Detection and identification of etiological agents (Liberibacter spp.) associated with citrus greening disease in Uganda

Abstract

Citrus Huanglongbing (HLB), commonly referred to as citrus greening, is one of the major disease challenges of citrus. In Uganda, there was limited information on Liberibacter spp., the causal organisms of HLB disease, and its psyllid vectors despite the presence of HLB symptoms on citrus. The objective of this study was to identify the Liberibacter spp. and psylla vector species responsible for HLB in Uganda. Other native plant species that could be hosts to pathogens and the psyllid vectors, but are not in the citrus genus, were also investigated. A survey was conducted in 15 citrus growing districts, and symptomatic citrus leaf samples collected, as well as citrus psyllid nymphs and adults for isothermal detection of pathogens in the laboratory. Two types of bacterial pathogens responsible for HLB were detected, namely Candidatus Liberibacter africanus (CLaf), known as the African type; and Candidatus Liberibacter asiaticus (CLas), known as the Asian type. CLaf was found in Mukono and Wakiso districts in Central Uganda and in Mbarara in Western Uganda; while CLas was found in Budaka and Tororo districts in Eastern Uganda. Citrus psyllids, which are the major known disease vectors were present in seven out of the fifteen districts. Psyllid vector identification by morphological means indicated Trioza erytreae, the African psyllid to be the insect vector. Psyllids were common on tangerines (66.7%), Sour Orange (13.3%) and Rough Lemon (13.3%); and least on Washington Navel (6.7%). Three non-citrus plants, Stephania abyssinica (Dill. & A. Rich) walp var. tomentella (Oliv.) Deils (Menispermaceae), Diospyros mespiliformis (Ebenaceae) and Ficus spp (Moraceae) were found to be alternative host plants for the psyllid.

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

“…We identified both Laf and LafCl in Uganda; the two Liberibacter previously found to be widely distributed in the citrus-growing areas in the western and central regions of the country 27 . In our study, LafCl was dominant in the eastern region of the country where Las had been previously misreported 25 . Our results support the hypothesis that the previous record of Las in Uganda was probably a misidentification 27 therefore, the presence of Las in Uganda remains unconfirmed.…”

Distribution, degree of damage and risk of spread of Trioza erytreae (Hemiptera: Triozidae) in Kenya

“…However, from molecular research by Khamis et al (), these psyllid species on S. abyssinica were grouped into ACT. Similarly, Kalyebi et al () claimed to have found ACT on S. abyssinica in Uganda based purely on presence of pit galls. Moreover, a study by Abate () on identity and bionomics of insect vectors of tristeza and greening diseases of citrus in Ethiopia found and grouped psyllids on S. abyssinica and other non‐rutaceae plants into ACT.…”

Habitat suitability and distribution potential of Liberibacter species (“Candidatus Liberibacter asiaticus” and “Candidatus Liberibacter africanus”) associated with citrus greening disease

“…The predicted optimal areas for the establishment of Las were the same areas where the pathogen was found in previous studies (Ajene et al, 2019; Saponari et al, 2010). In 2016, the presence of Las was reported in Uganda (Kalyebi et al, 2016). However, the presence points from Uganda were not included in the models as the report was shown to be a misidentification of LafCl (Roberts et al, (2017).…”