Boron deficiencies are common in fruit trees throughout the Pacific Northwest. Dry soils, particularly in the fall, aggravate the problem. Symptoms include twisted leaves, poorly developed stamens in the blossom, blast of pear blossoms, inadequate fruit set, low seed numbers, bark necrosis in apple, fruit cork, and sometimes fruit cracking.
Deficiencies in most orchards can be prevented or corrected by soil applications of boron, broadcast over the entire soil surface. One application should last up to three years. Because of the potential for serious injury and even loss of the crop or trees if too much boron is used, precautions should be taken. Do not apply more than 3 pounds of actual boron per acre unless higher rates are required as determined by soil tests, sampling to 3 feet. If an aircraft is used for soil application, apply only during the dormant season.
Spray applications can be used to prevent the development of deficiency symptoms or to correct deficiencies. An annual application at the maintenance rate should supply sufficient boron to prevent deficiencies. This may be applied at any time but is more effective in improving blossom quality and fruit set if applied shortly before full bloom. Sprays also can be applied early during the growing season or postharvest while the leaves are still green and active. Higher maintenance rates may be required for orchards planted on very sandy or calcareous soils or in the White Salmon area. Use the annual per acre deficiency rate at these locations, applying half the boron in a single prebloom spray and the remainder in one or more postbloom sprays. Boric acid- and polyborate-based spray products are equally effective, when applied as single-product sprays. With the exception of Mor-Bor 17 and B-17, all boron spray products tested to date at WSU produce alkaline spray tank solutions and may require acidifying adjuvents if used in pH-sensitive tank mixes. Tank mixes with Mor-Bor 17 or B-17 may require acidification when prepared using very alkaline well waters. Because boron product and water chemistries vary, the best practice is to measure and adjust the pH of the boron product-water-acidifier mix before adding pH-sensitive pesticides or growth regulators.
If deficiencies appear during the growing season, apply boron as soon as possible but do not use high rates after May because of the potential for fruit breakdown in storage. If deficiency symptoms occur frequently, make soil tests and consider ground applications.
Where pear "blossom blast" occurs, make spray applications in the fall after harvest but while the leaves are still green and active or in the spring during the first white to white blossom stages. Note: "blossom blast" is readily confused with false fire blight (Pseudomonas blight) and fire blight. See section on Diseases of Apples and Pears.
Caution: Both high rates and high concentrations of boron can cause shoot dieback and even tree death. High rates or late applications during the growing season can cause severe fruit loss in storage.