Detection of Candidatus Liberibacter asiaticus from branch collars of citrus trees

Candidatus Liberibacter asiaticus (Las) is a causal agent of citrus greening disease (CG; Huanglongbing) in Japan. Currently, the midribs of leaves are used to diagnose CG-infected trees. In this study, we sampled branch collars of trees to test for Las. The pathogen was identified more often in the shoot bases than in the petioles, midribs, or shoot tips. Las concentration in branch collars was the highest at the shoot bases. In branch collars, the Las concentration in the cortical tissues was higher than in the pith. The cortex of branch collars can be used to diagnose CG-infected trees. In addition, Las could be detected from dying trees in the branch collar cortex. Therefore, CG infection can be diagnosed in trees that do not produce leaves. In addition, a detection limit was determined when the branch collar cortex from one infected tree was mixed with those from multiple healthy trees. As a result, the pathogen could be detected in an infected tree when the cortex of a single infected tree was mixed with those of nine healthy trees. Based on this result, 10 trees can be assayed together to determine the incidence of CG in Japan.