Mobility within tree: Low

Uptake Form: H2BO^2-, HBO₃^2-

Function: translocation sugars, cell development, growth

Deficiency/Toxicity Symptoms:  Boron deficiency can occur in Okanagan orchards if maintenance applications are not applied. Boron promotes the translocation of sugars and cell development and is believed to be important for growth regulators (Maher, 2004). Boron is not mobile in plants. Consequently, B deficiency symptoms most often appear on the growing tip of the plant. In B-deficient plants, the growing tip is often deformed. Boron is important for pollen tube growth and thus, is needed at bloom to aid in fruit set. In apples and pears, the fruit does not develop normally, but has a gnarled misshapen appearance.  This defect is caused by depressions usually underlaid by hard corky tissue.  Cracks may also develop in some apple cultivars if both B and Ca are low.  Boron-deficient leaves are darker green, thick and brittle and abscise early first at the shoot tips.  Apple measles appears as purplish pimples on twigs and rough cracks on bark may be due to B deficiency.  Peach trees with B deficiency can show lenticel proliferation, corky callus and bark splitting.  B deficiency is particularly a problem in pears which need relatively high levels of B for adequate fruit set (Sallato et al. 2019). Pear trees that are B deficient may show fruit set failure or wilting and dying of blossoms (blossom blast).  Excess B is toxic to fruit trees, especially peaches, prunes and apricots. Leaf symptoms of B toxicity are yellowing along the midrib and large lateral veins followed by abscission. Apples with B deficiency will mature early and have shortened storage life.

Since the plant-available form of B is not positively charged it can be leached out of soils and is often lost from the plant root zone by overirrigation or high precipitation (Maher 2004). coarse  texture soils and soils with low organic matter are prone to B deficiency. Periods of drought or water-logging of soils can artificially result in a B deficiency in a tree due to poor root uptake from the soil.

Boron (B) Sources and Application

            Boron should be checked and balanced every few years.   Recent research indicated that orchards frequently have very low leaf B and Zn concentrations, and occasionally low leaf Mg and K levels. Boron is highly soluble in the soil, which can result in large differences in B levels under drip irrigation systems versus the alley ways in an orchard.  Soil testing for B can be used to determine toxicity and deficiency in medium- to fine-textured soils. Leaf tissue B levels usually give a good indication of tree B status (Sallato et al. 2019).  Leaves showing suspicious symptoms can be collected in May for analysis to compare to regular leaf analysis done in July.  Leaf analysis cannot be used to test B levels after post-bloom sprays have been applied to foliage.  To avoid this problem, B can be applied effectively in the fall before leaves start to senesce.  In these cases and where B toxicity is suspected, soil analysis is useful.  High B levels in the soil can be an indication of a potential soil drainage issue.  Issues could be a hard pan that is not allowing proper drainage or there is a problem of leaching or seepage from a higher property.  Applying B without testing could aggravate tree damage where B levels are already high. Common sources and application rates of B for fruit trees are presented in Table 23. 

            Where required in the soil, B ideally should be applied in August.  Dry or spray applications to soil should be distributed evenly over the main root zone areas and should not be made within one month of liming.  Boron is normally applied to the soil at very small rates that can be difficult to apply uniformly and thus has started to be incorporated as a coating on fertilizer mixes to allow easier even distribution.  Soil B levels should be balanced with a soil application once every three years on fine and medium textured soils or annually on coarse soils.  The best B application methods to the soil are through a blended fertilizer mix or as Solubor dissolved in water and applied with the herbicide sprayer.  Care should be taken not to exceed recommended rates with use of multiple products that contain B. Soil B values less than 0.4 ppm (µg/g) are considered low.  Soil values should not exceed 1.0 ppm for peaches, prunes and apricots or exceed 1.25 ppm for apples, cherries, and pears. 

            Foliar B sprays can be combined with sprays of chelated Zn, Mg, Mn and urea. Boron sprays may also be combined with some pesticides.  This may include a supplemental B spray of Solubor (20.3% B) at 2.8 kg/ha.  Boron can be applied in the fall with other nutrients on all crops.  In apples and Anjou pears, B can be applied with the tight cluster stage dormant oil spray or at the pink-bud stage if not used the previous fall.  Consult the container label to determine mineral-pesticide compatibility.  Foliar B sprays are not to be considered as substitutes for soil application in all years.

            Boron can also be applied by fertigation.  It is relatively easy to increase leaf and fruit B concentrations via fertigation at modest rates of 0.34 g B per tree per year at any time in the growing season.  The preferred time for fertigation applications though is within 4 weeks of bud break when leaf B concentrations have been chronically low.  Like N, B is very soluble and can be leached from the soil with excessive irrigation, especially for sandy soils.  As B has a narrow range of suitability for most fruit trees, it is not advisable to exceed recommended application rates or else toxicity and growth reduction will occur.

Table 1. Possible Boron application rates for all fruit trees