Diversity of strains in the Pseudomonas syringae complex causing bacterial stem blight of alfalfa (Medicago sativa) in the United States

Authors: LIPPS, SAVANA - University Of Minnesota; CASTELL-MILLER, CLAUDIA - University Of Minnesota; MORRIS, CINDY - National Research Institute For Agriculture, Food And Environment; ISHII, SATOSHI - University Of Minnesota; Samac, Deborah - Debby

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/31/2023
Publication Date: 3/15/2024
Citation: Lipps, S., Castell-Miller, C.V., Morris, C.E., Ishii, S., Samac, D.A. 2024. Diversity of strains in the Pseudomonas syringae complex causing bacterial stem blight of alfalfa (Medicago sativa) in the United States. Phytopathology. 114(4):802-812. https://doi.org/10.1094/PHYTO-02-23-0059-R.
DOI: https://doi.org/10.1094/PHYTO-02-23-0059-R

Interpretive Summary: Bacterial stem blight (BSB) of alfalfa has recently become more common, especially in the Intermountain West, and causes significant yield loss of the first forage harvest. Although related bacteria causing disease on annual crops have been well characterized, there has been little research on the two species of bacteria causing the disease on alfalfa. Both species caused similar symptoms on alfalfa but used different mechanisms to cause symptoms, and there was a large range in virulence among strains. The two species did not act synergistically to incite disease symptoms. The bacteria were genetically diverse, indicating that both pathogens are well established on alfalfa across a wide geographic range. Introduction or evolution of more aggressive strains as the cause for recent emergence of the disease is unlikely. Changes in cultivars, management methods, or environmental shifts may have played a role in the increased incidence of BSB.

Technical Abstract: Alfalfa growers in the Intermountain West of the United States have recently seen an increased incidence in bacterial stem blight (BSB), which can result in significant herbage yield losses from the first harvest. BSB has been attributed to Pseudomonas syringae pv. syringae and P. viridiflava; however, little is known about the pathogens causing BSB or their interaction with alfalfa plants. Here, we present a comprehensive phylogenetic and phenotypic analysis of P. syringae and P. viridiflava strains causing BSB on alfalfa. A multi-locus sequence analysis found that the BSB pathogens grouped exclusively with P. syringae PG2b and P. viridiflava PG7a. Symptoms caused by PG2a strains were indistinguishable from those caused by PG7a strains, although there was a large range in mean disease scores for individual strains. Overall, PG2b strains incited significantly greater disease scores than those caused by PG7a strains. Inoculated plants showed browning in the xylem and collapse of epidermal and pith parenchyma cells. Inoculation with a mixture of PG2b and PG7a strains did not result in synergistic activity. The population of PG2b and PG7a strains causing BSB on alfalfa showed genetic diversity within their clades and did not group by location, pathogenicity, or haplotype. The PG2b strains had genes for production of the phytotoxins syringolin A, syringopeptin, and syringomycin as well as for coronatine, which is unusual in PG2 strains. The results indicate that both pathogens are well established on alfalfa across a wide geographic range and that a recent introduction or evolution of more aggressive strains as the basis for emergence of the disease is unlikely.