FOLIAR B, CU AND ZN APPLICATIONS TO HASS AVOCADO TREES. PENETRATION, TRANSLOCATION AND EFFECTS ON TREE GROWTH AND CROPPING

Mature Hass avocado trees received Cu, Zn or B foliar sprays and their combinations in summer or early autumn. Sprayed trees had significantly higher copper contents in the fruit epicarp, inflorescences and spring shoots but not in the mesocarp, endocarp-seed or in mature leaves the following year. B and Zn concentrations were similar in all cases. Yield, fruit size and dry matter content of the mesocarp were not affected by any of the sprays. Solubor (20.8% B) sprayed to small branches at prebloom or full bloom did not increase B concentrations of mature leaves in September probably due to a dilution effect. When applied to whole trees B concentrations were significantly higher in mature spring grown leaves and slightly higher in summer grown leaves. Solubor treated half leaves exported most of the absorbed B in about 20 days to the rest of the tree and not to the neighbouring untreated half leaf. The ability to increase plant B concentration by Solubor, boric acid, borosilicate or trietanolamine-B in basic or acid solutions was tested in pot trees. When sprayed in early autumn to recently grown leaves only the acidified sprays showed a very small but significant increase in B concentration of spring grown leaves sampled the following year (21.5 vs. 19.1 ppm). Pot trees were sprayed at different stages of spring shoot development. Bark B content, measured in December, was slightly but significantly increased by the early sprays to young quickly expanding leaves.