Citrus Huanglongbing: Understanding the Vector-Pathogen Interaction for Disease Management.

Introduction Citrus Huanglongbing (HLB), better known as citrus greening disease, is a recently introduced disease to the Americas (Brazil and Florida). Numerous reviews on this important disease are available (2,5,6) and include a recent article by Gottwald et al. published jointly in Plant Health Progress and as an APSnet feature article (8). A PDF file has been developed that provides detailed information about the disease and its vectors. The article by Gottwald et al. addressed the pathogen and its impact, with an emphasis on the history and distribution of the disease, symptoms, the causal organism, its insect vectors, impacts for Brazil and Florida, and potential threats to other citrus producing regions in the Americas. The article extensively dealt with the epidemiology of the disease and the challenges that the temporal increase and spatial spread of the disease have on disease control and management. In this article we will discuss the currently known facts of vector (psyllid) transmission of the citrus greening pathogen and how an understanding of the vector-pathogen interaction affects the current management strategies for HLB. As previously reviewed by Gottwald et al. (8) HLB is putatively caused by an unculturable bacterium belonging to the α proteobacter. Three types of the disease—Asian, African and American (Brazil)— have been described, with causal bacteria tentatively named Candidatus Liberibacter asiaticus (2), Ca. L. africanus (2) and Ca. L. americanus (17,18,19), respectively. As described, the two known insect vectors of HLB are Diaphorina citri and Trioza erytreae, which inhabit different environmental niches, yet can apparently vector both the Asian or African HLB bacteria. In Brazil D. citri has been found to be the vector of the Ca. L. americanus bacterium (28), limited to Brazil thus far, as well as the Ca. L. asiaticus bacterium. In Florida, HLB is caused by Ca. L. asiaticus, which is vectored by D. citri.