Evidence That ‘<i>Candidatus</i> Liberibacter asiaticus’ Moves Predominantly Toward New Tissue Growth in Citrus Plants

‘Candidatus Liberibacter asiaticus’ (Las) is an unculturable, phloem-limited, insect-transmitted bacterium associated with the Asiatic form of huanglongbing (HLB), most destructive citrus disease. In Asia and Americas, it transmitted by Asian psyllid (Diaphorina citri Kuwavama). Despite considerable research, little known about processes involved in plant infection colonization Las. This study was conducted to determine whether basal portion (below girdling) important route for Las move laterally from a point inoculation on branch pathogen-free branches elsewhere canopy, quantify influence actively growing tissues vertical upward (acropetally) or downward (basipetally) movement Nongirdled fully partially girdled stems potted plants ‘Pera’ sweet orange, graft-inoculated above below girdling, were sampled distinct regions assessed qPCR, 6 months postinoculation. invaded all nongirdled but remained restricted inoculated plants, evidence that planta limited phloem. starch accumulated girdling site, probably because changes flow phloem sap. To tissues, ‘Valencia’ orange kept intact top- root-pruned force production new at 15-day intervals. migrated rapidly predominantly toward newly developing root leaf tissues. The rapid predominant developed shoots roots would explain failures canopy heat treatments pruning cure HLB-affected trees, reinforces need protect feeding D. order minimize transmission spread pathogen vector within between orchards.