Inconsequential Effect of Nutritional Treatments on Huanglongbing Control, Fruit Quality, Bacterial Titer, and Disease Progress

The use of enhanced nutritional programs (ENPs) to minimize the deleterious effects of the vector transmitted bacterial disease, citrus huanglongbing (HLB) caused by Candidatus Liberibacter asiaticus (Las), has been a topic of considerable discussion and debate since the discovery of HLB in Florida.  Most reports of the putative effects of ENPs are either anecdotal or based on non-replicated trials lacking untreated controls or proper experimental design and analysis.  Even so, Florida citrus producers use this unproven and non-validated approach for HLB management in lieu of integrated control of inoculum including roguing symptomatic trees to reduce inoculum and vector control using insecticides.  The composition of the formulation of the ENPs vary considerably, but usually consist of foliar applications of standard essential micronutrients, salts of phosphite, and in some programs, methyl salicylate.  Two field trials were conducted on Valencia sweet orange [Citrus sinensis (L.) Osbeck] to test efficacy of widely used ENPs.  The first trial consisting of a randomized complete block design with 3 blocks and 4 replicate trees/block was conducted from 2008 to 2010.  All trees were PCR+ for Las at the onset, but exhibited only mild HLB symptoms.  This stage of infection was chosen based on claims that the ENPs maintain health and productivity of HLB infected trees, thereby extending the orchard’s commercial viability.  Combinations of components were compared with a control consisting of a standard fertilization and control program for psyllids.  Additional treatments consisted of phosphite with Mn-carbonate, Mn-metalosate, Cu-metalosate, or Zn-metalosate, and injection treatments using soluble copper or silver mixed with a polymer.  After two seasons of three applications each, there were no significant differences in bacterial titer dynamics, fruit yield, or juice quality between treated and untreated control trees.  In a second trial, consisting of six commercial citrus blocks containing 40,885 trees, where enhanced vector control and rouging of diseased trees was practiced, the ENP in three blocks was compared to conventional fertilization in three blocks. In this trial, neither yields, disease progress, nor epidemic dynamics differed between the ENP and conventional fertilization treatments. Results of the large commercial trial corroborated the experimental results of the first trial. Considering both trials together, the ENP did not sustain tree health, yield, or fruit quality of HLB-symptomatic trees.  Moreover, since the nutritional supplements had no effect on Las titer, a major concern is that use of existing ENP strategies have promoted area-wide build-up of inoculum and increase disease spread.