Characterization of Dickeya fangzhongdai causing bacterial soft rot disease on Dendrobium nobile in India

Abstract

Soft-rot disease on Dendrobium nobile observed in a commercial farm in the Nilgiris district of Tamil Nadu, India was investigated. Disease etiology is reported along with phenotypic, molecular identification of the pathogen. Infected leaves showed rot symptoms characterized by irregular water-soaked lesions on the leaf margin that rapidly spread over the entire leaf. Milky exudation on the cut end of infected leaves indicated the bacterial etiology of the disease. The bacterial colonies isolated were irregular, whitish-yellow, spindle-shaped with a characteristic raised center. All isolates showed identical phenotypic traits and induced a hypersensitive reaction on Nicotiana tabacum leaves. Pathogenicity assay performed on D. nobile confirmed their pathogenic nature as all three bacterial isolates incited typical soft-rot within 12 h post-inoculation (hpi). In vitro host range studies revealed that all the isolates could cause rot on cut slices of other vegetable crops including potato, carrot, and onion. Additionally, the bacterium incited soft rot on Aloe vera, but not on chrysanthemum and banana transplants. The identity of the bacterial isolates was confirmed as Dickeya fangzhongdai using 16S rRNA gene sequences as well as by their pathogenic behavior. Complete understanding of the etiology of the disease and of the pathogen would be useful to devise suitable management practices for the containment of disease spread in the future.