Diversity of Strains in the Pseudomonas syringae Complex Causing Bacterial Stem Blight of Alfalfa ( Medicago sativa ) in the United States

• Published in: Phytopathology Vol. 114; no. 4; p. 802
• Main Authors: Lipps, Savana M, Castell-Miller, Claudia, Morris, Cindy E, Ishii, Satoshi, Samac, Deborah A
• Format: Journal Article
• Language: English
• Published: United States 01.04.2024
• Subjects: Genetic Variation; Medicago sativa - microbiology; Multilocus Sequence Typing; Phylogeny; Plant Diseases - microbiology; Plant Stems - microbiology; Pseudomonas syringae - genetics; Pseudomonas syringae - isolation & purification; Pseudomonas syringae - pathogenicity; United States; United States; pathogenicity; bacteriology; multilocus sequence analysis (MLSA); Pseudomonas syringae; Pseudomonas viridiflava
• ISSN: 0031-949X
• DOI: 10.1094/PHYTO-02-23-0059-R

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 pv and ; however, little is known about the genetic diversity and pathogenicity of these bacteria or their interaction with alfalfa plants. Here, we present a comprehensive phylogenetic and phenotypic analysis of and strains causing BSB on alfalfa. A multilocus sequence analysis found that they grouped exclusively with PG2b and PG7a. Alfalfa symptoms caused by both bacterial groups were indistinguishable, 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 populations of PG2b and PG7a strains were genetically diverse within their clades and did not group by location or haplotype. The PG2b strains had genes for production of the phytotoxin coronatine, which is unusual in PG2b 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.