Systemic infection of split root trees by Candidatus Liberibacter asiaticus suggests rapid movement between phloem sieve tubes
J. ORROCK (1), J. Orrock (1), E. Johnson (1) (1) Citrus Research and Education Center, University of Florida, U.S.A.
Huanglongbing (HLB; syn. citrus greening), caused by the phloem-limited bacterium Candidatus Liberbacter asiaticus (Las), poses one of the largest threats to citrus production. In ten years it has spread to most citrus trees in Florida. Citrus production in Florida is down >70% from the pre-HLB era. Las infects a tree when an infected Diaphorina citri (Asian Citrus Psyllid) feeds on phloem sap in a leaf. After infection of the leaf, Las rapidly colonizes the root system, but remains sectored in the canopy. Canopy sectoring is thought to occur because of limited lateral connections of phloem sieve tubes. To investigate if lateral movement of Las between sieve tubes is distance or tissue specific, root systems of grafted and seedling trees were split to three different heights: below crown, above crown, and above graft union (or equivalent on seedlings). The trees were then graft inoculated above one side of the split root system. Infection to both halves of the root system was monitored weekly. Contrary to our hypothesis; the graft union and crown do not play a unique role in lateral movement. Similar patterns of infection occurred in all tissue types of the grafted trees and seedling trees. Greater variability in the time between Las detection in the root systems on the inoculated side and the opposite side in all trees with the highest split suggests that vertical distance is important in lateral movement of the bacterium.