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 2018.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. 16S 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 14 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 20 and Tween 80. These strains produced indole and urease. The partial 16S rRNA gene sequence of strain MR1 was identical to that of Enterobacter hormaechei subsp. hoffmannii DSM 14563T. The maximum-likelihood phylogenic tree was created using concatenated sequences of gyrB (684 bp) and infB (906 bp) genes. The strains MR1, MR3, and MR5 were grouped with E. hormaechei subsp. hoffmannii DSM 14563T.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 های Enterobacter hormaechei subsp. hoffmannii, a new plant pathogen, associated with walnut decline:

Emerging pathogens ، Iran ، Juglans regia ، MLSA.