A Review of the 'Candidatus Liberibacter africanus' Citrus Pathosystem in Africa.

It has been nearly 100 years since citrus growers in two distinct regions in the northern provinces of South Africa noticed unusual symptoms in their citrus trees, causing significant crop losses. They had no idea that these symptoms would later become part of an almost global pandemic of a disease called greening or huanglongbing (HLB). The rapid spread of the disease indicated that it might be caused by a transmissible pathogen, but it took >50 years to identify the causative agent as 'Candidatus Liberibacter africanus'. Recently, the disease appeared in more African countries, spreading by both infected planting material and Trioza erytreae. To date, five 'Ca. L. africanus' subspecies have been identified in various rutaceous species, with 'Ca. L. africanus subsp. clausenae' the only subspecies for which a biovar was detected in citrus. Efforts to detect and differentiate HLB-causing Liberibacter species are ongoing, and recent developments are discussed here. This review focuses on aspects of the African form of HLB, including its specific bacterial species and subspecies, its main insect vector, its geographic distribution, and current management strategies.

Distribution and Genetic Diversity of Coguvirus eburi in South African Citrus and the Development of a Real-Time RT-PCR Assay for Citrus-Infecting Coguviruses.

Citrus virus A (CiVA), a novel negative-sense single-stranded RNA virus assigned to the species Coguvirus eburi in the genus Coguvirus, was detected in South Africa with the use of high-throughput sequencing after its initial discovery in Italy. CiVA is closely related to citrus concave gum-associated virus (CCGaV), recently assigned to the species Citrus coguvirus. Disease association with CiVA is, however, incomplete. CiVA was detected in grapefruit (C. paradisi Macf.), sweet orange [C. sinensis (L.) Osb.], and clementine (C. reticulata Blanco) in South Africa, and a survey to determine the distribution, symptom association, and genetic diversity was conducted in three provinces and seven citrus production regions. The virus was detected in 'Delta' Valencia trees in six citrus production regions, and a fruit rind symptom was often observed on CiVA-positive trees. Additionally, grapefruit showing symptoms of citrus impietratura disease were positive for CiVA. This virus was primarily detected in older orchards that were established prior to the application of shoot tip grafting for virus elimination in the South African Citrus Improvement Scheme. The three viral-encoded genes of CiVA isolates from each cultivar and region were sequenced to investigate sequence diversity. Genetic differences were detected between the Delta Valencia, grapefruit, and clementine samples, with greater sequence variation observed with the nucleocapsid protein (NP) compared with the RNA-dependent RNA polymerase (RdRp) and the movement protein (MP). A real-time detection assay, targeting the RdRp, was developed to simultaneously detect citrus-infecting coguviruses, CiVA and CCGaV, using a dual priming reverse primer to improve PCR specificity.

Monitoring Benzimidazole Resistance in Phyllosticta citricarpa Using a Molecular Assay Targeting Mutations in Codons 198 and 200 of the ß-Tubulin Gene.

Citrus black spot (CBS), caused by Phyllosticta citricarpa, is an economically important disease, which is effectively controlled by repeated fungicide applications to protect fruit from infection. Systemic fungicides such as benzimidazoles are widely used for controlling CBS in South Africa, but the molecular mechanisms of benzimidazole resistance in P. citricarpa had not been investigated. Analysis of the nucleotide sequence of the ß-tubulin gene in P. citricarpa revealed mutations inducing three amino acid replacements in benzimidazole-resistant isolates when compared with those of sensitive strains. Amino acid replacements in benzimidazole-resistant isolates included the change of glutamic acid to either alanine or lysine at codon 198 of the ß-tubulin gene and the change from phenylalanine to tyrosine at codon 200. All three mutations were previously implicated in benzimidazole resistance in several fungal pathogens. A PCR assay was designed to amplify a portion of the ß-tubulin gene, which is subsequently sequenced to identify benzimidazole resistance in P. citricarpa. This PCR and sequence assay was found to be a more rapid and reliable method for detecting resistance compared with the fungicide-amended plate tests and is valuable for monitoring the occurrence of benzimidazole-resistant P. citricarpa and for assessment of the need for alternative CBS management practices.

Towards the validation of high-throughput sequencing (HTS) for routine plant virus diagnostics: measurement of variation linked to HTS detection of citrus viruses and viroids.

BACKGROUND: High-throughput sequencing (HTS) has been applied successfully for virus and viroid discovery in many agricultural crops leading to the current drive to apply this technology in routine pathogen detection. The validation of HTS-based pathogen detection is therefore paramount. METHODS: Plant infections were established by graft inoculating a suite of viruses and viroids from established sources for further study. Four plants (one healthy plant and three infected) were sampled in triplicate and total RNA was extracted using two different methods (CTAB extraction protocol and the Zymo Research Quick-RNA Plant Miniprep Kit) and sent for Illumina HTS. One replicate sample of each plant for each RNA extraction method was also sent for HTS on an Ion Torrent platform. The data were evaluated for biological and technical variation focussing on RNA extraction method, platform used and bioinformatic analysis. RESULTS: The study evaluated the influence of different HTS protocols on the sensitivity, specificity and repeatability of HTS as a detection tool. Both extraction methods and sequencing platforms resulted in significant differences between the data sets. Using a de novo assembly approach, complemented with read mapping, the Illumina data allowed a greater proportion of the expected pathogen scaffolds to be inferred, and an accurate virome profile was constructed. The complete virome profile was also constructed using the Ion Torrent data but analyses showed that more sequencing depth is required to be comparative to the Illumina protocol and produce consistent results. The CTAB extraction protocol lowered the proportion of viroid sequences recovered with HTS, and the Zymo Research kit resulted in more variation in the read counts obtained per pathogen sequence. The expression profiles of reference genes were also investigated to assess the suitability of these genes as internal controls to allow for the comparison between samples across different protocols. CONCLUSIONS: This study highlights the need to measure the level of variation that can arise from the different variables of an HTS protocol, from sample preparation to data analysis. HTS is more comprehensive than any assay previously used, but with the necessary validations and standard operating procedures, the implementation of HTS as part of routine pathogen screening practices is possible.

Grapefruit Field Trial Evaluation of Citrus Tristeza Virus T68-Strain Sources.

Determination of virus genomes and differentiation of strains and strain variants facilitate the linkage of biological expression to specific genetic units. For effective management of stem pitting disease of citrus tristeza virus (CTV) by cross-protection, an understanding of these links is necessary. The deliberate field application of a biological agent such as a virus first requires a thorough assessment of the long-term impact before it can be applied commercially. Three CTV sources were genetically characterized as different variants of the T68 strain, and their long-term effects on stem pitting and production were investigated. The different CTV sources were inoculated to 'Star Ruby' grapefruit trees and evaluated for a number of biological parameters in a field trial in the Limpopo Province of South Africa over a 10-year period. Significant differences were observed in stem pitting severity, impact on tree growth, yield, and the percentage of small fruit produced. These T68 variants were also associated with different stem pitting phenotypes. The variants differed in only 44 nucleotide positions across their genomes, and these minor genetic differences can therefore be used to identify possible genome regions affecting stem pitting.

Citrus Tristeza Virus Isolates of the Same Genotype Differ in Stem Pitting Severity in Grapefruit.

Two isolates of the T68 genotype of citrus tristeza virus (CTV) were derived from a common source, GFMS12, by single aphid transmission. These isolates, named GFMS12-8 and GFMS12-1.3, induced stem pitting with differing severity in 'Duncan' grapefruit (Citrus × paradisi [Macfad.]). Full-genome sequencing of these isolates showed only minor nucleotide sequence differences totaling 45 polymorphisms. Numerous nucleotide changes, in relatively close proximity, were detected in the p33 open reading frame (ORF) and the leader protease domains of ORF1a. This is the first report of full-genome characterization of CTV isolates of a single genotype, derived from the same source, but showing differences in pathogenicity. The results demonstrate the development of intragenotype heterogeneity known to occur with single-stranded RNA viruses. Identification of genetic variability between isolates showing different pathogenicity will enable interrogation of specific genome regions for potential stem pitting determinants.

First report of 'Candidatus Liberibacter africanus' associated with African Greening of Citrus in Angola.

Huanglongbing (HLB, Asian Citrus Greening), the most devastating disease of citrus has not been detected in southern Africa (Gottwald, 2010). HLB is associated with 'Candidatus Liberibacter asiaticus' (CLas), a phloem-limited bacterium vectored by Diaphorina citri Kuwayama (Hemiptera: Liviidae), the Asian Citrus Psyllid (ACP). African Citrus Greening, associated with 'Candidatus Liberibacter africanus' (CLaf) and its vector the African Citrus Triozid, Trioza erytreae (Del Guercio) (Hemiptera: Triozidae), are endemic to Africa, although not previously reported from Angola. African Greening is less severe than HLB, largely due to heat sensitivity of CLaf and its vector. Introduction of HLB into southern Africa would be devastating to citrus production in commercial and informal sectors. Concern was raised that CLas or ACP might hae inadvertently been introduced into Angola. In July 2019, a survey was conducted in two citrus nurseries in Luanda and Caxito and in different orchards on 7 farms surrounding Calulo and Quibala. Yellow sticky traps for insects were placed at the various localities and collected after c. 3 weeks. Breeding signs of T. erytreae (pit galls) were observed on citrus in some locations, but no insect vectors were detected on traps. Trees were inspected for signs and symptoms of citrus pests and diseases, particularly those that resemble HLB (foliar blotchy mottle, shoot chlorosis, vein yellowing and corking, lopsided fruit with aborted seeds and colour inversion) and its vectors (pit galls on leaves or waxy exudates). Leaves and shoots with suspect symptoms were sampled for laboratory analysis (43 samples). DNA was extracted from petiole and midrib tissue of leaves using a modified CTAB extraction protocol of Doyle and Doyle (1990). Real-time PCR was done using universal Liberibacter primers of Roberts et al. (2015), CLaf specific primers of Li et al. (2006) and CLas specific primers of Bao et al. (2019). All real-time PCR protocols indicated the presence of CLaf in 6 samples (Tab. S1). CLas or other citrus Liberibacter species were not detected. The presence of CLaf in sample 37 was confirmed by constructing a library (NEXTFLEX® DNA Sequencing Kit, PerkinElmer) with extracted DNA and performing high-throughput sequencing on an Ion Torrent™ S5™ platform (Central Analytical Facility, Stellenbosch University). To improve the quality of the reads, all 233,617,700 obtained reads were trimmed from the 3' end to a maximum length of 240 nt using Trimmomatic (Bolger et al. 2014). The high quality reads were mapped to the Citrus sinensis reference genome (NC_023046.1) using Bowtie 2.3.4 (Langmead and Salzberg 2012) to subtract all the reads that had high identity to the host plant (number of mismatches allowed in the seed was set to 1). The 14,691,369 unmapped reads (6.2% of original data) were mapped to the CLaf reference genome NZ_CP004021.1 using CLC Genomics Workbench 10.1.1 (Qiagen) (Length fraction = 0.8; Similarity fraction = 0.9). A CLaf consensus genome was generated that spanned 99.7% of the reference genome and the 163001 mapped reads had a 22.9 mean read coverage. The consensus sequence was 99.7% identical to NZ_CP004021.1 and was submitted to Genbank as accession: CP054879. The positive CLaf detections were from trees with typical HLB or African Citrus Greening symptoms, viz. lopsided fruit with green stylar ends, aborted seed and stained columella at base of fruit button; yellow shoots with leaves showing symptoms of blotchy mottle and vein yellowing and corking (Fig. S1) in a commercial citrus farm outside Calulo and included 2 'Ponkan' mandarin (C. reticulata), 2 Valencia and 1 'Navelina' tree (C. sinensis), and a citrus nursery in Luanda (1 lime tree; C. aurantifolia) (Tab. S1). This first report of CLaf in Angola highlights the need to prevent spread by removing infected trees and managing the insect vector, as well as the need for further surveys to determine the occurrence of African Greening and its vectors in other provinces and to confirm the absence of exotic citrus pests and diseases. References Bao, M. et al. 2020. Plant Dis. 104:527 Bolger, A. M. et al. 2014. Bioinformatics. 30:2114-2120. Doyle, J.J. and Doyle, J.L. 1990. Focus 12:13 Gottwald, T.R. 2010. Annu. Rev. Phytopathol. 48:119 Langmead, B. and Salzberg, S. 2012. Nature Methods. 9:357-359. Li, W. et al. 2006. Jnl. Microbiol. Methods 66:104 Roberts, R. et al. 2015. Int. J. Syst. Evol. Micr. 65:723.

In silico analysis of the grapefruit sRNAome, transcriptome and gene regulation in response to CTV-CDVd co-infection.

BACKGROUND: Small RNA (sRNA) associated gene regulation has been shown to play a significant role during plant-pathogen interaction. In commercial citrus orchards co-infection of Citrus tristeza virus (CTV) and viroids occur naturally. METHODS: A next-generation sequencing-based approach was used to study the sRNA and transcriptional response in grapefruit to the co-infection of CTV and Citrus dwarfing viroid. RESULTS: The co-infection resulted in a difference in the expression of a number of sRNA species when comparing healthy and infected plants; the majority of these were derived from transcripts processed in a phased manner. Several RNA transcripts were also differentially expressed, including transcripts derived from two genes, predicted to be under the regulation of sRNAs. These genes are involved in plant hormone systems; one in the abscisic acid, and the other in the cytokinin regulatory pathway. Additional analysis of virus- and viroid-derived small-interfering RNAs (siRNAs) showed areas on the pathogen genomes associated with increased siRNA synthesis. Most interestingly, the starting position of the p23 silencing suppressor's sub-genomic RNA generated a siRNA hotspot on the CTV genome. CONCLUSIONS: This study showed the involvement of various genes, as well as endogenous and exogenous RNA-derived sRNA species in the plant-defence response. The results highlighted the role of sRNA-directed plant hormone regulation during biotic stress, as well as a counter-response of plants to virus suppressors of RNA-silencing.

Resolution of the Identity of 'Candidatus Liberibacter' Species From Huanglongbing-Affected Citrus in East Africa.

'Candidatus Liberibacter asiaticus', the bacterium associated with citrus Huanglongbing (HLB), was reported from Uganda and tentatively from Tanzania, posing a threat to citriculture in Africa. Two surveys of citrus expressing typical HLB symptoms were conducted in Uganda, Kenya, and Tanzania to verify reports of 'Ca. L. asiaticus' and to assess the overall threat of HLB to eastern and southern African citrus production. Samples were analyzed for the presence of 'Candidatus Liberibacter' species by real-time PCR and partial sequencing of three housekeeping genes, 16S rDNA, rplJ, and omp. 'Ca. L. africanus', the bacterium historically associated with HLB symptoms in Africa, was detected in several samples. However, samples positive in real-time PCR for 'Ca. L. asiaticus' were shown not to contain 'Ca. L. asiaticus' by sequencing. Sequences obtained from these samples were analogous to 'Ca. L. africanus subsp. clausenae', identified from an indigenous Rutaceae species in South Africa, and not to 'Ca. L. asiaticus'. Results indicate a nontarget amplification of the real-time assay and suggest that previous reports of 'Ca. L. asiaticus' from Uganda and Tanzania may be mis-identifications of 'Ca. L. africanus subsp. clausenae'. This subspecies was additionally detected in individual Diaphorina citri and Trioza erytreae specimens recovered from collection sites. This is the first report of 'Ca. L. africanus subsp. clausenae' infecting citrus and being associated with HLB symptoms in this host.

Characterization of Citrus tristeza virus Single-Variant Sources in Grapefruit in Greenhouse and Field Trials.

Citrus tristeza virus (CTV) is endemic to southern Africa and the stem pitting syndrome that it causes was a limiting factor in grapefruit production prior to the introduction of cross-protection in the Citrus Improvement Scheme. This disease mitigation strategy, using various field-derived CTV sources, has significantly extended the productive lifespan of grapefruit orchards in South Africa. CTV commonly occurs as a population of various strains, masking the phenotypic effect of individual strains. Likewise, current South African CTV cross-protection sources are strain mixtures, obscuring an understanding of which strains are influencing cross-protection. The severity of various CTV strains has mostly been assessed on sensitive indicator hosts, but their effect on commercial varieties has seldom been investigated. Single-variant CTV isolates were used to investigate the phenotypic expression of CTV strains in commercial grapefruit varieties as well as CTV indicator hosts. They were biologically characterized for their ability to cause stem pitting and their rate of translocation and titer in the different hosts, monitored by enzyme-linked immunosorbent assay. Complete genome sequences for three CTV strain variants were generated. Isolates of CTV strains VT, T68, RB, and HA16-5 did not induce severe stem pitting in four grapefruit hosts in a glasshouse trial. Viral titers of the strains differed in the grapefruit hosts, but the RB isolate reached a higher titer in the grapefruit hosts compared with the VT, T68, and HA16-5 isolates. Additionally, horticultural assessment of two grapefruit varieties inoculated with the RB isolate in two field trials demonstrated that mild stem pitting did not negatively influence the horticultural performance of the grapefruit trees over an eight-year assessment period. 'Star Ruby' trees containing the CTV source GFMS35 showed less stem pitting than trees inoculated with the RB isolate, but had smaller canopy volumes and lower yields than trees containing the RB isolate. This suggests that the influence of CTV sources on tree performance is not limited to the effect of stem pitting.

Reproducibility and Sensitivity of High-Throughput Sequencing (HTS)-Based Detection of Citrus Tristeza Virus and Three Citrus Viroids.

The credibility of a pathogen detection assay is measured using specific parameters including repeatability, specificity, sensitivity, and reproducibility. The use of high-throughput sequencing (HTS) as a routine detection assay for viruses and viroids in citrus was previously evaluated and, in this study, the reproducibility and sensitivity of the HTS assay were assessed. To evaluate the reproducibility of HTS, the same plants assayed in a previous study were sampled again, one year later, and assessed in triplicate using the same analyses to construct the virome profile. The sensitivity of the HTS assay was compared to routinely used RT-PCR assays in a time course experiment, to compensate for natural pathogen accumulation in plants over time. The HTS pipeline applied in this study produced reproducible and comparable results to standard RT-PCR assays for the detection of CTV and three viroid species in citrus. Even though the limit of detection of HTS can be influenced by pathogen concentration, sample processing method and sequencing depth, detection with HTS was found to be either equivalent or more sensitive than RT-PCR in this study.

Citrus Tristeza Virus Genotype Detection Using High-Throughput Sequencing.

The application of high-throughput sequencing (HTS) has successfully been used for virus discovery to resolve disease etiology in many agricultural crops. The greatest advantage of HTS is that it can provide a complete viral status of a plant, including information on mixed infections of viral species or virus variants. This provides insight into the virus population structure, ecology, or evolution and can be used to differentiate among virus variants that may contribute differently toward disease etiology. In this study, the use of HTS for citrus tristeza virus (CTV) genotype detection was evaluated. A bioinformatic pipeline for CTV genotype detection was constructed and evaluated using simulated and real data sets to determine the parameters to discriminate between false positive read mappings and true genotype-specific genome coverage. A 50% genome coverage cut-off was identified for non-target read mappings. HTS with the associated bioinformatic pipeline was validated and proposed as a CTV genotyping assay.