At high, but nontoxic, concentrations, leaf symptoms are similar to those caused by excessive salt, deficient potassium or biuret toxicity from foliar urea sprays not using low biuret urea. Thus, leaf tissue analysis is important for delineating cause. Excessive levels of boron produce a yellowing of the leaf tip and yellow spotting of the leaf surface. Death of the leaf tissue may occur along the margins. Higher levels of boron may cause brownish, resinous gum spots on undersides of leaves, although this symptom is not always present.
Boron accumulates in the leaves as they age so symptoms usually appear on older leaves first.
Older leaves with high concentrations of boron are relatively short lived compared to leaves that have boron at optimum concentrations. Often excessive boron and sodium appear together in leaf tissue analyses. Trees with high leaf tissue boron concentrations appear to be less vigorous with shorter branches. On branches, boron is associated with a decreased distance between leaf nodes. Discussion of boron levels which would be considered excessive in September sampled spring-flush leaf tissue may be misleading.