Citrobacter braakii, a new plant pathogen, causal agent of walnut decline

BACKGROUND AND OBJECTIVES Walnut canker is characterized by brown to blackish roundish blotches on the trunks and main branches, necrosis of inner bark and bleeding with dark brown to black-colored exudates. The present study aimed to identify the causative agents of walnut decline by their phenotypic features, approval of pathogenicity, the partial sequencing of the housekeeping genes in Razavi Khorasan.MATERIAL AND METHODSTen Symptomatic samples were collected from walnut orchards of Razavi Khorasan in ۲۰۱۹. Pathogenicity of all isolated strains was carried out on walnut immature fruits cv. ‘Hartley’ and young green twigs of cv. ‘Chandler’. All pathogenic strains were subjected to physiological, morphological and biochemical tests. ۱۶S rRNA and housekeeping genes (fusA, leuS, and pyrG) were partially amplified and sequenced.RESULTS AND DISCUSSIONEight strains were able to cause necrosis and a dark-colored region in the mesocarp of immature walnut fruits and three representative strains caused necrosis on young inoculated twigs. Strains utilized starch, however, did not utilized esculin, Tween ۲۰, Tween ۸۰, and gelatin. The partial ۱۶S rRNA gene sequence of strain KH۷ indicated ۹۹.۶۳ % similarity to that of Citrobacter braakii ATCC۵۱۱۳T. The phylogenetic analyses based on the partial sequencing of three housekeeping genes, fusA (۶۳۳ bp), pyrG (۳۰۵), and leuS (۶۴۰ bp) demonstrated that strains KH۱, KH۳, and KH۷ belong to C. braakii species in a monophyletic clade with high bootstrap support.CONCLUSIONTo the best of our knowledge, this is the first report of C. braakii as a new plant pathogen which cause walnut decline. Identification of bacteria associated with walnut decline will eventually improve our understanding of the etiology of the disease and may result in improved management techniques for control.

کلیدواژه ها:

Emerging pathogens ، Iran ، Juglans regia ، MLSA