Short communication.: Detection by multiplex PCR and characterization of nontoxigenic strains of Pseudomonas syringae pv. phaseolicola from different places in Spain

The efficient control of halo blight, caused by Pseudomonas syringae pv. phaseolicola, is primarily based on the use of pathogen-free seed. Detection of the pathogen in seeds is currently carried out with high-sensitive methods based on the detection by PCR of genes involved in the biosynthesis of phaseolotoxin, which was believed to be produced by all strains of the pathogen with epidemiological importance. However, field epidemics of halo blight in the county of Castilla y Leon, Spain, are often associated to nontoxigenic isolates of P syringae pv. phaseolicola, which cannot be detected using current molecular and serological methods. The results presented in this work show the existence of nontoxigenic isolates of P syringae pv. phaseolicola in areas other than Castilla y Leon, indicating the need to establish a reliable methodology for seed certification. A simple two-step methodology is presented with the aim to identify both types of isolates that is based on a multiplex enrichment PCR of seed soakates and on pathogenicity assays.