2018 ASHS Annual Conference

Previous research has shown that Huanglongbing (HLB) -affected trees have a reduced root system and a higher rate of root turnover. The diminished root system cannot support the existing above ground canopy and fruit production; as a result, the tree enters into a continuous carbohydrate stress cycle and the tree declines in overall health. Therefore, the objective of this study was to evaluate different levels of pruning as methiod to correct for root to shoot ratio and improve growth and productivity of HLB-affected trees. In January of 2015, a 3 year trial was initiated, 15-year-old grove of ‘Hamlin’ on ‘Swingle’ rootstock were divided for four pruning treatments. The four pruning treatments were: (1) 0%, no canopy removal; (2) 25% reduction; (3) 50 % reduction; (4) 80% reduction. The two sources of fertilizer used were: (1) conventional fertilizer (dry granular; CNV) applied at 200 lb/acre nitrogen in 5 split applications and (2) controlled release fertilizer (CRF) applied at150 lb/acre nitrogen, split in 3 applications. Within each pruning treatment, half of the trees received conventional fertilizer (CNV) and the other half received CRF. All the trees that were pruned produced new flush that looked healthy with no HLB symptoms (initially). The 80% pruned trees grew vigorously over the course but are still significantly smaller than the canopy of control trees (0% pruning) for both CRF and CNV. The 25% and 50 % pruned tree canopies grew back and were similar in canopy size as 0% control pruning treatment by end of year 2. In the first year, the yield for 25%, 50%, and 80% were significantly lower. In the second and third year, the yields of all pruned trees were significantly improved and there was no significant difference between 0, 25, and 50% pruning. A significant correlation was observed between canopy volume and yield. The brix value of juice from the fruit was observed to decrease with a decrease in canopy volume. A clear inverse relationship between canopy volume and pre-harvest fruit drop was observed. Overall, our results indicate that pruning did not improve the productivity HLB-affected trees over the course of 3 year. Therefore, pruning may be viable option to rejuvenate the HLB-affected trees. No significant differences were observed between the two forms of fertilizer for any of the measured parameters, therefore with use of CRF amount nitrogen and frequency of application can be reduced.