Huanglongbing has a complex pathosystem (an ecosystem based on parasitism). There are multiple strains, diverse hosts, several insect vectors, and different environmental conditions that affect the expression and spread of the disease. Three forms of the disease are known (Asian, African, and American), and these are associated with different species and strains of Liberibactors that are disseminated by different species of citrus psyllid insect vectors.
Vectors of the Liberibacters include citrus psyllids Diaphorina citri and Trioza erytreae. Huanglongbing can also be spread by grafting. Transmission can occur in the nursery and in the orchard.
Non-citrus hosts have been identified as alternate hosts for the Liberibacters and the citrus psyllids, but their role is still unclear in the epidemiology of the disease.