Detection and identification of Pseudomonas savastanoi pv. phaseolicola, the causal agent of the bacterial halo blight of snap bean

In March-April of 2020-2022, round and water-soaked spots surrounded by a brown border were observed on the green bean pods delivered to the markets of Isfahan from the southern provinces of the country. A bacterium with a fluorescent appearance and a spindle-shaped structure with irregular edges was routinely isolated from the 81 specimens collected. The genetic fingerprinting employing rep-PCR revealed a resemblance between the PCR product profiles of the isolates, which is consistent with their pathogenic and biochemical characteristics. The PCR reaction of representative isolates produced the expected-size fragments of 16Sr DNA through genus-specific primer sets. Subsequent analysis of the sequences of the fragments confirmed the affiliation of the isolates with Pseudomonas spp. The expected-sized amplicons were amplified and sequenced in nested-PCR tests using phaseolotoxin gene based primer pairs. The results showed that the nucleotide sequences were 99% homologous to Pseudomonas savastanoi pv. phaseolicola (Psp). By conducting a phylogenetic analysis with sequences of housekeeping genes gyrB and rpoD, the isolates were clustered with reference strains of Psp, confirming their identity. Considering the isolates’ morphological, biochemical and molecular characteristics, Psp was identified as the cause of halo blight disease in beans imported to Isfahan from the southern regions. In laboratory conditions, the persistence of the pathogen on seed was evaluated for more than one year. The long-term viability of Psp on seeds may lead to the ongoing prevalence of bean halo blight disease in the country in the forthcoming years, due to the cool and moist conditions resulting from spring rains.