Rio Grande Gummosis

Young Valencia orange trees were grown for three years in large outdoor sand cultures on complete nutrient solu tions that varied differentially only in the amount of boron. Three rates of boron were applied to single-tree plots. The plots were replicated four times. No difference in tree size resulted from the differential treatments. Rather large differences in the boron content of the leaves were induced. The low-boron plants showed mild foliage deficiency symptoms during the second year but not in the first or third years of growth. The high-boron plants showed slight leaf symptoms of toxicity throughout the three-year period. Mature leaves showed virtually no differences in mineral composition other than the 10to 24-fold difference in boron. Phosphorus tended to be present in slightly greater concentration when boron was low. Young leaves showed this same rela tionship with phosphorus in a more pro nounced manner. When the boron sup ply was low, potassium accumulation in the leaf was retarded and magnesium accumulation accentuated. The rate of calcium accumulation was depressed at the highest boron level. These differ ences appear to diminish as the leaf approaches maturity. The only consistent difference in the quality of the fruit produced during the third year was a slight reduction in the ascorbic acid content in the low-boron cultures.