Managing Plant Nutrients on Selected Rootstocks of Sweet Orange (Citrus sinensis L.) and Their Impact on Tree Growth, Fruit Yield, and Postharvest Quality of Huanglongbing-affected Hamlin Trees

Crop protection measures, cultural practices, nutrient and irrigation management, rootstock selection, and other studies have been conducted to alleviate the devastating bacterial disease of Huanglongbing (HLB; also known as citrus greening) in recent years. However, there is no single cure for HLB caused by Candidatus Liberibacter asiaticus ( C Las), which is spread by the Asian citrus psyllid Diaphorina citri Kuwayama . The objectives of the study were to assess whether tree growth, soil and leaf tissue nutrient concentrations, fruit yield, and postharvest quality can be improved through improved nutrient management of sweet orange grown on selected rootstocks. The study of ‘Hamlin’ citrus trees [ Citrus sinensis (L.) Osbeck] budded on Cleopatra mandarin ( Citrus reshni ) or Swingle rootstocks [ Citrus paradisi Macf. × Poncirus trifoliata (L.) Raf.] was conducted from 2019 to 2021. The study was conducted in a split-plot design with two rootstocks as main plots, three nitrogen (N) rates (168, 224, and 280 kg·ha −1 ) as sub-plots, and secondary macronutrients [control, calcium (Ca), or magnesium (Mg) thiosulfates at 45 kg·ha −1 each and a combination of Ca and Mg at 22.5 kg·ha −1 each)] as a sub-subplots replicated four times. Results indicated that soil ammonium nitrogen (NH 4 -N) and nitrate nitrogen (NO 3 -N) significantly accumulated in an oscillating (between spring and summer seasons) pattern for soil NH 4 -N and a persistent accumulation of NO 3 -N on both rootstocks over time from <1.0 mg·kg −1 at the beginning; thereafter, it stabilized at approximately 4.0 mg·kg −1 on both rootstocks. Citrus trees budded on Swingle rootstocks responded to macronutrient applications more than those on Cleopatra rootstocks. Leaf micronutrients [manganese (Mn) and zinc (Zn)], as opposed to leaf boron (B), iron (Fe), and copper (Cu) concentrations, were within the lowest ranges. Hence, supplementary micronutrients (Mn and Zn) are a mandatory cultural practice for HLB-affected Hamlin citrus trees. Control trees had a lower leaf area index (LAI) than those that received either Ca and/or Mg treatments, and the impact was significant on Swingle, rather than on Cleopatra, rootstocks. Trees on Cleopatra were inherently greater in canopy volume compared with Swingle rootstocks. However, the impact of secondary macronutrients manifested only after the third year of the study. Fruit yield significantly increased by a magnitude of 1.8-times in the trees that received combined Ca and Mg treatments compared with control trees. Greater total soluble solids were observed in trees that received Mg or combined Ca and Mg treatments. Thus, we conclude that nutrient management enhanced tree growth, fruit yield, and postharvest quality and should be considered as an immediate solution for mitigating HLB until it can be permanently cured or eradicated.