LAUSR

Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas), is a devastating disease of citrus. After initial infection, CLas quickly colonizes the root system before canopy symptoms develop. There is limited understanding of CLas movement from roots to canopy and local and systemic effects on root dynamics. Using split-root rhizoboxes and late summer below-the-split bud inoculation, effects of local infection on systemic disease development were studied. Upward bacterial movement from roots is linked to seasonal flushes and CLas population in roots. CLas stayed isolated to one side of the roots for at least 8 months, until the spring flush. HLB caused differential root responses depending on tree age at infection. Systemic effects, independent of CLas movement, occur very early after infection. Stimulation of root growth occurred on non-infected roots prior to CLas detection in 1.5 years old trees but decreased in 2.5 years old trees. Independent of tree age, root growth was stimulated during spring root flushes after CLas population stabilized. Root dieback began simultaneously with detection of CLas in roots (6 weeks post inoculation). Infection and tree age altered root lifespan. 1.5 years old CLas-infected roots from summer and fall flushes had 3 and 6 weeks reduced lifespan. In contrast, 2.5 years old CLas-infected plants lifespan was unaffected. Season affected root lifespan with late summer root flush lifespan 3x shorter than fall or spring root flushes. Split root inoculation allowed study of local and systemic effects of CLas infection in roots, information crucial to prolonging the productivity of HLB-affected trees.