Characterization of recently introduced HLB and CTV isolates

Abstract

Severe stem-pitting isolates of Citrus tristeza virus (CTV) were found in Florida citrus 5 years ago, followed by the discovery 2 years later of the citrus greening disease (huanglongbing: HLB) caused by the bacterium Candidatus Liberibacter asiaticus. The new CTV isolates are members of the VT group, which contains isolates more severe than the mild and decline isolates now common in Florida. These new isolates are apparently moving rapidly into commercial citrus. Individual VT isolates from the same field vary considerably in severity, suggesting genetic variability in the virus population. Grapefruit seedlings inoculated with some of the VT isolates have shown severe reactions in the greenhouse, but effects on larger trees remain unknown. Additionally, severe CTV-like symptoms were observed in some of the trees initially discovered with HLB infections, suggesting that additional severe isolates of CTV may have been co-imported with the citrus greening pathogen. The similarity of these CTV isolates is under investigation. The establishment of greening in Florida poses a major threat to future citrus production, so development of effective management strategies is urgently needed. Using information and techniques developed for studying CTV, we are examining the movement and distribution of Liberibacter to optimize sampling times and patterns for better detection, especially in trees without definite visual symptoms. We also are examining the relationship between symptom expression and Liberibacter titers in infected trees.