Citrus Huanglongbing (HLB) caused by ‘ Candidatus Liberibacter asiaticus’ (CLas) is the most devastating citrus disease worldwide. CLas induces systemic and chronic reactive oxygen species (ROS) production, which has been suggested to be a primary cause of cell death in phloem tissues and subsequent HLB symptoms. Mitigating oxidative stress caused by CLas using horticultural approaches has been suggested as a useful strategy to reduce HLB damages. To provide information regarding the application timing to mitigate ROS, we investigated monthly dynamics of CLas concentration, CLas-triggered ROS, and phloem cell death in the bark tissues of asymptomatic and symptomatic branches of HLB-positive Hamlin and Valencia sweet orange trees in the field. Healthy branches in the screenhouse were used as controls. CLas concentration exhibited significant variations over the course of the year, with two distinct peaks observed in Florida citrus groves—late spring/early summer and late fall. Within both Hamlin and Valencia asymptomatic tissues, CLas concentration demonstrated a negative correlation with the deviation between the monthly average mean temperature and the optimal temperature for CLas colonization in plants (25.7°C). However, such a correlation was not evident in symptomatic tissues of Hamlin or Valencia sweet oranges. ROS levels were consistently higher in symptomatic or asymptomatic branches than in healthy branches in most months. ROS concentrations were higher in symptomatic branches than in asymptomatic branches in most months. CLas triggered significant increases in ion leakage in most months for asymptomatic and symptomatic branches compared with healthy controls. In asymptomatic branches of Hamlin, a positive correlation was observed between CLas concentration and ROS concentrations, CLas concentration and ion leakage levels, as well as ROS and ion leakage. Intriguingly, such a relationship was not observed in Valencia asymptomatic branches or in the symptomatic branches of Hamlin and Valencia. This study sheds light on the pathogenicity of CLas by providing useful information on the temporal dynamics of ROS production, phloem cell death, and CLas growth, as well as provides useful information in determining the timing for application of antioxidants and antimicrobial agents to control HLB.