Genome-Enabled Phylogeographic Investigation of the Quarantine Pathogen Ralstonia solanacearum Race 3 Biovar 2 and Screening for Sources of Resistance Against Its Core Effectors.

Phylogeographic studies inform about routes of pathogen dissemination and are instrumental for improving import/export controls. Genomes of 17 isolates of the bacterial wilt and potato brown rot pathogen Ralstonia solanacearum race 3 biovar 2 (R3bv2), a Select Agent in the United States, were thus analyzed to get insight into the phylogeography of this pathogen. Thirteen of fourteen isolates from Europe, Africa, and Asia were found to belong to a single clonal lineage while isolates from South America were genetically diverse and tended to carry ancestral alleles at the analyzed genomic loci consistent with a South American origin of R3bv2. The R3bv2 isolates share a core repertoire of 31 type III-secreted effector genes representing excellent candidates to be targeted with resistance genes in breeding programs to develop durable disease resistance. Toward this goal, 27 R3bv2 effectors were tested in eggplant, tomato, pepper, tobacco, and lettuce for induction of a hypersensitive-like response indicative of recognition by cognate resistance receptors. Fifteen effectors, eight of them core effectors, triggered a response in one or more plant species. These genotypes may harbor resistance genes that could be identified and mapped, cloned, and expressed in tomato or potato, for which sources of genetic resistance to R3bv2 are extremely limited.