Abstract
Common bacterial blight is a devastating seed-borne disease of common beans that also occurs on other legume species including lablab and Lima beans. We sequenced and analyzed the genomes of 26 strains of Xanthomonas axonopodis pv. phaseoli and X. fuscans subsp. fuscans, the causative agents of this disease, collected over four decades and six continents. This revealed considerable genetic variation within both taxa, encompassing both single-nucleotide variants and differences in gene content, that could be exploited for tracking pathogen spread. The bacterial strain from Lima bean fell within the previously described Genetic Lineage 1, along with the pathovar type strain (NCPPB 3035). The strains from lablab represent a new, previously unknown genetic lineage closely related to strains of X. axonopodis pv. glycines. Finally, we identified more than 100 genes that appear to have been recently acquired by Xanthomonas axonopodis pv. phaseoli from X. fuscans subsp. fuscans.
Highlights
Common bacterial blight (CBB) is a devastating, widespread and seed-borne disease of common beans (Phaseolus vulgaris)
The non-fuscous strains are classified as Xanthomonas axonopodis pv. phaseoli (Xap) while the fuscous strains are classified into a different species as X. fuscans subsp. fuscans (Xff ) (Schaad et al, 2005; Bull et al, 2012), though some authors consider the species X. fuscans to be a subclade within X. axonopodis (Rodriguez-R et al, 2012; Mhedbi-Hajri et al, 2013)
All strains had been deposited as Xap, except for those marked with an asterisk (*), which had been deposited as “X. axonopodis pv. phaseoli variant fuscans.”
Summary
Common bacterial blight (CBB) is a devastating, widespread and seed-borne disease of common beans (Phaseolus vulgaris). Lablab bean has been reported to be the main leguminous fodder crop used in Sudan around Khartoum, where it is known as hyacinth bean, bonavist bean or, in Arabic, lubia afin (Schaaffhausen, 1963) It is grown in Sudan as a pulse legume (Mahdi and Atabani, 1992). Infection by Xap has been observed when lablab was sown during the rainy months (Tarr, 1958) It is not currently clear whether a single bacterial population moves frequently between host species or to what extent CBB agents colonizing different plant species represent distinct and genetically isolated populations or distinct taxa. Do the strains from Lima bean and lablab bean belong to the same genetic lineages as do strains from common bean?
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