DOMESTIC, EXOTIC, AND EMERGING DISEASES OF CITRUS, VEGETABLES, AND ORNAMENTALS (DEED)
Location: Subtropical Plant Pathology Research
Title: Inconsequential Effect of Nutritional Treatments on Huanglongbing Control, Fruit Quality, Bacterial Titer and Disease Progress
Submitted to: Crop Protection Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 11, 2012
Publication Date: March 29, 2012
Citation: Gottwald, T.R., Graham, J.H., Mccollum, T.G., Wood, B.W. 2012. Inconsequential Effect of Nutritional Treatments on Huanglongbing Control, Fruit Quality, Bacterial Titer and Disease Progress. Crop Protection Journal. 36:73-82.
Interpretive Summary: The use of an enhanced nutritional programs (ENPs) to minimize the effects of citrus huanglongbing (HLB) commonly called greening, has been a topic of considerable discussion and debate since the discovery of HLB in Florida. Most reports of the effects of ENPs are anecdotal. Even so, Florida citrus producers are using this unproven approach for HLB management instead of conventional integrated control practices of inoculum which includes removing symptomatic trees to reduce inoculum and vector control. Two field studies were conducted on Valencia sweet orange to investigate the effect in replicated trials. Considering both trials together, the ENP did not sustain tree health, yield, or fruit quality of symptomatic trees. Moreover, there is a major concern that use of ENP strategies have promoted area-wide build-up of inoculum and increase disease spread within and between citrus orchards, worsening the epidemic.
The use of an 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/analysis with sufficient statistical rigor. Even so, Florida citrus producers are using this unproven and non-validated approach for HLB management in lieu of conventional integrated control of inoculum which includes rouging symptomatic trees to reduce inoculum and vector control. 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 of the trial, but exhibited only mild HLB symptoms. This stage of infection was chosen based on claims that the ENPs maintain the 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 (number of fruit/tree, kg fruit/tree, proportion of fruit dropped), or juice quality (Brix, acid, Brix:acid ratio) between treated trees 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 NP in three blocks was compared to conventional fertilization in three blocks. In this commercial trial, neither yields, disease progress, nor epidemic dynamics differed between the NP and conventional fertilization treatments. Results of the large commercial trial corroborated the experimental results of the first trial with more diverse micronutrient treatments. Considering both trials together, the ENP did not sustain tree health, yield, or fruit quality of Las-infected HLB-symptomatic trees. Moreover, since the nutritional supplements had no effect on Las titer, a major concern is that use of existing NP strategies have promoted area-wide build-up of inoculum and increase disease spread within and between citrus orchards.