Characterization of the bacterial stem blight pathogen of alfalfa, Pseudomonas syringae pv. syringae ALF3

Abstract

Bacterial stem blight of alfalfa occurs sporadically in the central and western U.S. Yield losses of up to 50% of the first harvest can occur with some cultivars. Developing resistant cultivars is hampered by lack of information on the pathogen and a standard test for evaluating plant germplasm. Bacteria producing a fluorescent pigment were isolated on King’s B agar from alfalfa with symptoms of bacterial stem blight from near Cheyenne, WY. The strain ALF3 was tentatively identified as Pseudomonas syringae pv. syringae based on 16S rDNA sequence and PCR amplification of syrB for lipodepsinonapeptide toxin production. Multilocus sequence analysis indicated that ALF3 falls within a clade containing strains of P. syringae pv. syringae with closest affinity to FF5 from pear. Comparison of a draft whole-genome sequence of ALF3 further confirmed that ALF3 most closely resembles FF5 (~96% sequence identity) and P. syringae pv. aptata DSM50252 from beet. Approximately 60 genes were unique to ALF3, including several predicted genes in the T3SS cluster such as a type III helper protein HrpZ (Pto) and phage-associated genes. ALF3 was highly pathogenic to snapbean pods but caused only mild symptoms on leaves of snapbean, pear, and sugarbeet. A standardized method for evaluating disease resistance in alfalfa was developed. Cultivars with fall dormancy ratings of 1 and 2 had higher percentages of resistant plants than cultivars with fall dormancy ratings of 8-11.