Boron Nutrition in Deciduous Tree Fruit Orchards

Written by Frank J. Peryea, Orchard Soils & Fruit Tree Mineral Nutrition Research Scientist (emeritus), TFREC, WSU Wenatchee. Written in March 2004; revised in April 2018.

Introduction

Boron (B) is a microelement that is essential only in vascular plants and diatoms.

The essentiality of B resulted from evolution of xylem, passive transport of B in the transpiration stream, and accumulation of B to levels of metabolic significance in the shoot apices. Acquisition of an essential role for B in the normal functioning of the apical meristem served as a means of preventing B toxicity.

Roots originated from stems and have a similar requirement for B. Roots are more sensitive to B deficiency because they do not receive B through the transpirational stream and must absorb B from the surrounding medium.

The primary effect of B deficiency appears to be disruption of the normal functioning of the apical meristem (evidence suggests a fundamental role in pyrimidine metabolism), with changes in auxin metabolism, lignification, phenol accumulation, and sucrose transport being secondary effects.

Excessive accumulation of B can cause B phytotoxicity.

Boron Chemistry in Water and Soil

Adsorbed B and soil solution B are in equilibrium in soil, with most of the B occurring being adsorbed.

Most well-drained soils are low in B. High soil B occurs primarily in semi-arid and arid areas because of high water tables coupled with little or no drainage or because of additions of B in irrigation water.

Boron dissolved in water occurs primarily as monomers of orthoboric acid, B(OH)3. As pH increases, orthoboric acid spontaneously reacts with an additional water molecule to create a hydroxyborate anion, B(OH)4-. Boron species other than B(OH)3 and B(OH)4- can be ignored for most practical purposes in soils.

Boron is specifically adsorbed to clay minerals, hydrous metal oxides, and organic matter in soils. Boron can also be coprecipitated with calcium carbonate.

In low B soils, a large portion of total B is associated with the organic matter. Boron is released to the soil solution by mineralization of the organic matter and becomes available to plant roots.

Soil Boron Availability to Plant Roots

Plants respond mainly to the concentration of B in soil solution. Because B moves passively in the transpirational stream, plant uptake of B usually increases with increasing temperature.

Soil B availability decreases with:

Decreasing total soil B;
Increasing clay mineral, hydrous metal oxide, organic matter, and lime contents in soil;
Increasing pH above pH 6.5;
Very wet or very dry soils;
Increased leaching;
Cold soil temperature.

Foliar Boron Availability

Foliar application of B solutions is an efficient way to increase the B content of fruit trees.

The efficiency of B uptake by fruit surfaces is lower than that of leaves and does not appear to be influenced by the presence of lenticels. Boron uptake through bark is negligible.

Foliar B uptake increases the longer the applied B solution remains as a fine film on the leaf or fruit surface. Leaf and fruit injury can result from salt scorching on hot clear days when evaporation rates are high. Leaf injury can also result on cool humid days when evaporation rates are low because of B toxicity resulting from excessive uptake of B from foliar sprays.

Some of the B applied in foliar sprays appears to be washed from the tree to the soil surface by rainfall or sprinkler irrigation water where it can then be absorbed by tree roots.

Boron Deficiency Symptoms

Boron deficiency was very common in deciduous tree fruit orchards until the 1920s, when B was identified as an essential element and corrective fertilizer programs were developed and implemented.

Apples

Fruit symptoms:

Small, flattened or misshapen fruit;
Drought spot;
Internal cork;
Cracking and russet;
Premature ripening;
Increased fruit drop;
Seed count may be low.

Vegetation symptoms

Internal bark necrosis (bark measles);
Dead terminal buds and shoot dieback, sometimes with witches-
broom effect as sidebuds break and start developing
Shortened internodes;
Dwarfed, stiff, thick, brittle leaves with smooth margins.

Pears

Blossom symptoms

blossom blast.

Fruit symptoms

Reduced fruit set;
External and internal cork;
Cracking.

Vegetation symptoms

Internal bark necrosis (bark measles);
Dead terminal buds and shoot dieback, sometimes with witches-
broom effect as sidebuds break and start developing;
Shortened internodes;
Dwarfed, stiff, thick, brittle leaves with smooth margins.

Apricots, Peaches, Cherries

Fruit symptoms

Cracking, shriveling, deformation;
Internal and external browning;
Cork formation around pit and in flesh;
Differential ripening within a single fruit;
Increased fruit drop.

Vegetative symptoms

Usually appears after growth has started in the spring;
Buds fail to break or break and fail to develop normally;
Blossom blast;
Death of terminal buds and twig dieback; retarded shoot growth;
Dwarfed, narrow leaves with upturned edges, often with thickened midribs; may blacken and fall off.

Boron Toxicity Symptoms

Boron toxicity was recognized in the 1860s but did not become a problem in deciduous tree fruit orchards in the PNW until B fertilizer programs were initiated.

Apples

Fruit symptoms

Reduced or no yield;
Increased internal breakdown after harvest;
Increased watercore development after harvest;
Premature ripening.

Vegetation symptoms

Dead terminal buds and shoot dieback;
Marginal leaf chlorosis and necrosis; defoliation.

Pears

Fruit symptoms

Reduced or no yield.

Vegetation symptoms

Dead terminal buds and shoot dieback;
Late developing leaves;
Small, curled, and elongated or cup-shaped leaves;
Marginal leaf chlorosis and necrosis; defoliation.

Apricots, Peaches, Cherries

Fruit symptoms

Reduced or no yields;
Malformation;
Poor pit development;
Earlier maturation;
Poor flavor.

Vegetation symptoms

Tips of new shoots wither and die-back;
Cankers and gummosis develop along stems;
Brittle, partially deformed leaves; may have small necrotic
spots along midrib that may drop out, creating a shot-holed
effect, and small cankers on the underside of midribs and
petioles;
Enlarged nodes at base of buds may be present.

Diagnosis of Boron Nutritional Status

Fruit and blossoms are more likely to exhibit symptoms associated with B deficiency or excess than are the vegetative portions of the trees. Fruit and blossom B content are more sensitive indices of tree B status than are soil or leaf B content. Boron fertilizer treatments usually result in relatively greater increases in fruit B concentrations than in leaf B concentrations. Differences in fruit B concentrations usually persist throughout the growing season, while differences in leaf B concentrations often disappear later in the growing season. Midsummer leaf and at-harvest fruit B concentrations are often poorly correlated. Tree B content or horticultural responses are usually poorly correlated with soil B analyses.

Diagnostic Guidelines for Soil Boron

Most studies have found poor relationships between soil B and fruit tree B content or B malnutrition symptoms; however, soil B test guidelines for orchard soils based on the hot water extraction procedure have been published (N.B., 1 mg/kg = 1 ppm).

Tree B Level	Soil B Content
Deficient	<0.5 mg/kg
Optimal	0.5 to 1.0 mg/kg
High	1.0 to 2.0 mg/kg
Excessive	>2.0 mg/kg

To my knowledge, these guidelines have never been critically evaluated.

Soils should be sampled by depth increments of 0-6, 6-18, and 18-30 inches (most of the soil B is likely to be in topsoil; however, toxic levels can occur in subsoil and will be missed by sampling only the surface).

Tentative Diagnostic Guidelines for Boron in Apple and Pear Tissues

All analyses are dry matter basis (N.B., 1 mg/kg = 1 ppm).

Apple fruit (whole fruit samples at harvest)
	Deficient		<10 mg/kg
	Excessive
		Jonathan, McIntosh	>25 mg/kg^a
		Red Delicious, Golden Delicious	>60 mg/kg^a
		Others	intermediate^a
Pear fruit (whole fruit samples at harvest)
	Deficient		<20 mg/kg^b
	Excessive
		D’Anjou	>45 mg/kg^c
		Bartlett	>55 mg/kg^c
Pear flower (full bloom)
	Deficient		<15 mg/kg
	Excessive
		D’Anjou	>90 mg/kg^c
		Bartlett	>115 mg/kg^c
Apple and Pear leaves (midsummer)
	Deficient		<20 mg/kg^d
	Excessive		>80 mg/kg^e
^a Apple fruit quality impaired. ^b Criteria for blossom blast control. Maintaining fruit at this level should eliminate B-related cork. ^c Associated with vegetative phytotoxicity symptoms; fruit quality was not impaired. ^d Deficiency symptoms can be present at higher concentrations. ^e Temporarily high leaf levels may occur without ill effect after foliar B sprays.

Tentative Diagnostic Guidelines for Boron in Apricot, Peach, and Sweet Cherry Tissues

All analyses are dry matter basis (N.B., 1 mg/kg = 1 ppm).

Fruits (at harvest)
	Deficient
		Apricot	<20 mg/kg^a
	Excessive
		Apricot	>200 mg/kg^a
		Peach	>60 mg/kg^a
Leaves (midsummer)
	Deficient		<20 mg/kg^b
	Excessive		>50 mg/kg^b
^a Associated with impaired fruit quality. ^b Associated with tree phytotoxicity and impaired fruit quality.

Guidelines for Fertilizer Boron Application

The commercial guideline for soil application of B in Washington is a surface-broadcast application of 3 lb actual B per acre, made once every three years. An argument has been made that this infrequent application of a high B rate creates large and potentially deleterious fluctuations in soil and tree B; hence, an annual spray application at a low B rate would better benefit fruit production by providing a more constant supply at lower concentration. This hypothesis has not been tested except for pears grown in non-irrigated areas in Washington, where it was found to be valid. The suggested foliar spray rate is 1.0 lb B/acre for B-deficient orchards. Soil application is suggested if the soil test level is below 0.5 mg/kg or if B deficiency symptoms are present. Suitable forms of B fertilizer for soil application include sodium borates, borax, and boric acid.

Aircraft applications of B should be made during the dormant season to enhance likelihood of application uniformity.

If the soil test level is 0.5 to 1.0 mg/kg and B deficiency symptoms are lacking, a single annual maintenance foliar spray of 0.5 lb B/acre is suggested for the irrigated apple, pear and stone fruit production areas. Boron fertilizer specifically formulated for foliar sprays are available. Preferred application timing is early fall after harvest but the spray can also be applied with possible lesser effect in the early spring at the prepink-to-pink blossom stage. Most foliar B fertilizers are reported to be compatible with most pesticides used in cover sprays (consult the label). The timing of maintenance B applications does not appear to be critical for apple trees; applying the B in the first cover spray or split between the first two cover sprays appears to be acceptable. Applying B in concentrate sprays does not appear to reduce the efficiency of B uptake; however, slight marginal scorching or leaves has been reported for sprays containing 0.4 lb B/100 gal, and severe leaf injury at 0.8 or more lb B/100 gal. There is some anecdotal evidence suggesting that a single annual spray is inadequate for trees grown on sandy soils; multiple applications at low B rates may be preferred.

Pears have a higher B requirement than other tree fruit species in the early spring. A slightly different annual B maintenance program is suggested for non-irrigated pear orchards, where B deficiency is more likely to occur. Sprays applied for blossom blast control tend to have little effect on fruit B. The WSU spray guide therefore suggests increasing the maintenance rate to 1.0 lb B/acre; however, a possibly more effective alternative program is to apply one postharvest or first-white to full-white spray of 0.5 lb B/acre, followed by a second application at the same rate in the first or second cover-spray.

No B fertilizer applications are suggested if soil B exceeds 1.0 mg/kg or if B toxicity symptoms are observed. If B toxicity is confirmed, soil reclamation may be necessary.

Boron phytotoxicity in Washington orchards is rare. An informal but critical review of grower anecdotes suggests that B phytotoxicity is usually associated with two preventable management errors: (1) mistaking the bag of B fertilizer for the nitrogen fertilizer, and (2) overlap of B applications flown on by airplane.

Application of B through the irrigation system is not currently recommended because of the likelihood of nonuniform application and consequent risk of B phytotoxicity.