Rehabilitation of Huanglongbing-affected Citrus Trees Using Severe Pruning and Enhanced Foliar Nutritional Treatments

Abstract

<jats:p>Citrus trees affected by huanglongbing (HLB) become diminished, weak, and develop dieback resulting in reduced production. Decline in fruit yield ultimately prevents economically acceptable commercial citrus production. The objectives of this study were to evaluate the effects of severe pruning in combination with an enhanced foliar nutritional treatment on growth, yield, and juice quality of HLB-affected orange trees. The bacterial titer within the trees was monitored before and after treatments, and a cost–benefit analysis provided an economic evaluation of the treatments. Fifteen-year-old ‘Valencia’ orange (<jats:italic>Citrus sinensis</jats:italic> Macf.) trees on Swingle citrumelo rootstocks [<jats:italic>C. paradisi</jats:italic> × <jats:italic>Poncirus trifoliata</jats:italic> (L.) Raf.] with 100% incidence of HLB, confirmed by real-time polymerase chain reaction (PCR), were severely pruned back to the main scaffold branches. Between 2010 and 2015, foliar nutrients were sprayed on both pruned and nonpruned trees to target new flush growth. Three enhanced nutritional foliar treatments were evaluated and compared with a control foliar nutritional treatment that was considered to be a standard practice before endemic HLB. The enhanced nutritional treatments included a mixture of micro- and macronutrients commonly known as the “Boyd cocktail,” a micronutrient package labeled Fortress © (Florida Phosphorus LLC, Key Largo, FL) sprayed with potassium nitrate (KNO<jats:sub>3</jats:sub>), and the Fortress © micronutrient package sprayed with urea. The experiment was a split-plot with seven replications, with pruning as the main plots, and a foliar nutritional treatment as subplots. Tree pruning was performed in Feb. 2010 before the spring flush. Pruned trees grew longer shoots than the controls the year after pruning. Canopy volume and leaf area were greater with nonpruned trees, but the chlorophyll content per cm<jats:sup>2</jats:sup> leaf area was higher in the pruned trees compared with nonpruned trees in 3 years of the 5-year experiment. Pruned and nonpruned trees bloomed and set fruit the first year of the experiment in the spring of 2010–11. The fruit crop for the 2010–11 and 2014–15 seasons, and the overall total fruit crop for the 2010–15 season on pruned trees were significantly lower than those on nonpruned trees. However, no significant yield differences were found between pruned and nonpruned trees in the 2011–12, 2012–13, and 2013–14 growing seasons. Fruit yields from pruned trees never surpassed the yields from nonpruned trees, and this was possibly due to the severe-pruning treatment. Thus, severe pruning, as used in this trial, was not cost effective through the first 5 years after pruning. The rapid regrowth response of the pruned trees, however, may indicate that a reduced pruning approach could be effective at rejuvenating the HLB-affected trees, and an alternative to tree removal and replanting. Enhanced foliar nutrition treatments provided some yield benefits, especially in the early years of the trial. However, the enhanced foliar nutrition treatments did not prove to be cost effective.</jats:p>

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

“…Moreover, according to recent studies, the asian citrus psyllids in the citrus production region of Florida have become resistant to systemic insecticides (Langdon et al, 2018); therefore, the spread of infection is difficult to contain via vector control. Comparable to our results, Rouse et al (2017) and Lopes et al (2007) reported that the pruned trees showed symptomless new growth but were never detected free of CLas. Similarly, Daugherty et al (2018) reported that the severe pruning of Pierce's disease (Xylella fastidiosa)affected grapevines (Vitis vinifera) did not affect the infection, and the symptoms of the disease reappeared within two seasons.…”

“…Even though significant growth in canopy was observed during the 3 years, it was still not sufficient to counteract the effect of intense pruning (50% and 80%) on the root and shoot system. Likewise, Rouse et al (2017) had reported that ''buck horn'' pruning resulted in vigorous growth in trees; nonetheless, untreated trees had larger canopy volume and leaf area. Altogether, this suggests that high pruning treatment can have a detrimental effect on tree growth and can take years to grow to prior or improved condition, potentially making them uneconomical.…”

“…Therefore, pruning to adjust the root-to-shoot ratio seems like a logical and rational choice. Nevertheless, Rouse et al (2017) reported that severe (''buck horn'') pruning of HLB-affected trees did not result in tree health or yield improvement over a period of 5 years, whereas enhanced foliar nutritional treatment had an effect on yield but was not economically profitable. Overall, they speculated that a moderate pruning treatment might be beneficial for improving health and productivity of HLB-affected trees.…”

Efficacy of In-field Thermotherapy in Comparison and Combination of Defoliation for Mitigating Huanglongbing in Sweet Orange

“…Within 6 months, the combination treatments (DFTT, DFTT 6, TTTT 6, DFDF 6) resulted in reduced tree health, as indicated by DI rating and canopy density, suggesting that consecutive loss of canopy (via thermotherapy or defoliation) resulted in accelerated decline of already diseased (HLB-affected) trees. Similarly, it has been reported that severe pruning of HLB-affected trees does not result in any significant improvement in health (Rouse et al, 2017;Vashisth and Gainey, 2017). Over the course of 2 years, all the trees in the experiment showed a decline in health, most likely owing to the HLB epidemic.…”

Performance of Two Citrus Species Grafted to Different Rootstocks in the Presence of Huanglongbing Disease in Puerto Rico

“…From 2009 to 2010, the citrus trees were fertilized and managed as suggested in the "Conjunto Tecnológico para la producción de Cítricos en Puerto Rico" [23]. To overcome the effects of HLB on tree phloem, the fertilization program was modified since 2011-2016, with slow release fertilizers and supplementary foliar nutritional cocktail, both with micronutrients as recommended it by Rouse [24] in the state of Florida, USA. The supplementary foliar nutritional cocktail with micronutrients consisted of slow release nitrogen (30-0-0), Phosphite ® (0-29-26), Recover Rx ® (3-18-18) and a biological fungicide (Companion ® ) were applied in monthly basis.…”