Modified Bean Pod Assay to Detect Strains of Pseudomonas syringae pv. syringae That Cause Bacterial Brown Spot of Snap Bean

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Abstract
Approximately 99%, of 1,310 strains of Pseudomonas syringae pv. syringae isolated from bacterial brown spot (BBS) lesions on beans in New York State, Wisconsin, and Colorado [USA] caused a unique symptom when assayed on excised bean pods. This symptom could be distinguished from those caused by 45 of 46 strains of P. s. pv. syringae recovered from 15 other crop species. The exception was a strain isolated from almond in California, which caused symptoms indistinguishable from those caused by strains from BBS lesions. None of 68 strains of 17 other pathovars of P. syringae, including 34 strains of P. s. pv. phaseolicola, or eight strains of of P. viridiflava incited symptoms that could be confused with those caused by strains of P. s. pv. syringae recovered from BBS lesions. Of 337 epiphytic strains of P. syringae recovered from leaves of 51 weed species, 23 strains incited symptoms on detached pods that were identical to those caused by strains from BBS lesions. These strains were recovered only from sites where BBS occurred that year. Consistent results were obtained with a procedure that involves placing a smear of bacterial cells on the pod surface and inserting an insect-mounting pin through the cells and laterally underneath the epidermis of greenhouse-grown snap bean pods (cultivar Bush Blue Lake 274). Since the procedure does not require calculated inoculum concentrations, it is useful in evaluating large numbers of strains (e.g., epiphyte collections) for their ability to cause BBS. Pod assays were more rapid and reliable than leaf assays of whole plants in distinguishing strains of P. s. pv. syringae capable of causing BBS.