Identification of 'Candidatus Liberibacter asiaticus' the Huanglongbing Bacterium in citrus from Colombia.

Candidatus Liberibacter spp is the most prevalent microorganism in the citrus plant, associated with Citrus Huanglongbing (HLB), which is transmitted by the psyllid vector. In Colombia, the vector Diaphorina citri Kugayama has been reported in different regions, but "Ca. Liberibacter asiaticus" (CLas) has only been detected in insect vectors, not in citrus host plants. To identify the presence and quantify the pathogen in citrus tissues, we employed a combined strategy that involved three techniques based on polymerase chain reaction (PCR). First, we used endpoint PCR with specific primers for CLas (OI1-OI2c) to confirm the infection. Second, we used qPCR with specific primers CIT295a - CIT298 designed on 16S rDNA gene regions to quantify the pathogen load. Finally, we employed droplet digital PCR (ddPCR) to determine the copy number of the pathogen in citrus tissues using the ß-subunit of ribonucleotide reductase (RNR) gene (nrdB) that is specific to CLas. We identified the presence of CLas in citrus plants for the first time in Colombia and quantified its titer in the plant tissue. We employed ddPCR and qPCR to provide crucial information for the country's disease management, control strategies, and general crop health.