 fixation. Copper is part of oxidase enzymes that reduce molecular O2, including those required for lignin formation (shepherd’s crook symptom). It is required for seed and fruit formation; deficiency reduces pollen viability.

Cu deficiency symptoms

Copper deficiency has been considered uncommon in the PNW, where it has usually been associated with trees planted on barnyard or feedlot soils that are high in organic matter. There is anecdotal evidence that Cu deficiency is becoming more common because of increased purity of fertilizers and reduced use of Cu-containing fungicides.

The principal deficiency symptom is “wither tip”.  Shoots grow normally early in the season. About mid-June, terminal leaves turn yellow, wither and die. Twigs with dead and withered tips occur over part or most of the tree. Copper deficiency causes poor seed set in wheat; we are researching if the same effect occurs in apples.

Correcting and Preventing Cu deficiency

Airblast foliar sprays for deficiency
  1. apply 1 lb Cu as CuSO4 per acre, postharvest (can cause severe russet if applied to fruit); or
  2. apply label rate of Cu-chelate or Cu-oxysulfate (basic Cu sulfate), during the growing season or postharvest (Note: this practice is poorly documented).
Maintenance program
  1. occasionally use a Cu-containing fungicide.
  2. add a small amount of Cu sulfate or Cu-oxysulfate to postharvest nutrient sprays or to the dormant zinc spray (Note: this practice is not extensively tested).

Cu toxicity

Excessive application of Cu-containing fertilizer, manures, sewage sludges, industrial sludges, or fungicides can induce Cu phytotoxicity. Symptoms include leaf chlorosis, reduced shoot growth, abnormal root development, and wilting. The symptoms are attributed to Cu-induced Fe deficiency. Remediation of excessive Cu levels in soil is difficult; seek professional help.

Molybdenum (Mo)

Molybdenum is a component of two major plant enzyme systems:  (a) nitrogenase, which converts dinitrogen gas to ammonia; and (b)  nitrate reductase, which converts nitrate to nitrite. The nitrite is then converted to ammonium by a different enzyme.  Molybdenum requirement is reduced by increased availability and utilization of ammonium.

Mo deficiency symptoms

Molybdenum deficiency has never been documented in deciduous tree fruits under field conditions. Leaf symptoms on apples and apricots grown under hydroponic conditions included: (a) uniform chlorosis of younger leaves; (b) tip and marginal burning of older leaves, followed by defoliation; and (c) high nitrate levels in the leaves with marginal necrosis.

Chlorine (Cl)

Chlorine is involved in the evolution of oxygen in photosystem II of the photosynthetic process.  It raises cell osmotic pressure, affects stomatal regulation, and increases plant tissue hydration.

Cl deficiency symptoms

Chlorine deficiency has never been documented in deciduous tree fruits under field conditions.  Symptoms in other plant species include chlorosis of younger leaves, wilting, and reduced growth.

Specific chloride toxicity

Chlorine occurs as the chloride anion (Cl-) ion in soil, water, and plants. Excessive amounts of chloride and other salts in soil create osmotic stress, resulting in nonspecific salt injury in most plants. Certain plant species, particularly woody plants, are injured specifically by high chloride levels, a phenomenon called specific ion toxicity. Specific chloride toxicity has been documented on stone fruits in California but not in the PNW. Deciduous fruit trees in the PNW have been injured by excessive applications of chloride-containing fertilizers; presumably, part of the injury may have been caused by specific chloride toxicity. Symptoms include premature yellowing of leaves, burning of leaf tips and margins, and bronzing and abscission of leaves.

 

Washington State University