Plant Science

Citrus greening or Huanglongbing (HLB) is one of the main devastating citrus diseases worldwide. HLB causing infectious bacterium “Candidatus Liberibacter” is a phloem specific and fastidious agent, having three different species. A comparison of conventional PCR and real-time quantitative PCR (qPCR) was done by universal 16S rDNA marker. In the current study conventional PCR was used to early screening of Candidatus Liberibacter spp and real-time quantitative PCR used for the quantification of HLB causing agent in citrus trees. Total 35 citrus cultivars were selected from mandarins and sweet oranges groups from an orchard “Citrus Research Institute, Sargodha,” Pakistan. The DNA was extracted from HLB asymptomatic and symptomatic leaves. By conventional PCR, the HLB infectious bacterium has identified in 11 sweet oranges samples (31%) in which an amplicon of 1174 bp was generated. While qPCR subsequently reported to quantify the titer of Ca. L.asiaticus in host plant. In the present study qPCR results showed the pathogen could be detected with a Cq value getting in ranged of 26.58to 38.17. Samples with higher Cq ratio showed lower copy number of targeted gene. There was no amplification detected with healthy citrus samples and water control. A robust and sensitive qPCR assay can be used for the screening and quantify the Ca. L. asiaticus strain to confirm the HLB infection in Pakistan. This diagnostic test will be very helpful for implementing a wide range of diagnostics research programs aiming at ultimately developing strategies for the control of HLB in Pakistan. Keywords: Citrus greening, Conventional PCR, Candidatus Liberibacter asiaticus, Huanglongbing, cytochrome oxidase, real-time qPCR.

‘Candidatus Liberibacter asiaticus’ (CLas) is the predominant causal agent of citrus huanglongbing, the most devastating citrus disease worldwide. CLas colonizes phloem tissue and causes phloem dysfunction. The pathogen population size in local tissues and in the whole plant is critical for the development of disease symptoms by determining the load of pathogenicity factors and metabolic burden to the host. However, the total population size of CLas in a whole plant and the ratio of CLas to citrus cells in local tissues have not been addressed previously. The total CLas population size for 2.5-year-old ‘Valencia’ sweet orange on ‘Kuharske’ citrange rootstock trees was quantified using quantitative PCR to be approximately 1.74 × 109 cells/tree, whereas 7- and 20-year-old sweet orange trees were estimated to be 4.3 × 1010 cells/tree, and 6.0 × 1010 cells/tree, respectively. The majority of CLas cells were distributed in leaf tissues (55.58%), followed by those in branch (36.78%), feeder root (4.75%), trunk (2.39%), and structural root (0.51%) tissues. The ratios of citrus cells to CLas cells for branch, leaf, trunk, feeder root, and structural root samples were within approximately 39 to 79, 44 to 124, 153 to 1,355, 191 to 1,054, and 561 to 3,760, respectively, representing the metabolic burden of CLas in different organs. It was estimated that the ratios of phloem cells to CLas cells for branch, leaf, trunk, feeder root, and structural root samples are approximately 0.39 to 0.79, 0.44 to 1.24, 1.53 to 13.55, 1.91 to 10.54, and 5.61 to 37.60, respectively. Approximately 0.01% of the total citrus phloem volume was estimated to be occupied by CLas, explaining the difficulty to observe CLas in most tissues under transmission electron microscopy. The CLas titer inside the leaf was estimated to be approximately 1.64 × 106 cells/leaf or 9.2 × 104 cells cm–2 in leaves, approximately 104 times less than that of typical apoplastic bacterial pathogens. This study provides quantitative estimates of phloem colonization by bacterial pathogens and furthers the understanding of the biology and virulence mechanisms of CLas.

Streptomycin (STR) has been used to control citrus huanglongbing (HLB) caused by ‘Candidatus Liberibacter asiaticus’ (CLas) via foliar spray. Here, we studied the residue dynamics of STR and its effect on CLas titers in planta applied by foliar spray and trunk injection of 3-year-old citrus trees that were naturally infected by CLas in the field. After foliar spray, STR levels in leaves peaked at 2 to 7 days postapplication (dpa) and gradually declined thereafter. The STR spray did not significantly affect CLas titers in leaves of treated plants as determined by quantitative PCR. After trunk injection, peak levels of STR were observed 7 to 14 dpa in the leaf and root tissues, and near-peak levels were sustained for another 14 days before significantly declining. At 12 months after injection, moderate to low or undetectable levels of STR were observed in the leaf, root, and fruit, depending on the doses of STR injected, with a residue level of 0.28 µg/g in harvested fruit at the highest injection concentration of 2.0 µg/tree. CLas titers in leaves were significantly reduced by trunk injection of STR at 1.0 or 2.0 g/tree, starting from 7 dpa and throughout the experimental period. The reduction of CLas titers was positively correlated with STR residue levels in leaves. The in planta minimum effective concentration of STR needed to suppress the CLas titer to an undetectable level (cycle threshold ≥36.0) was 1.92 µg/g fresh weight. Determination of the in planta minimum effective concentration of STR against CLas and its spatiotemporal residue levels in planta provides the guidance to use STR for HLB management.