Abstract:
Citrus greening disease (CG) in South Africa is associated with the phloem-limited bacterium, ‘Candidatus Liberibacter africanus’ (Laf). This disease has been known to cause yellowing leaf symptoms as well as the formation of unprofitable fruit for nearly a century in this country. In addition to Laf, Liberibacters have been described from indigenous trees belonging to the Rutaceae family in this country. The first of these to be described was ‘Ca. L. africanus subsp. capensis’ and was followed by the description of ‘Ca. L. africanus subsp. clausenae’ (LafCl), ‘Ca. L. africanus subsp. vepridis’ (LafV) and ‘Ca. L. africanus subsp. zanthoxyli’ (LafZ). It has been speculated that either one of these Laf-subspecies may have given rise to Laf senso stricto associated with commercial citrus in South Africa. This dissertation aimed to expand on the indigenous rutaceous host species evaluated for the presence of Laf either as alternative hosts, or harbouring close relatives of Laf. From Oricia, Teclea and Agathosma spp sampled, a novel Liberibacter was only described from a Teclea gerrardii collected in Southern KwaZulu-Natal. This Liberibacter was named ‘Ca. L. africanus subsp. tecleae (LafT)’. It was further assessed whether any of these Laf-subspecies infect commercial citrus species in South Africa. Previous epidemiological studies on CG in South Africa found that Laf was the sole agent associated with this disease, with the current study supporting these findings. Finally, the genetic diversity of Laf was assessed across four provinces from different citrus types using microsatellite markers. This analysis indicated that the genetic diversity of Laf in South Africa is comparatively high, and that the genetic populations observed were mainly influenced by geographic distribution rather than citrus type. This study gave support to the hypothesis that Laf originated on the African continent from an indigenous species present in the country. The study further supports that Laf is well adapted to its commercial citrus host as genetically distinct populations are formed based on geographical populations.
