Boron requirement of young 'Sharwil' avocado trees

Possible boron (B) deficiency symptoms were observed on avocado (Persea ame ricana Mill. 'Sharwil') grown in Kona, Hawaii. To determine the B requireme nt of young,'Sharwil' avocado trees, two greenhouse experiments were conduc ted. In a soil study, seven B treatments (0, 3.7, 11, 22, 44, 89, and 178 m g.kg(-1) soil fines) were applied to 1-year-old grafted 'Sharwil' avocado t rees 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 concentratio n did not appear to be a good predictor of B requirements by avocados. Adeq uate 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 ar e 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 mu M). At 11 months after B treatment initi ation, leaves with deformed margins and a "shot-hole" appearance were first observed at a solution level of 0 mu M B, At 14 months after B treatment i nitiation, foliar B concentrations that were associated with 12% to 14% inc idence of deformed lea, es ranged from 9.8 to 13.5 mg.kg(-1) (dry-weight ba sis). Although 'Sharwil' avocados are reportedly susceptible to B deficienc y, foliar B concentrations required for adequate growth and those associate d with B deficiency symptoms are similar to those for other cultivars.