Interaction of Huanglongbing and Foliar Applications of Copper on Growth and Nutrient Acquisition of Citrus sinensis cv. Valencia

Abstract

The following study was conducted in 2016 and 2017 to determine the impact of frequent foliar copper (Cu) applications on Huanglongbing (HLB)-affected Citrus sinensis cv. Valencia orange. The experiment was conducted in a psyllid-free greenhouse with HLB-positive and non-HLB control trees grown in an Immokalee fine sand soil. The trees were well-maintained to promote health. Cu was applied to the foliage at 0x, 0.5x, 1x, and 2x the commercially recommended rates, which were 0, 46, 92, and 184 mM, respectively, with applications made 3x in both 2016 and 2017. The impact of HLB and Cu treatments on leaf and root Cu concentrations, vegetative growth, Candidatus Liberibacter asiaticus (CLasiaticus) genome copy number, and acquisition of other essential nutrients were determined. HLB caused the roots to acidify the soil more than non-HLB controls, which promoted Cu availability and promoted greater Cu concentrations in leaves and roots. HLB and Cu application treatments suppressed leaf area and total root length observable in rhizotron tubes such that, by the end of the experiment, leaf, stem, root, and whole-plant dry weights were reduced. HLB reduced foliar concentrations of calcium (Ca), magnesium (Mg), manganese (Mn), zinc (Zn) and possibly iron (Fe), but HLB did not affect root concentrations of these same essential nutrients. Cu application treatments did not affect leaf or root concentrations of other essential nutrients except foliar concentration of Fe, which may have been suppressed. Foliar applications of Cu are used to suppress Xanthomonas citri ssp. citri (Xcc) the causal agent of citrus canker, and the frequency of its use may need to be reconsidered in commercial groves.

Interaction of Huanglongbing and Foliar Applications of Copper on Water Relations of Citrus sinensis cv. Valencia

“…The first paper in this series demonstrated that the pH of the soil in which HLB-affected trees grew was lowered compared with the soil in which non-HLB controls were grown. Reduced soil pH solubilized soil Cu and promoted higher uptake in HLB-affected trees than the non-HLB control [13]. The low pH-induced solubilization of soil cations has a reason for nutrient uptake and improved fertilization management of commercial citrus groves with HLB.…”

“…This approach removed bias in tree size across all treatments when the experiment was initiated. Total leaf area was determined by counting leaves and measuring 10 leaves per tree, as described by Ebel et al [13].…”

“…Leaf samples of 10 mature, fully expanded leaves were randomly collected from each tree and rinsed to remove residues in 0.2 M HCl and dried for 72 h at 60 °C. Once the tissues reached a constant weight, they were ground in a mill until all tissue could pass through a 60-mesh sieve and mixed thoroughly [13]. Tissue Cu concentrations were determined using a dry ash combustion digestion method [20].…”

“…However, T sa , as measured here, is an overestimation of true T sa , which is the total surface area of the stomatal pores. Total leaf area was determined every two to four weeks, as described by Ebel et al [13].…”

“…In the first part of the research in this series, we demonstrated that HLB-affected trees are more sensitive to foliar applications of Cu than non-HLB trees [13]. Cu was applied to the foliage and a few days later, excess Cu was washed off leaves and onto the soil surface where it moved into the soil profile.…”