Skip to main content
ARS Home » Midwest Area » St. Paul, Minnesota » Plant Science Research » Research » Publications at this Location » Publication #416003
Research Project: Expanding Resiliency and Utility of Alfalfa in Agroecosystems

Location: Plant Science Research

Title: Bacterial Stem Blight: A re-emerging disease of alfalfa caused by Pseudomonas syringae phylogroup (PG) 2b and PG7a

Author
item Samac, Deborah - Debby
item LIPPS, SAVANA - University Of Minnesota
item MORRIS, CINDY - Inrae
item ISHII, SATOSHI - University Of Minnesota

Submitted to: Plant Pathogenic Bacteria International Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 6/16/2024
Publication Date: 7/8/2024
Citation: Samac, D.A., Lipps, S.M., Morris, C.E., Ishii, S. 2024. Bacterial Stem Blight: A re-emerging disease of alfalfa caused by Pseudomonas syringae phylogroup (PG) 2b and PG7a. Plant Pathogenic Bacteria International Conference, July 8-12, 2024, Blacksburg, Virginia.

Interpretive Summary:

Technical Abstract: Bacterial stem blight (BSB) was first described in 1904 in the high-altitude alfalfa production areas in the state of Colorado and subsequently in Utah, New Mexico, Nebraska, and Kansas. Yield losses of 80% from the first forage harvest were recorded. Later, BSB was observed causing significant yield losses in Missouri, Nevada, and Indiana as late as 1957. Researchers then stressed the need for disease resistant cultivars and identified germplasm with moderate levels of resistance. However, there is no record of BSB resistant cultivars, although cultivars with resistance to bacterial wilt, caused by Clavibacter insidiosus, were developed during the same period. The causal agent of BSB was identified as Pseudomonas medicaginis, now P. syringae. The disease was reported only sporadically until 2016 when it was found in California, Utah, Oregon, Minnesota, and Ohio causing severe foliar blight. A collection of bacteria from diseased plants were characterized by phylogenetic analysis of cts, gyrB, and gapA and identified as P. syringae PG2b and P. syringae PG7a (P. viridiflava). Both bacteria cause similar foliar disease symptoms but do not act synergistically. In 2024, rot of crown buds and interior crown tissues, which had been described in 1904, was associated with BSB from plants in Utah, leading to premature stand decline. Screening of currently grown cultivars indicates low levels of BSB resistance, suggesting that changes in alfalfa germplasm has led to re-emergence of the disease. Complete genome sequences of PG2b and PG7a strains were assembled and will form the basis of understanding pathogenicity on alfalfa.