Many citrus trees in the southern end of the San Joaquin Valley are grown on moderately calcareous soils and frequently have high levels of boron in the leaf tissue.
Citrus is sensitive to boron.
Boron, when excessive, may cause defoliation and significant yield loss. 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.