Boron requirement of young 'Sharwil' avocado trees

Abstract

Possible boron (B) deficiency symptoms were observed on avocado (Persea americana Mill. 'Sharwil') grown in Kona, Hawaii. To determine the B requirement of young, 'Sharwil' avocado trees, two greenhouse experiments were conducted. In a soil study, seven B treatments (0, 3.7, 11, 22, 44, 89, and 178 mg·kg-1 soil fines) were applied to 1-year-old grafted 'Sharwil' avocado trees grown for 13 weeks in a Tropofolist soil. Due to the low and variable fractions of soil fines in this rocky soil, extractable soil B concentration did not appear to be a good predictor of B requirements by avocados. Adequate foliar B concentrations in 'Sharwil' avocado trees based on dry weight and area of new leaves ranged from 37 (±3) to 65 (±4) and from 31 (±10) to 78 (±13) mg·kg-1 (dry-weight basis), respectively. (Means are followed by standard errors of the mean in parentheses.) In a hydroponics study, 6-month-old grafted 'Sharwil' avocado trees were supplied with four levels of B (0, 1, 10, and 100 μM). At 11 months after B treatment initiation, leaves with deformed margins and a 'shot-hole' appearance were first observed at a solution level of 0 μ B. At 14 months after B treatment initiation, foliar B concentrations that were associated with 12% to 14% incidence of deformed leaves ranged from 9.8 to 13.5 mg·kg-1 dry-weight basis). Although 'Sharwil' avocados are reportedly susceptible to B deficiency, foliar B concentrations required for adequate growth and those associated with B deficiency symptoms are similar to those for other cultivars.