The emerging pathogen Gibbsiella quercinecans was found in walnut and oak trees, posing a threat to Persian walnut cultivation in central provinces of Iran.

Abstract

The cultivation of Persian walnuts in Iran is concentrated in the mountainous regions of Zagros and Alborz, encompassing Kohgiluyeh and Boyer-Ahmad, Isfahan and Fars provinces. Historically, these areas were renowned for their abundant growth of oak trees. However, due to environmental stressors, oak populations in the Zagros region have been declining, leading to an increased risk of diseases caused by new and aggressive pathogens, exacerbated by climate change. Understanding the distribution and association of pathogenic bacteria in the environment, especially for less common or uncommon species, has become crucial. In this study, 80 bacterial strains were isolated from 84 symptomatic walnut and 16 symptomatic oak trees to investigate bacterial canker agents in primary walnut cultivation regions of Iran. Following the hypersensitivity test and pathogenicity assays, 21 strains were classed as ‘Brenneria nigrifluens’ or ‘Brenneria-like’, based on their similarities to the reference strain B. nigrifluens ICMP 20120 using phenotypic techniques and specific primers (F1/C3, B. nigrifluens). Varying biochemical characteristics were exhibited by the ‘Brenneria-like’ group in comparison to the ‘B. nigrifluens’ group. Multilocus sequence analysis was performed using the gyrB, rpoB, infB and atpD genes to determine the taxonomic classification of this group, revealing that it belonged to Gibbsiella quercinecans. The increasing reports of this bacterium from different woodland tree hosts suggest its opportunistic role as an individual causative agent, necessitating monitoring of its host expansion and morbidity.