2018 ASHS Annual Conference

The following study was conducted to determine the impact of frequent foliar Cu applications, which are used to suppress citrus canker caused by Xanthmonas citri subsp. citri in commercial groves, on Huanglongbing (HLB)-affected Citrus sinensis cv. ‘Valencia’. The experiment was conducted in a psyllid-free greenhouse with HLB positive and non-HLB control trees grown in Immokalee fine sand soil and well-maintained to promote health. Cu was applied to the foliage at 0x, 0.5x, 1x, and 2x of the commercially-recommended rate, 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 titer, acquisition of other essential nutrients, and water relations were determined. HLB acidified the soil more than non-HLB controls, which promotes Cu availability and promoted higher Cu contents 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 Ca, Mg, Mn, Zn and possibly 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 content of Fe which may have been suppressed. HLB suppressed stem water potential slightly compared to the non-HLB controls. Stomatal conductance and transpiration flux were reduced at the higher rates of Cu applied. Traditional recommendations for Cu applications to control HLB may have to be reconsidered in commercial citrus groves due to the impact on growth and development.