South African Avocado Growers ’ Association Yearbook 1996. 19:1-7 Boron Nutrition of Avocados

Boron is an essential micronutrient for normal plant growth, but is deficient in many soils that support avocado cult ivation. In avocado, deficiency symptoms include yellowing and deformation of leaves, thickening of nodal regions on branches, loss of geotropism, reduced root growth, branch and trunk lesions, reduced pollen viabil i ty, and deformed and smaller fruit. Avocado trees are particularly recalcitrant in respect to correcting deficiency problems and tolerate much higher soil levels of boron than other fruit trees such as citrus, macadamia and mangoes. Some foliar uptake of boron has been demonstrated, but when deficiency symptoms are present in trees, soil applications of boron fert i l izers are the most effective method of correcting the problem. Supplemental fol iar sprays during f lowering have been shown to increase fruit set. Leaf boron concentrations of 40-60 mg/kg in mature summer f lush leaves prior to inflorescence development are considered optimal for normal growth and development. Caution is required when using soil applications of boron ferti l izers, especially on sandy soils, as toxicity can be induced. Observation of symptoms and monitoring of leaf concentrations are the most useful tools for managing this nutrient