A Technical Report from The Bartlett Tree Research Laboratories 341
Boron is an essential micronutrient to plants, but many species vary in their requirements for boron; there is a narrow range between adequate and toxic levels of boron. Symptoms of toxicity include marginal leaf chlorosis and necrosis, followed by scorching, burning, and premature drop of interior foliage, leaving plants with only the current season's growth. This perennial defoliation leads to plant stress and decline. Symptoms are most pronounced in broadleaf evergreens, but can occur in other species.