Abstract
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.
Highlights
Citrus is the most extensively cultivated fruit crop in Kenya[18,19] with annual production ranging from 61 to 135 tons between 2012 and 201620
Infected trees which were subsequently shown to be Liberibacter africanus (Laf)-infected showed milder yellowing on the leaves and in most cases, infected leaves were observed on one side of the tree
We assessed the distribution of Liberibacter species in three Eastern African countries, as well as determined the potential spread of Liberibacter asiaticus (Las) in the region
Summary
Citrus is the most extensively cultivated fruit crop in Kenya[18,19] with annual production ranging from 61 to 135 tons between 2012 and 201620. Recent studies have established that D. citri is established in Kenya and Tanzania[26,28]. This could potentially have even more impact than envisaged as D. citri possesses a superior ability as a Las vector[15,29]. Current models on the potential spread of HLB are based on suitable climate conditions for the psyllid vector D. citri[30,31]. This study aimed to asses the status of HLB in three Eastern Africa countries (Ethiopia, Kenya and Uganda) by identifying the associated Liberibacter species in citrus plants and their psyllid vectors, establishing their occurrence, and determining the potential spread to other citrus-producing parts of the region
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