An Improved Method to Track Changes of Candidatus Liberibacter asiaticus Titer in HLB-affected Citrus Trees: Extended Persistence of Detectable CLas DNA after Cell Death

Abstract

Tracking changes in Candidatus Liberibacter asiaticus (CLas) qPCR titer (Ct) as a means to track treatment effects HLB-affected citrus trees has the inherent uncertainty as to how much dead-CL as DNA has contributed to the overall Ct values. The potential inclusion of dead CLas DNA in Ct estimates would overestimate live cell counts undermining the effect of the treatment under investigation. Using the leaf disc sampling method, we followed leaf CLas titer in potted Citrus sinensis ‘Valencia’ trees for five months after heat-treatment, shown to eliminate viable CLas from an HLB-infected citrus tree. Although titer slowly declined, CLas remained detectable after five months of treatment, the duration dependent on the initial Ct values. These results warn extreme caution in interpreting experiments aimed at curtailing HLB where recovery is measured in terms of CLas titer. Measurements made prior to six months after treatment could contain a significant background titer of disabled CLas causing significant overestimation of live bacterial populations in the plant.