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Occurrence of bacterial pathogens, including non-toxigenic strains of Pseudomonas syringae pv. phaseolicola, in bean seed crops in Washington State

Michael Derie: Washington State University Mount Vernon NWREC

<div>Bean processing and seed crops are grown on 12,000 and 2,700 ha annually, respectively, in the Columbia Basin of Washington. A bean seed quarantine and phytosanitary inspection program was instituted in the region in 1968 to ensure production of pathogen-free seed. In 2009, five bean seed lots grown in the Columbia Basin were destroyed after <em>Xanthomonas axonopodis</em> pv. <em>phaseoli</em> (common bacterial blight) or <em>Pseudomonas syringae</em> pv. <em>phaseolicola </em>(<em>Psp</em>) (halo blight) were detected. A bean seed crop in each of 2012 and 2014 was destroyed after detection of <em>Psp</em>. Five bean seed crops were destroyed in 2016 to 2017 after detection of quarantine bacterial pathogens during inspections. In 2016 and 2017, bacteria were isolated from severely symptomatic crops. The 2016 isolates tested positive for presence of the phaseolotoxin genes, but lesions on inoculated bean plants did not develop the chlorotic halo linked to phaseolotoxin production, i.e., the highly virulent isolates were non-toxigenic (Tox¯). The 2017 isolates also were Tox¯. Phylogenetic analyses of the <em>gapA</em>, <em>gltA</em>, <em>gyrB</em>, and <em>rpoD</em> genes grouped the 2016 isolates with <em>Psp</em> and the 2017 isolates with <em>P. syringae</em> pv. <em>syringae </em>(bacterial brown spot). Thus, three quarantine bacterial pathogens have been confirmed on bean seed crops in Washington, including highly virulent Tox¯ isolates of <em>Psp</em> causing atypical halo blight.</div>