First Report of 'Candidatus Liberibacter asiaticus' Associated with Huanglongbing in Sweet Orange in Ethiopia

Identification and Distribution of the 'Candidatus Liberibacter asiaticus'-Asian Citrus Psyllid Pathosystem in Saudi Arabia

Citrus greening disease was first reported in Saudi Arabia during the 1970s when characteristic foliar and fruit symptoms were observed in commercial citrus groves. However, 'Candidatus Liberibacter asiaticus' (CLas) was not detected in symptomatic trees until 1981 to 1984 when CLas-like cells were observed by transmission electron microscopy in leaves collected from symptomatic citrus groves in southwestern Saudi Arabia. Despite the anticipated establishment of the CLas-Asian citrus psyllid (ACP) (Diaphorina citri Kuwayama) pathosystem, CLas presence has not been verified in suspect trees nor have ACP infestations been documented. Given the recent expansion of citrus production in Saudi Arabia, a systematic country-wide survey was carried out to determine the potential CLas distribution in the 13 citrus-growing regions of the country. Citrus trees were surveyed for the presence of CLas-psyllid vector(s) and characteristic disease symptoms in commercial and urban citrus trees. Adult psyllids collected from infested citrus trees were identified as ACP based on morphological characteristics. Real-time quantitative PCR amplification (qPCR) of the CLas β-subunit of the ribonucleotide reductase (RNR) gene from citrus leaf and fruit samples and/or ACP adults revealed that trees were positive for CLas detection in 10 of the 13 survey regions; however, CLas was undetectable in ACP adults. Phylogenetic and single nucleotide polymorphism (SNP) analyses of a PCR-amplified, cloned fragment of the CLas 16S rRNA gene (∼1.1 kbp) indicated Saudi Arabian isolates were most closely related to Florida, U.S.A., isolates. Analysis of climate variables indicated that the distribution of the ACP-CLas pathosystem observed in Saudi Arabia was consistent with published predictions of terrains most likely to support establishment.

Crossing the Gateless Barriers: Factors Involved in the Movement of Circulative Bacteria Within Their Insect Vectors

Plant bacterial pathogens transmitted by hemipteran vectors pose a large threat to the agricultural industry worldwide. Although virus-vector relationships have been widely investigated, a significant gap exists in our understanding of the molecular interactions between circulative bacteria and their insect vectors, mainly leafhoppers and psyllids. In this review, we will describe how these bacterial pathogens adhere, invade, and proliferate inside their insect vectors. We will also highlight the different transmission routes and molecular factors of phloem-limited bacteria that maintain an effective relationship with the insect host. Understanding the pathogen-vector relationship at the molecular level will help in the management of vector-borne bacterial diseases.

Report on the first detection of Asian citrus psyllid Diaphorina citri Kuwayama (Hemiptera: Liviidae) in the Republic of Benin, West Africa

The Asian citrus psyllid (ACP), Diaphorina citri, was detected for the first time in the Republic of Benin, West Africa. The ACP is a known vector of Candidatus Liberibacter asiaticus (CLas), the putative causal agent of the devastating Huanglongbing (HLB; citrus greening disease). During visual surveys, ACP was only observed on residential citrus trees in southern Benin, but not in residential areas or commercial groves in the central and northern parts of the country. Its identity was confirmed morphologically and molecularly via DNA barcoding with published primers. Analysis of the obtained sequences showed that the ACP recorded in Benin clustered with the ones previously reported from Nigeria, suggesting a common origin of both populations. The ACP samples from Benin also carried Ca. Carsonella ruddii and Ca. Profftella armatura, two commonly found ACP endosymbionts. However, all the sampled ACP individuals tested negative for Ca. Liberibacter africanus, Ca. Liberibacter americanus, and CLas by quantitative polymerase chain reaction. This is the second report of the ACP in West Africa after Nigeria, the eastern bordering country of the Republic of Benin. Benin has an expanding commercial citrus industry, especially in the southern part of the country. Although the ACP samples tested negative for the HLB associated bacteria, the detection of ACP in the country requires swift actions including area-wide surveys to determine the extent of spread of this pest and the implementation of eradication or control efforts to prevent its establishment and spread of HLB in the country.

Integrated Pest Management Strategies for Asian Citrus Psyllid Diaphorina citri Kuwayama (Hemiptera: Psyllidae) and Huanglongbing in Citrus for Sarawak, East Malaysia, Borneo

The Asian citrus psyllid (ACP), Diaphorina citri Kuwayama, transmits ‘Candidatus Liberibacter asiaticus’ (CLas), a phloem-limited bacterium associated with the severe Asian form of huanglongbing (HLB), and the most destructive disease of citrus. The pathogen and the psyllid, both of South Asian origin, are now widespread in citrus regions of Asia and the Americas. There is no cure for the disease. Application of synthetic pesticides, in some instances more frequently than fortnightly, to minimise incidence of ACP in citrus orchards, has not prevented inevitable impacts of the disease in regions of Asia where CLas is present. Despite the inevitable spread of the disease, significant progress has been made in Sarawak since the mid-1990s towards effectively implementing integrated pest management (IPM) programs for stemming the impact of the disease and detrimental consequences of over-reliance on synthetic pesticides. Growers are encouraged to plant pathogen-free trees, remove diseased trees, monitor incidence of the psyllid, and to use pesticides judiciously to reduce their detrimental impacts on natural enemies. Knowledge has been enhanced through research on seasonal incidence of the psyllid, use of mineral oils, development of protocols and iodine−starch test kits for detecting infected trees, PCR for confirming the presence of CLas in symptomatic leaves, methods for monitoring incidence the psyllid, and training extension staff and growers. However, major impediments to increasing the average longevity of trees beyond <5 years in poorly managed orchards, based on marcotting (air layering), and >12 years in well-managed orchards, based on pathogen-free trees, still need to be addressed. These include grower knowledge, marcotting, aggressive marketing of synthetic pesticides, high prices of mineral oils, spray application procedures, and better reliance on natural enemies of the psyllid.

Genetic diversity of Diaphorina citri (Hemiptera: Liviidae) unravels phylogeographic structure and invasion history of eastern African populations

The Asian citrus psyllid (Diaphorina citri Kuwayama) is a key pest of Citrus sp. worldwide, as it acts as a vector for Candidatus Liberibacter asiaticus, the bacterial pathogen that causes citrus Huanglongbing. Diaphorina citri has been reported in Kenya, Tanzania, and more recently in Ethiopia. This study assessed the genetic diversity and phylogeographic structure of the pest to gain insights into the potential sources of its introduction into Africa. Population structure and differentiation of D. citri populations from China, Ethiopia, Kenya, Tanzania, and the USA were assessed using 10 microsatellite loci. Additionally, five new complete mitogenomes of D. citri collected in China, Ethiopia, Kenya, Tanzania, and the USA were analyzed in the context of publicly available sequences. Genotype data grouped the D. citri populations from Kenya and Tanzania in one cluster, and those from Ethiopia formed a separate cluster. The two genetic clusters inferred from genotype data were congruent with mitochondrial sequence data. The mitogenomes from Kenya/Tanzania/China had 99.0% similarity, and the Ethiopia/USA had 99.9% similarity. In conclusion, D. citri populations in eastern Africa have different sources, as the Kenyan and Tanzanian populations probably originated from southeastern Asia, while the Ethiopian population most probably originated from the Americas.

Cultural Management of Huanglongbing: Current Status and Ongoing Research

Huanglongbing (HLB), formerly known as greening, is a bacterial disease restricted to some Asian and African regions until two decades ago. Nowadays, associated bacteria and their vectors have spread to almost all citrus-producing regions, and it is currently considered the most devastating citrus disease. HLB management can be approached in terms of prevention, limiting or avoiding pathogen and associated vectors to reach an area, or in terms of control, trying to reduce the impact of the disease by adopting different cultural strategies depending on infestation/infection levels. In both cases, control of psyllid populations is currently the best way to stop HLB spread. Best cultural actions (CHMAs, TPS system) to attain this goal and, thus, able to limit HLB spread, and ongoing research in this regard is summarized in this review.

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.

Genome Sequence Resource of 'Candidatus Liberibacter asiaticus' from Diaphorina citri Kuwayama (Hemiptera: Liviidae) from La Réunion

'Candidatus Liberibacter asiaticus' is an insect-transmitted, phloem-restricted α-proteobacterium associated with huanglongbing. Here, we provide the whole genome sequence of 'Ca. L. asiaticus' strain ReuSP1 from its insect vector Diaphorina citri (Hemiptera: Liviidae) collected in La Réunion. The genome is composed of 1,230,064 bp and has a 36.5% G+C content. This study reports the first 'Ca. L. asiaticus' genome sequence from La Réunion, which will add to 'Ca. L. asiaticus' genome resources and help elucidate our understanding of the introduction pathway into La Réunion.

Detection of Asian Citrus Psyllid (Hemiptera: Psyllidae) in Ethiopia: A New Haplotype and its Implication to the Proliferation of Huanglongbing

Diaphorina citri Kuwayama, also known as the Asian citrus psyllid, is a pest of citrus known for its transmission of Candidatus Liberibacter asiaticus (Ca. L. asiaticus), the causal bacterium of Huanglongbing. The African citrus triozid Trioza erytreae (Del Guercio) (Hemiptera: Triozidae) has been the putative vector of Candidatus Liberibacter africanus (Ca. L. africanus) which causes the African citrus greening disease, until the recent detection of D. citri on the continent. Following reports of D. citri in Kenya and Tanzania, we surveyed citrus plants to establish the presence/absence of D. citri in Ethiopia in citrus-growing regions ranging from 900 to 2,460 m above sea level (masl). Diaphorina citri adults were detected in five of the surveyed sites in Ethiopia. Adult insects encountered were collected using an aspirator and stored in 97% ethanol. The mitochondrial cytochrome oxidase 1 (mt COI) gene of the collected insects was amplified using LepF1/LepR1 primers, and sequences obtained showed low variation, which fell within the acceptable range of species. BLAST was used to query the sequences obtained, and all the sequences linked to D. citri accessions that are available in GenBank. The analysis of the sequences revealed a new haplotype of the species that differs from haplotypes previously reported. Phylogenetic relationships of our samples and other D. citri reference sequences was inferred using the Maximum-likelihood method. Monophyly was observed between the samples and the publicly available sequences from global accessions. This is the first report of the presence of D. citri in Ethiopia.

Microbiome diversity in Diaphorina citri populations from Kenya and Tanzania shows links to China

The Asian citrus psyllid (Diaphorina citri) is a key pest of Citrus spp. worldwide, as it acts as a vector for “Candidatus Liberibacter asiaticus (Las)”, the bacterial pathogen associated with the destructive Huanglongbing (HLB) disease. Recent detection of D. citri in Africa and reports of Las-associated HLB in Ethiopia suggest that the citrus industry on the continent is under imminent threat. Endosymbionts and gut bacteria play key roles in the biology of arthropods, especially with regards to vector-pathogen interactions and resistance to antibiotics. Thus, we aim to profile the bacterial genera and to identify antibiotic resistance genes within the microbiome of different populations worldwide of D. citri. The metagenome of D. citri was sequenced using the Oxford Nanopore full-length 16S metagenomics protocol, and the “What’s in my pot” (WIMP) analysis pipeline. Microbial diversity within and between D. citri populations was assessed, and antibiotic resistance genes were identified using the WIMP-ARMA workflow. The most abundant genera were key endosymbionts of D. citri (“Candidatus Carsonella”, “Candidatus Profftella”, and Wolbachia). The Shannon diversity index showed that D. citri from Tanzania had the highest diversity of bacterial genera (1.92), and D. citri from China had the lowest (1.34). The Bray-Curtis dissimilarity showed that China and Kenya represented the most diverged populations, while the populations from Kenya and Tanzania were the least diverged. The WIMP-ARMA analyses generated 48 CARD genes from 13 bacterial species in each of the populations. Spectinomycin resistance genes were the most frequently found, with an average of 65.98% in all the populations. These findings add to the knowledge on the diversity of the African D. citri populations and the probable introduction source of the psyllid in these African countries.

First report of the occurrence of Asian citrus psyllid Diaphorina citri (Hemiptera: Liviidae), an invasive species in Nigeria, West Africa

The Asian citrus psyllid (ACP; Diaphorina citri) is the vector of Candidatus Liberibacter asiaticus (CLas) that is associated with the devastating Huanglongbing (HLB; citrus greening disease). This pest of Asian origin has spread into the Americas and more recently into a few countries in East Africa. During recent surveys, suspect ACP adults and nymphs were recorded for the first time infesting citrus trees in southwest Nigeria. Morphological identification and DNA barcoding confirmed the samples to be D. citri. Analysis of the obtained sequences revealed that the ACP recorded in Nigeria clustered with other taxa in the previously identified B1 clade that consists of populations from different continents. The presence of the endosymbionts Ca. Carsonella ruddii and Ca. Profftella armatura in ACP from Nigeria was also confirmed by PCR and Sanger sequencing. The ACP individuals were assayed for the presence of CLaf, CLam and CLas by qPCR, but none of the insects tested positive for any of the Liberibacters. The prolific nature of ACP and the tropical climate prevailing in the citrus-producing areas of Nigeria and other West African countries may favor its rapid spread and population increase, thus posing a grave threat to the sustainability of citriculture in these countries.

Mitochondrial genetic variation reveals phylogeographic structure and cryptic diversity in Trioza erytreae

Trioza erytreae is the main vector for ‘Candidatus Liberibacter africanus’, the causative agent of African Citrus Greening disease. The insect is widespread in Africa, and has recently disseminated to Southwestern Europe. This study aimed at generating reference mitogenome sequences for T. erytreae, as a background for future genetic diversity surveys. Complete mitochondrial sequences of three specimens collected in Ethiopia, Uganda and South Africa were recovered using Ion Torrent technology. The mitogenomes of T. erytreae from Uganda and Ethiopia were highly similar, and distinct from that found in South Africa. The phylogeographic structure of T. erytreae was assessed using genetic clustering and pairwise distances, based on a dataset of public COI sequences recorded as T. erytreae. The dataset revealed ten haplotypes with strong phylogeographic structure in Africa and Europe. Three haplotypes found in Kenya on Clausena anisata belonged to pairs separated by distances as high as 11.2%, and were basal to all other sequences. These results indicate that not all sequences identified as T. erytreae belong to the same species, and that some degree of specificity with different plant hosts is likely to exist. This study provides new baseline information on the diversity of T. erytreae, with potential implications for the epidemiology of African Citrus Greening disease.

Distribution of Candidatus Liberibacter species in Eastern Africa, and the First Report of Candidatus Liberibacter asiaticus in Kenya

Huanglongbing (HLB) is a serious disease of Citrus sp. worldwide. In Africa and the Mascarene Islands, a similar disease is known as African citrus greening (ACG) and is associated with the bacterium Candidatus Liberibacter africanus (Laf). In recent years, Candidatus Liberibacter asiaticus (Las) associated with the severe HLB has been reported in Ethiopia. Thus, we aimed to identify the Liberibacter species affecting citrus, the associated vectors in Eastern Africa and their ecological distribution. We assessed the presence of generic Liberibacter in symptomatic leaf samples by quantitative PCR. Subsequently, we sequenced the 50 S ribosomal protein L10 (rplJ) gene region in samples positive for Liberibacters and identified the species by comparison with public sequence data using phylogenetic reconstruction and genetic distances. We detected generic Liberibacter in 26%, 21% and 66% of plants tested from Uganda, Ethiopia and Kenya, respectively. The rplJ sequences revealed the most prevalent Liberibacters in Uganda and Ethiopia were LafCl (22%) and Las (17%), respectively. We detected Las in Kenya for the first time from three sites in the coastal region. Finally, we modelled the potential habitat suitability of Las in Eastern Africa using MaxEnt. The projection showed large areas of suitability for the pathogen in the three countries surveyed. Moreover, the potential distribution in Eastern Africa covered important citrus-producing parts of Ethiopia, Kenya, Uganda and Tanzania, and included regions where the disease has not been reported. These findings will guide in the development of an integrated pest management strategy to ACG/HLB management in Africa.

Molecular Characterization and Detection of 16SrIII Group Phytoplasma Associated with Huanglongbing Symptoms

When huanglongbing (HLB) was found in Brazil in 2004, 'Candidatus Liberibacter americanus' was infecting most of the trees while 'Ca. L. asiaticus' was present in a minor proportion. Currently, 'Ca. L. asiaticus' is the predominant bacterium associated with HLB in citrus trees in São Paulo (SP) and Minas Gerais (MG) States, the major citrus-growing regions in Brazil. A phytoplasma from the 16SrIX group was associated with HLB symptoms in Brazil in 2007, in plants free of Liberibacter spp. In this report, HLB samples testing negative for 'Ca. L. asiaticus', 'Ca. L. americanus', and 16SrIX phytoplasma were infected with 16SrIII phytoplasmas. Coinfection with 'Ca. L. asiaticus' and 16SrIII was also found. The 16S ribosomal RNA (rRNA) gene sequences from 22 samples were obtained and sequenced, confirming that the 16SrIII group phytoplasma is associated with HLB symptoms in SP and MG States. Ten single-nucleotide polymorphisms (SNPs) were found in the 1,427-bp 16S rRNA gene sequences from 16SrIII phytoplasmas from citrus, whereas none was detected in 16S rRNA gene sequences among 16SrIX phytoplasma from citrus. Ribosomal protein (rp) rpsSrplVrpsC gene sequences were amplified with 16SrIII group-specific primers, sequenced from a subset of nine samples, and assembled into three groups based on eight SNPs. SNPs in 16S rRNA gene and rp gene sequences are common in 16SrIII phytoplasmas from other hosts and this phytoplasma group is widespread in South America. 16SrIII phytoplasmas highly related are commonly found in Melia azedarach, a widespread tree in Brazil and Argentina. The finding of a new phytoplasma associated with HLB symptoms belonging to the 16SrIII group reinforces the need to develop diagnostic tools to assess HLB-associated microbiomes.

DNA Barcode Reference Library for the African Citrus Triozid, Trioza erytreae (Hemiptera: Triozidae): Vector of African Citrus Greening

Citrus (Citrus spp.) production continues to decline in East Africa, particularly in Kenya and Tanzania, the two major producers in the region. This decline is attributed to pests and diseases including infestation by the African citrus triozid, Trioza erytreae (Del Guercio) (Hemiptera: Triozidae). Besides direct feeding damage by adults and immature stages, T. erytreae is the main vector of 'Candidatus Liberibacter africanus', the causative agent of Greening disease in Africa, closely related to Huanglongbing. This study aimed to generate a novel barcode reference library for T. erytreae in order to use DNA barcoding as a rapid tool for accurate identification of the pest to aid phytosanitary measures. Triozid samples were collected from citrus orchards in Kenya, Tanzania, and South Africa and from alternative host plants. Sequences generated from populations in the study showed very low variability within acceptable ranges of species. All samples analyzed were linked to T. erytreae of GenBank accession number KU517195. Phylogeny of samples in this study and other Trioza reference species was inferred using the Maximum Likelihood method. The phylogenetic tree was paraphyletic with two distinct branches. The first branch had two clusters: 1) cluster of all populations analyzed with GenBank accession of T. erytreae and 2) cluster of all the other GenBank accession of Trioza species analyzed except T. incrustata Percy, 2016 (KT588307.1), T. eugeniae Froggatt (KY294637.1), and T. grallata Percy, 2016 (KT588308.1) that occupied the second branch as outgroups forming sister clade relationships. These results were further substantiated with genetic distance values and principal component analyses.

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.

A novel subspecies of 'Candidatus Liberibacter africanus' found on native Teclea gerrardii (Family: Rutaceae) from South Africa

The phloem limited bacterium 'Candidatus Liberibacter africanus' is associated with citrus greening disease in South Africa. This bacterium has been identified solely from commercial citrus in Africa and the Mascarene islands, and its origin may lie within an indigenous rutaceous host from Africa. Recently, in determining whether alternative hosts of Laf exist amongst the indigenous rutaceous hosts of its triozid vector, Trioza erytreae, three novel subspecies of Laf were identified i.e. 'Candidatus Liberibacter africanus subsp. clausenae', 'Candidatus Liberibacter africanus subsp. vepridis' and 'Candidatus Liberibacter africanus subsp. zanthoxyli' in addition to the formerly identified 'Candidatus Liberibacter africanus subsp. capensis'. The current study expands upon the range of indigenous rutaceous tree species tested for liberibacters closely related to Laf and its subspecies. A collection of 121 samples of Teclea and Oricia species were sampled from Oribi Gorge and Umtamvunu nature reserves in KwaZulu Natal. Total DNA was extracted and the presence of liberibacters from these samples determined using a generic liberibacter TaqMan real-time PCR assay. Liberibacters from positive samples were further characterised through amplification and sequencing of the 16S rRNA, outer-membrane protein (omp) and 50S ribosomal protein L10 (rplJ) genes. A single Teclea gerrardii specimen tested positive for a liberibacter and, through phylogenetic analyses of the three genes sequenced, was shown to be unique, albeit closely related to 'Ca. L. africanus' and 'Ca. L. africanus subsp. zanthoxyli'. We propose that this newly identified liberibacter be named 'Candidatus Liberibacter africanus subsp. tecleae'.

Three novel lineages of ‘Candidatus Liberibacter africanus’ associated with native rutaceous hosts of Trioza erytreae in South Africa

Greening disease of citrus in South Africa is associated with ‘Candidatus Liberibacter africanus’ (Laf), a phloem-limited bacterium vectored by the sap-sucking insect Trioza erytreae (Triozidae). Despite the implementation of control strategies, this disease remains problematic, suggesting the existence of reservoir hosts to Laf. The current study aimed to identify such hosts. Samples from 234 trees of Clausena anisata, 289 trees of Vepris lanceolata and 231 trees of Zanthoxylum capense were collected throughout the natural distribution of these trees in South Africa. Total DNA was extracted from samples and tested for the presence of liberibacters by a generic Liberibacter TaqMan real-time PCR assay. Liberibacters present in positive samples were characterized by amplifying and sequencing rplJ, omp and 16S rRNA gene regions. The identity of tree host species from which liberibacter sequences were obtained was verified by sequencing host rbcL genes. Of the trees tested, 33 specimens of Clausena, 17 specimens of Vepris and 10 specimens of Zanthoxylum tested positive for liberibacter. None of the samples contained typical citrus-infecting Laf sequences. Phylogenetic analysis of 16S rRNA gene sequences indicated that the liberibacters obtained from Vepris and Clausena had 16S rRNA gene sequences identical to that of ‘Candidatus Liberibacter africanus subsp. capensis’ (LafC), whereas those from Zanthoxylum species grouped separately. Phylogenetic analysis of the rplJ and omp gene regions revealed unique clusters for liberibacters associated with each tree species. We propose the following names for these novel liberibacters: ‘Candidatus Liberibacter africanus subsp. clausenae’ (LafCl), ‘Candidatus Liberibacter africanus subsp. vepridis’ (LafV) and ‘Candidatus Liberibacter africanus subsp. zanthoxyli’ (LafZ). This study did not find any natural hosts of Laf associated with greening of citrus. While native citrus relatives were shown to be infected with Laf-related liberibacters, nucleotide sequence data suggest that these are not alternative sources of Laf to citrus orchards, per se.

Spatial imaging of Zn and other elements in Huanglongbing-affected grapefruit by synchrotron-based micro X-ray fluorescence investigation

Huanglongbing (HLB) is a highly destructive, fast-spreading disease of citrus, causing substantial economic losses to the citrus industry worldwide. Nutrient levels and their cellular distribution patterns in stems and leaves of grapefruit were analysed after graft-inoculation with lemon scions containing 'Candidatus Liberibacter asiaticus' (Las), the heat-tolerant Asian type of the HLB bacterium. After 12 months, affected plants showed typical HLB symptoms and significantly reduced Zn concentrations in leaves. Micro-XRF imaging of Zn and other nutrients showed that preferential localization of Zn to phloem tissues was observed in the stems and leaves collected from healthy grapefruit plants, but was absent from HLB-affected samples. Quantitative analysis by using standard references revealed that Zn concentration in the phloem of veins in healthy leaves was more than 10 times higher than that in HLB-affected leaves. No significant variation was observed in the distribution patterns of other elements such as Ca in stems and leaves of grapefruit plants with or without graft-inoculation of infected lemon scions. These results suggest that reduced phloem transport of Zn is an important factor contributing to HLB-induced Zn deficiency in grapefruit. Our report provides the first in situ, cellular level visualization of elemental variations within the tissues of HLB-affected citrus.