Enterobacter hormaechei subsp. hoffmannii, a new plant pathogen, associated with walnut decline

BACKGROUND AND OBJECTIVESPersian walnut (Juglans regia) has a considerable economic importance worldwide. However, the vigor and vitality of walnut trees were heavily affected by bark canker during the last few years. Stem tissue necrosis, and bleeding with black-colored exudates walnut trees were observed in Markazi province in ۲۰۱۸.MATERIAL AND METHODSA total of eight symptomatic samples were collected from affected walnut trees. The pathogenicity of all strains was proved by inoculating a suspension of the bacterial strains under the bark of immature walnut fruits cv. ‘Hartley’. Conventional phenotypic assays were performed. ۱۶S rRNA, gyrB, and infB genes were partially amplified and sequenced.RESULTS AND DISCUSSIONSixteen strains with a metallic green sheen were isolated on EMB-agar medium. Eleven strains caused necrosis and a dark-colored region in the mesocarp around the inoculation site ۱۴ days post-inoculation. Moreover, three representative strains induced necrotic and black- colored tissues in the bark of young green twigs of two-year old walnut seedling cv. ‘Chandler’. No symptoms were detected in the fruits and young branches infiltrated with sterile distilled water. Strains hydrolyzed gelatin, however, did not utilize starch, esculin, Tween ۲۰ and Tween ۸۰. These strains produced indole and urease. The partial ۱۶S rRNA gene sequence of strain MR۱ was identical to that of Enterobacter hormaechei subsp. hoffmannii DSM ۱۴۵۶۳T. The maximum-likelihood phylogenic tree was created using concatenated sequences of gyrB (۶۸۴ bp) and infB (۹۰۶ bp) genes. The strains MR۱, MR۳, and MR۵ were grouped with E. hormaechei subsp. hoffmannii DSM ۱۴۵۶۳T.CONCLUSION To the best of our knowledge, this is the first report of E. hormaechei as plant pathogen. This study highlights the fact that walnut decline has to be considered a complex disease in which several bacteria are involved.

کلیدواژه ها:

Emerging pathogens ، Iran ، Juglans regia ، MLSA.