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Two phases of root loss caused by citrus huanglongbing are independent of root growth
E. G. JOHNSON (1), K. M. Gerberich (1), J. Wu (2), J. H. Graham (1). (1) University of Florida, Citrus Research and Education Center, Lake Alfred, FL, U.S.A.; (2) University of Florida, Lake Alfred, FL, U.S.A.

Huanglongbing (HLB) is a devastating disease of citrus, caused by phloem limited <i>Candidatus</i> Liberibacter spp. Early symptoms of disease include fibrous root loss and blotchy mottle on leaves followed by yield declines, leaf drop, and dieback. Early root loss was previously described as a 30-50% reduction in root density that begins before foliar symptoms develop. Early root loss is dependent on local bacterial infection and occurs before carbohydrate starvation caused by phloem plugging in the canopy. Continued sampling has identified a second phase of root loss (70-80%) that begins at the early stages of canopy thinning from leaf drop. During this phase carbohydrate supply to the roots is severely limited by canopy disease. Surprisingly, in both phases of root loss, root growth was not significantly different from presumed healthy trees through all root flushes sampled to date. This suggests that root density reductions result from the shortened lifespan of fibrous roots. In the absence of anatomical effects of infection on roots, such as phloem-plugging, the cause of this shortened lifespan is still under investigation.

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