Trioza erytreae (Del Guercio) (Hemiptera: Triozidae), is a minor pest of citrus, but the significance of this triozid is attributed to its ability to vector the African citrus greening disease pathogen, ‘Candidatus’ Liberibacter africanus. The population fluctuation of T. erytreae is correlated with the flushing rhythm of the citrus host. However, seasonal fluctuation in the infection potential of populations carrying the greening pathogen has been difficult to monitor in the past due to limited detection methodologies. This study explored the fluctuation in infectivity in an orchard severely infected by African greening. Two seasons' data are presented from a small Citrus aurantium (L.) (sour orange) orchard in the Nelspruit district in South Africa. Individual T. erytreae caught weekly on sticky traps placed in the orchard were tested using PCR. Additionally, a T. erytreae outbreak was observed in 2011 in another C. sinensis (L.) Osbeck (sweet orange) orchard which displayed a high incidence of greening also in the Nelspruit district. T. erytreae collected here were used in a challenge study and some transmission data are presented. T. erytreae population fluctuations followed the citrus growth flush cycles and were influenced by specific climatic factors. Fluctuations in the percentage infectivity of the T. erytreae populations were observed, with infectivity peaking with or just after the citrus flush seasons, but with peaks in infectivity differing to peaks in population of the vector. In a challenge study, the percentage pathogen transmission to citrus was much lower than the detected percentage infection of the triozids used for the study.