2018 ASHS Annual Conference

Huanglongbing (HLB) is a disease complex that in Florida putatively involves the gram-negative bacteria Candidatus Liberibacter asiaticus (CLasiaticus), the psyllid vector Diaphorina citri Kuwayama that introduces CLasiaticus into sieve tube elements of phloem, and secondary stresses that take advantage of the weakened tree. In commercial groves, the combined factors promote the most serious symptoms of decline. In greenhouses where psyllid vectors are excluded and secondary stresses minimized, a single inoculation of trees promotes only mild symptoms. The better growing conditions in greenhouse studies have also demonstrated that CLasiaticus is capable of moving into new growth, but the pathway is currently not known. Four hypotheses on intra-plant movement of CLasiaticus are presented and discussed, including 1) sieve tubes naturally develop into a network that extend the full length of the tree and develop across growth flushes, 2) the presence of CLspecies promotes development of a network of sieve tubes that extend the full length of the tree and develop across growth flushes, 3) the host’s cell walls become debilitated allowing CLasiaticus to traverses the apoplast, and 4) sieve tube injury by mechanical or pathogenic processes allows CLasiaticus to move from infected to uninfected sieve tubes. Elucidating the mode of intra-plant movement of CLspecies will facilitate development of strategies that impede movement and coupled with strong psyllid-vector suppression programs in commercial groves should help citrus trees “grow” out of HLB by restricting CLasiaticus to old phloem.