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Differential response of citrus rootstock breeding lines to Huanglongbing-induced root loss
E. G. JOHNSON (1), J. H. Graham (1), J. W. Grosser (1). (1) University of Florida, Lake Alfred, FL, U.S.A.

Commercial citrus rootstocks used in Florida are susceptible to Huanglongbing (HLB)-associated root loss caused by <i>Candidatus </i>Liberibacter asiaticus (Las).  Las stimulates new fibrous root growth, while significantly reducing root lifespan, resulting in an overall loss of root mass.  This dynamic of increased root growth and dieback shifts the carbohydrate balance in the tree, reducing the sugar available for fruit development and ripening and amplifying carbohydrate disruption caused by phloem plugging.  New genetically complex rootstock breeding lines have slowed or delayed scion decline in one field trial even though their root systems were infected with Las.  If these rootstocks are less susceptible to Las-induced root loss, they may reduce allocation of carbohydrate for root regeneration in favor of vegetative growth and fruit development.  While most rootstock breeding lines had similar root loss compared to the commercial rootstocks, UFR-4 increased root mass as Las infection progressed through the tree and disease symptoms became more severe.  UFR-4 was also the breeding line with the least symptomatic sweet orange scion after multiple years of infection.  Las still stimulates root growth in UFR-4, but root lifespan is either not affected or less affected than other rootstocks evaluated.  While natural sources of resistance remain elusive, reduced susceptibility to HLB root loss might sustain yields until trees become profitable.

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