Genetic Mechanisms of the Devious Intruder Candidatus Liberibacter in Citrus

Abstract

Citrus Huanglongbing (HLB), or citrus greening disease, is the most important, destructive and dangerous disease of citrus in the world It is caused by endogenous, sieve vascularrestricted liberibacters (Candidatus liberibacter; Ca.L.). Ca.L. can infect most citrus cultivars, species and hybrids and even some citrus relatives. HLB symptoms are similar in all three types: (Ca. L. asiaticus, Ca. L. africanus, and Ca. L. americanus). The affected leaves develop a pattern of yellow and green areas lacking clear limits between the colors, giving a "blotchy mottle" appearance Ca.L. is mainly present in leaf veins and petioles; however, it is has also been detected in bark, roots, and fruit peel. The disease causes fruit malformation and altered phenotype. Fruits are colored green to yellow/orange, usually small, asymmetric, and lopsided, with a bent fruit axis and small or aborted seeds. Symptoms are similar to those of zinc deficiency, although Huanglongbing disease causes asymmetric leaf yellowing. After symptoms appear, leaves generally drop and twig dieback occurs. Another typical effect of HLB infection is excessive fruit drop Over 100 million HLB-infected citrus trees have been destroyed to limit disease spread since HLB was first recognized