An analysis of the phenotypic, phylogenetic, and genomic characteristics of Pseudomonas syringae phylogroup 2 and Pseudomonas viridiflava phylogroup 7 strains causing bacterial stem blight of alfalfa in the United States

Abstract

In recent years, alfalfa growers in the Western United States have noted an increase of frost damage on their stands in correlation with increased bacterial stem blight (BSB) disease symptoms including necrotic lesions on stems, chlorosis and necrosis of leaves, and shepherd’s crooking. BSB symptoms have been attributed to strains grouping in Pseudomonas syringae phylogroup 2 (PG2) and Pseudomonas viridiflava phylogroup 7 (PG7). The symptoms can result in significant yield loss up to 50% during the first cutting, so managing this disease is critical for farmers to obtain optimal yields. In order to eventually manage and control BSB, it is necessary to characterize the pathogenic population causing the disease. In chapter one, an extensive literature review of broad aspects of P. syringae and P. viridiflava is presented. Chapter two contains the first comprehensive phenotypic, phylogenetic, and genomic study of a population of 123 strains of P. syringae PG2 and P. viridiflava PG7 causing BSB on alfalfa. Results of this research show that the population of PG2 and PG7 strains causing BSB on alfalfa are diverse both in phenotypes, such as pathogenic potential on alfalfa and ice nucleation capability, and in genetic diversity within their clades. Genomic analysis indicated that PG7 strains contain the S-PAI pathogenicity island polymorphism, and that for both PG2 and PG7 strains, single gene sequences related to virulence may not be able to explain differences in virulence phenotypes.