Calcium and magnesium input did not decrease huanglongbing progress and yield loss of sweet orange trees

Abstract

Enhanced nutritional programs have been applied by growers in an attempt to reverse crop losses caused by huanglongbing (HLB), despite the limited evidence available on their effects on HLB progress and damage mitigation in symptomatic sweet orange trees. Therefore, a long-term trial was set up in a 10-year-old ‘Hamlin’ sweet orange orchard to verify the effects of an additional supply of calcium and magnesium on HLB incidence, severity progress, and symptomatic tree yield. The trial had four treatments in four randomized blocks (ammonium nitrate or calcium nitrate, with or without magnesium sulfate). In each plot and treatment, 30 trees with ∼33.7% of canopy with HLB symptoms (HLB severity) were selected in November 2012 (group 1 trees), and 20 trees with ∼5.2% HLB severity were selected in March 2014 (group 2 trees). Additionally, 20 asymptomatic trees were selected in each plot and treatment. Those selected trees were individually harvested every year. HLB incidence, HLB severity, and yield of individual trees and of total area were assessed from 2012 to 2018. There were no significant effects of calcium and/or magnesium inputs on the reduction of HLB incidence progress (increased from 18.3 to 53.8%) or on HLB severity progress (increased from 33.7 to 60.7% for group 1 and from 5.2 to 61.3% for group 2). In all plots, symptomatic trees never recovered their yield to the same yield of asymptomatic trees, producing 60 and 48% less for groups 1 and 2, respectively, at the end of the trial. The total plot yield also declined over time.