2017 ASHS Annual Conference

Phosphorus (P) induced iron (Fe) chlorosis is a common problem in Hawaiian grown macadamia (Macadamia integrifolia) orchards. Leaf P concentrations in excess of 0.15% are associated with the development of Fe deficiency symptoms. Trees with advanced Fe chlorosis can appear weak and can have reduced production. The current recommendation is to eliminate P application in the orchard until the P:Fe ratios return to normal, which can take several years. Despite following this recommendation, P-induced Fe chlorosis is persistent in the fields at Royal Hawaiian Orchards (RHO) in Kau, Hawaii. The objective of this study was to evaluate five iron treatments to increase the Fe absorption in deficient trees. Treatments included tree injection (MIN-jet), foliar application (Ligno Fe), soil drench 1 (Iron Sulfate with citric acid), soil drench 2 (Humega), soil drench 3 (Rebar), and a control. We applied treatments to six trees in December 2015 at RHO. We collected tissue samples from each tree before (2 months) treatment application, at 4, 8 and 15 months after treatment (MAT). We collected yield data during the 2016-2017 harvest season. Before treatment application and 4 MAT there was no difference in Fe levels between the treated and control trees. At 8 and 15 MAT the only treatment that significantly increased Fe concentrations above the control was the Foliar spray treatment. Iron concentrations in the Tree Injection and Humega treatments at 15 MAT were not significantly different from the Foliar treatment, and were the only other treatments that increased Fe from pre-application levels, suggesting some effectiveness, but were not different from the control. Throughout the study the Fe rates of all the treatments were above the recommended value of 50 ppm, however, the P levels in all of the trees were double the recommended range of 0.08-0.11%, inducing the iron chlorosis. Fe:P ratios less than 0.07 also suggest iron chlorosis, and all trees were below this level throughout the study. Yields were highest in Iron Sulfate trees, and although still below 0.07, the Fe:P ratio was highest in the Iron Sulfate treatment. These results suggest that although Fe levels of Iron Sulfate treated trees were not different from control trees, the Fe:P ratio was higher than the other treatments and could explain the higher yields. Longer term studies of the effects of Fe applications on yields are needed to further substantiate the most effective treatments to overcome P-induced Fe chlorosis in macadamia in Hawaii.