Assessment of Pruning and Controlled-release Fertilizer to Rejuvenate Huanglongbing-affected Sweet Orange

Abstract

<jats:p>Previous research has shown that Huanglongbing {HLB [causal agent <jats:italic>Candidatus</jats:italic> Liberibacter asiaticus (<jats:italic>C</jats:italic>Las)]}-affected sweet orange (<jats:italic>Citrus sinensis</jats:italic>) trees have a reduced root-to-shoot ratio, potentially due to the high rate of root death. The diminished root system cannot support the existing aboveground canopy and a cycle of imbalance begins. As a result, the tree enters into a continuous carbohydrate stress cycle and, eventually, the tree declines. Therefore, the goal of this study was to evaluate pruning as a strategy to adjust the root-to-shoot ratio to improve growth and productivity of HLB-affected trees. In Jan. 2015, a 3-year trial was initiated on a 14-year-old grove of ‘Hamlin’ sweet orange on Swingle citrumelo (<jats:italic>Citrus paradisi</jats:italic> × <jats:italic>Poncirus trifoliate</jats:italic>) rootstock that was symptomatic of HLB and produced less than 180 lb of fruit per tree. The four pruning treatments were as follows: 1) 0% pruning (no canopy removal), 2) 25% pruning (canopy removed), 3) 50% pruning (canopy removed), and 4) 80% pruning (canopy removed). In a split-plot design, two sources of fertilizer were evaluated in combination with the pruning: 1) conventional fertilizer [CNV (dry granular)] applied at 200 lb/acre nitrogen (N) in five split applications per year, and 2) controlled-release fertilizer (CRF) applied at 150 lb/acre N, split in three applications per year. Within each pruning treatment, half of the trees received CNV and the other half received CRF. The fertilizer treatments were applied in each of the 3 years; however, pruning was performed only once in the beginning of the experiment. The trees that were pruned produced new vegetative growth that looked healthy with no visual HLB symptoms (initially); however, the trees remained positive for CLas throughout the study as determined by quantitative real-time polymerase chain reaction. The 80% pruned trees grew vigorously over the course of 3 years but remained significantly smaller in canopy than control trees (0% pruning) for both CRF and CNV treatments. The 25% and 50% pruned tree canopies grew back and were similar in canopy size as 0% pruning (control) treatment by the end of year 2. At the end of the study, the use of CRF on 25% pruned trees resulted in a significantly higher leaf area index as compared with trees receiving CNV. A significant positive linear correlation was observed between canopy volume and root density; the root density decreased with intensive pruning. A significant positive correlation was also observed between canopy volume and yield, and a negative correlation between canopy volume and fruit drop. There were no significant increases in yield resulting from any pruning or fertilization treatments compared with controls (0% pruning). However, with the use of CRF, the amount of N and frequency of application were reduced. Overall, our results indicate that pruning did not improve the productivity of HLB-affected trees over the course of 3 years. Therefore, severe pruning is not a viable option to rejuvenate the HLB-affected trees.</jats:p>

Nutrient Uptake in Huanglongbing-affected Sweet Orange: Transcriptomic and Physiological Analysis

“…To obtain HLB-positive trees, the trees were budded with CLas-infected buds, whereas the healthy trees were mock-inoculated with healthy buds. Three months after inoculation, trees were confirmed for CLas infection by quantitative real-time polymerase chain reaction (qRT-PCR), as described by Vashisth and Livingston (2019). Uniform looking HLB-positive and healthy trees were selected and were kept in greenhouse for 6 months where they were not fertilized to create nutrient-deficient conditions.…”

Overview of citrus huanglongbing spread and management strategies in Brazil

“…Some of these practices have also been evaluated as a possible alternative to the recommended preventive management in Brazil. Pruning of symptomatic branches (Lopes et al 2007;Vashisth and Livingston 2019), steam thermotherapy (Hoffman et al 2013;Lopes et al 2014;Kelley and Pelz-Stelinski 2019), enhanced nutritional programs (ENP) (Xia et al 2011;Gottwald et al 2012;Stansly et al 2014;Bassanezi et al 2016Bassanezi et al , 2019, potential resistance inducers (Li et al 2016 Bassanezi et al 2016;Hu et al 2018), phytohormones (Canales et al 2016;Hu et al 2018), and antibiotic application (Zhang et al 2014;Shin et al 2016;Yang et al 2016;Hu and Wang 2016;Hu et al 2018) are among the measures studied. Although some of these control measures have produced positive results, the level of control was not high enough or would not be economically feasible to be adopted on a large scale in the field.…”

“…For ENP, resistance inducers, phytohormones, and antibiotic treatments have no direct effects or very mild effects on HLB bacteria titers, maintenance of tree health, yield and fruit quality. ENP only corrects mineral deficiencies in the healthy parts of diseased trees but does not reduce HLB symptoms of leaf mottle and lopsided fruit (Gottwald et al 2012;Stansly et al 2014;Bassanezi et al 2016Bassanezi et al , 2019Vashisth and Livingston 2019). In 2016, the antibiotics streptomycin sulfate, oxytetracycline hydrochloride, and oxytetracycline calcium complex were approved for emergency field use in foliar sprays to treat HLB in Florida but after two years the results were inconclusive (Blaustein et al 2018).…”

Effect of Irrigation Water pH on the Performance of Healthy and Huanglongbing-affected Citrus

“…The healthy trees were grafted with healthy CLas-negative buds and kept in the greenhouse. All trees were confirmed for CLas using quantitative real-time polymerase chain reaction (qPCR) twice as described by Vashisth and Livingston (2019). The first confirmation was performed 3 months after bud inoculation, and the second was performed 1 month before proceeding with the experiment.…”