Publications

4529

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.

Areca catechu palm is an important cash plant in Hainan Island of China and also in the tropical regions of the world. A. catechu palm yellow leaf (AcYL) disease caused by phytoplasmas is a devastating disease for plant production. In the study, the phytoplasmas associated with the AcYL disease were identified and characterized based on their conserved genes, and genetic variation and phylogenetic relationship of the phytoplasma strains in the 16SrXXXII group were demonstrated. The results indicated that A. catechu palm plants showing yellow leaf symptoms were infected by ‘Candidatus Phytoplasma malaysianum’-related strains belonging to the 16SrXXXII-D subgroup. BLAST and multiple sequence alignment analysis based on 16S rRNA and secA genes showed that the AcYL phytoplasmas shared 100% sequence identity and 100% homology with the ‘Ca. P. malaysianum’-related strains. Phylogenetic analysis indicated that the AcYL phytoplasmas and ‘Ca. P. malaysianum’-related strains belonging to the 16SrXXXII group clustered into one clade with a 100% bootstrap value. Based on computer-simulated digestions, six kinds of restriction fragment length polymorphism patterns within the 16SrXXXII group were obtained, and a novel subgroup in the 16Sr group was recommended to propose and describe the relevant strains in this 16Sr subgroup. To our knowledge, this is the first study to report that A. catechu palm showing yellow leaf symptoms was infected by ‘Ca. P. malaysianum’-related strains belonging to the 16SrXXXII group. A novel 16Sr subgroup, 16SrXXXII-F, was proposed based on the systematical analysis of genetic variation of all phytoplasmas within the 16SrXXXII group. The findings of this study will support references for monitoring the epidemiology and developing effective prevention strategies for AcYL disease.

Huanglongbing (HLB), which is associated with the phloem-limited bacteria Candidatus Liberibacter asiaticus (CLas), is a devastating disease that affects citrus trees worldwide. Because of the pervasiveness of the bacteria and psyllid vector, the disease is considered endemic in Florida. Although the effects of CLas on tree growth and physiology have been investigated for decades, most studies compared infected and noninfected trees under greenhouse conditions. This study used newly planted field-grown ‘Valencia’ sweet orange (Citrus sinensis) trees on two different rootstocks to monitor the distribution and accumulation of CLas in aboveground and belowground tissues following natural psyllid colonization and assess tree physiological responses and biomass reductions under HLB-endemic conditions. Trees were transplanted into the field with individual protective covers (IPCs), which are used to exclude psyllids and prevent infection. Openings were cut in the IPCs of half of the trees; to promote infection, these IPCs were temporarily removed during the main vegetative flushing period when psyllid populations were high. All trees that were exposed to psyllids became infected and displayed the symptoms typically associated with HLB. Throughout the study, higher levels of CLas were detected in the leaves compared with those in the fibrous roots. Trees that were not exposed to psyllids remained noninfected and healthy. After 18 months, a subset of trees was excavated to assess biomass differences between infected and noninfected trees. Infected trees had root system reductions of 37% and shoot system reductions of 20%, thereby significantly reducing the belowground-to-aboveground biomass ratio. Fibrous root loss was 49% and more severe than the loss of the rest of the root tissue. This study is the first to demonstrate the full extent of damage caused by CLas infection under natural HLB-endemic conditions. The results confirm previous observations that suggested fibrous root loss as one of the major consequences of infection and colonization with CLas. They also reinforce the benefits of using IPCs to prevent infection of young citrus trees during the first years of growth in the field.