Submitted to: Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/6/2013
Publication Date: 7/15/2013
Citation: Nelson, W.R., Munyaneza, J.E., McCue, K.F., Bove, J.M. 2013. The Pangean origin of 'Candidatus Liberibacter' species. Journal of Plant Pathology. 95:455-461; doi: 10.4454/jpp.v9513.001. Interpretive Summary: Liberibacter is an economically important bacterium that severely damages several crops, including citrus, potato, tomato, pepper, and carrot. Researchers at USDA-ARS Wapato, WA and Albany, CA, the New Zealand Institute for Plant and Food Research, and the French National Agricultural Research Institute assessed the origin and spread of this plant pathogen. It was determined that the current species of Liberibacter evolved prior to 300 million years ago and spread to different parts of the world following the earth plate tectonic movements that resulted into formation of the different continents. Information from this research will help researchers and growers predict the spread of this bacterium and develop management strategies to minimize damages caused by this plant pathogen.
Technical Abstract: There are six currently recognized “Candidatus Liberibacter” species. Three are associated with Huanglongbing of citrus, and one with Zebra Chip and Psyllid Yellows in Solanaceous crops and Yellows Decline in carrots. Another is an apparently asymptomatic infection of apple, pear and related species and the final, culturable, species is also asymptomatic in Babaco (hybrid mountain papaya). These have become prominent within the last 100 years because of their impact on economically important crops. Psyllids are the only known vector. Liberibacter species are ancient organisms that have opportunistically developed their obligate alternating insect/plant host lifestyle since the evolution of flowering plants and psyllids. The common ancestor of these species evolved prior to 300million years ago, probably associated with the Central Pangaean Mountains, and sensitive to heat (above 30oC). Phylogeny of these Liberibacter species, and estimates for dates of separation of some of the species, suggests unusual linkages predating the evolution of angiosperms and insects. Plate tectonic movements from about 300 million years ago can explain the geographic spread of the currently known species from a Pangaean origin to their currently assumed natural geographic origin. It seems likely that the presence of “Ca. Liberibacter americanus” (Lam) in Brazil is a recent incursion event, most likely from a Palearctic source such as North America rather than Asia. The fairly generic “yellows” and “decline” nature of plant symptomology suggests a physiological rather than a virulent pathology. More species are likely to be discovered, and the introduction of species into more plant crop/psyllid vector combinations is inevitable. Solanaceae, Apiaceae, and Rutaceae crops are already affected, significant Eucalyptus and Acacia forestry investments are likely highly susceptible as they host many psyllid species. Of these, the most problematic is likely to arise if the more heat tolerant species spreads to more crop species.