Characterization of Xanthomonas hortorum isolates causing leaf spot on Primula vulgaris

Abstract

The bacterial leaf spot of Primula vulgaris was reported from forest locations in Mazandaran province, Iran in 1995. The causative bacterium was initially identified as Xanthomonas campestris based on sequences of ITS and 16S rRNA regions. On the basis of sequences of gyrB and dnaK housekeeping genes, the isolates resembled strains of Xanthomonas hortorum. The present study was performed to clarify the taxonomic status of the incitant bacterium. Samples of P. vulgaris with leaf spot symptoms were collected from the Darabkola forest (Mazandaran province) and the bacterial strains were isolated on plates of glucose nutrient agar. Some isolates only induced hypersensitive reaction on geranium (Pelargonium × hortorum) leaves but a few isolates incited leaf spot symptoms when inoculated on P. vulgaris, P. heterochroma and P. × polyantha plants. The isolates were Gram stain, oxidase and urease negative but positive in tests for catalase and hydrolysis of gelatin, aesculin and Tween 80. Several unique nonpathogenic isolates were recovered with varying carbon source utilization profiles and different molecular features, including distinct ERIC‐PCR profiles. Representative gyrB, rpoD, dnaK and fyuA housekeeping genes were amplified by PCR and sequenced for comparison with Xanthomonas species in the GenBank database using maximum‐likelihood, maximum‐parsimony and Bayesian algorithms to construct phylogenetic trees. The pathogenic and nonpathogenic strains were closely related to X. hortorum pathovars. Therefore, it is likely that these isolates belong to a new pathovar of X. hortorum. Based on previous studies and our findings, we propose naming the strain causing primrose leaf spot as X. hortorum pv. primulicola.