ASHS 2025 Conference

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, aka citrus greening) in recent years. However, there is no single cure for HLB caused by Candidatus Liberibacter asiaticus, 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 using selected rootstocks. The study was conducted on ‘Hamlin’ citrus trees [Citrus sinensis (L.) Osbeck] budded on Cleopatra mandarin (Citrus reshni) or Swingle rootstocks [Citrus paradisi Macf. Poncirus trifoliata (L.) Raf.] from 2019 – 2021. The study was conducted in a split plot design with two rootstocks as main plots, three nitrogen rates (168, 224, and 280 kg ha-1) as sub-plots, and secondary macronutrients (control, Ca or Mg-thiosulfates at 45 kg ha-1 each, and a combination of Ca and Mg at 22.5 kg ha-1 each) replicated four times. Results indicated that soil ammonium nitrogen (NH4-N) and nitrate nitrogen (NO3-N) were significantly accumulated in an oscillating (between spring and summer seasons) pattern for soil NH4-N and persistent accumulation of NO3-N on both rootstocks. Leaf Ca and Mg nutrient concentrations were significantly greater in Cleopatra than Swingle rootstocks. Trees on Swingle rootstocks react greater than those on Cleopatra rootstocks. Control trees had a lower LAI than those that received either Ca and/or Mg treatments, and the impact is significantly prevalent on Swingle than on Cleopatra rootstocks. Trees on Cleopatra had greater canopy volumes than Swingle rootstocks. However, the impact of secondary macronutrients manifested only after the third year of the study. Fruit yield significantly increased with a magnitude of 1.8× in the trees that received combined Ca and Mg treatments than a control tree. Greater fruit total soluble solids were detected in trees that received Mg or combined Ca and Mg treatments. Hence, nutrient management can enhance tree growth, fruit yield, and postharvest quality and can be used as an immediate solution until the HLB can be permanently eradicated.