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.

Aim: To quantify current and predict future distribution of the citrus greening pathogens “Candidatus Liberibacter asiaticus” (Las) in Africa and “Candidatus Liberibacter africanus” (Laf) globally.Location: Africa.Methods: Three species distribution models (MaxEnt, BIOCLIM and Boosted Regression Trees) were used to predict the current and future potential distribution of Las in Africa, and the potential global distribution of Laf, using long‐term bioclimatic variables. Two climate change scenarios (moderate and extreme) were employed to determine how future climate alterations may affect the potential distribution of Las in Africa. Presence data from global reports of Las, as well as the new positional points obtained in this survey, were used to predict the habitat suitability of the pathogen in Africa, while the presence data points of Laf were used to predict the global habitat suitability. Testing data comprised 25% of the presence only points.Results: Consensus of the three models predicted a potential distribution of Las in large areas of Western, Eastern and sub‐Saharan Africa. North Africa was mostly unsuitable for Las, except for the northern fringes. The potential distribution of Laf included South and Central America, Asia and Australia. In Europe, the United Kingdom and the Iberian Peninsula showed marginal suitability for Laf. The projections under the future climate change scenarios showed an increase in the Laf habitat suitability hotspots under the extreme scenario.Main conclusions: This study highlights the potential establishment and distribution in Africa of Las‐associated Huanglongbing and globally for Laf‐associated with African citrus greening disease. The ensemble modelling approach for the distribution of plant pathogens is a valuable tool for the development of strategies for crop protection. These results constitute an early alert for citrus‐producing regions that should inform strategies for surveillance and preventive management against the invasion and spread of this destructive disease.

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.

African Citrus greening is a destructive disease of citrus that has been reported in South Africa since the 1920s. The disease is associated with “Candidatus Liberibacter africanus” (Laf) and is transmitted by Trioza erytreae (McClean 1974, Cook et al. 2014). The related bacteria “Candidatus Liberibacter asiaticus” (Las), which is associated with the much more severe Huanglongbing disease and is transmitted by Diaphorina citri, was recently reported in Ethiopia (Saponari et al. 2010). Experimentally, T. erytreae has been proven to transmit Las (Massonié et al., 1976), but natural occurrence of Las in field populations of this psyllid has not been reported. A survey was conducted for the citrus greening vector T. erytreae in the Amhara region of Ethiopia in November 2017. Trioza erytreae adults were identified as per the descriptions of OEPP/EPPO (2005). Sampling sites included large and small-scale citrus orchards (in both the highland and lowland areas), and citrus trees grown in backyard gardens. T. ery...

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.

African citrus greening disease (ACGD) is considered as one of the major diseases of citrus threatening citrus production in East Africa. Our study aimed for the first time to assess the incidence, severity, and distribution patterns of ACGD in Kenya and Tanzania. In total, 105 citrus orchards were assessed in 13 regions representing low, mid, and high altitude areas. In each backyard and orchard, trees were randomly selected and rated for visual ACGD symptoms; then leaves and insect samples collected for analysis of ‘Candidatus Liberibacter africanus’ (CLaf), the presumptive causal agent of ACGD. Endpoint PCR, sequencing, and molecular phylogenetic tools were employed to confirm the identity of potential circulating pathogens. Incidence and severity of ACGD varied significantly among the different regions. Both Trioza erytreae (Del Guerico) (Hemiptera: Triozidae) and the invasive Asian citrus psyllid vector Diaphorina citri (Kuwayama) (Hemiptera: Liviidae) were found to co-occur in upper and lower midland regions. Molecular characterization identified ‘Candidatus Liberibacter africanus spp. Clausenae’ (CLafCl) as the main causal agent of ACGD in most of the citrus plants and insect samples. No instances of Candidatus Liberibacter asiaticus infection were found. These findings provide valuable insights into understanding and management of ACGD by employing stringent and early disease detection tools to curb the spread of the disease.