What should you do to try and revive a citrus tree affected with HLB/Greening? Researchers at UF/IFAS wanted to know if hard pruning and fertilizing would bring HLB-affected trees back into production. HLB kills the roots in citrus trees causing the root:shoot ratio to be out of balance. The hypothesis was that if you prune the tree back to a more balanced root:shoot ratio and fertilize the tree, the tree could recover.
Researchers at the UF/IFAS Citrus Research and Education Center tested pruning techniques on 14-year-old ‘Hamlin’ sweet oranges on Swingle citrumelo rootstock over three years. In the first year, trees were either not pruned (control) or 25%, 50%, or 80% of the canopy removed. In addition, every year the trees either received conventional dry fertilizer at 200 lb N/acre in five split applications per year or controlled-release fertilizer (CRF) at 150 lb N/acre split in three applications per year. Trees were evaluated for leaf area index, root density, fruit drop and yield.
Pruned trees regrew with healthy-looking leaves, but all trees remained infected with HLB. Trees with 80% canopy removal grew vigorously but remained smaller than control trees with either fertilizer treatment. Trees that had 25-50% of their canopy removed grew back to be similar in size to the control trees by the end of the second year. Intensive pruning actually reduced root density rather than bring the root:shoot ratio into better balance. They found the larger the canopy volume, the larger the yield. The smaller the canopy volume, the more fruit that dropped before maturity. Yield was not significantly increased by pruning treatment or fertilizer compared with controls, but 25% less fertilizer was used with CRF with similar growth and yield results. Severe pruning is therefore not considered a good option to try and rejuvenate HLB-affected trees, but there is a savings in labor (3 applications instead of 5) and in amount of nitrogen put into the environment (150 lbs instead of 200 lbs) when using controlled release fertilizer.
Tripti Vashisth and Taylor Livingston. Assessment of Pruning and Controlled-release Fertilizer to Rejuvenate Huanglongbing-affected Sweet Orange. HortTechnology. December 2019 29(6).
For more information on growing citrus: Citrus Production Guide