Phylogeny, phenotypic characteristics and pathogenicity of Sphingomonas sp. and Erwinia persicina as bacterial causal agents of lettuce diseases in southwest of Iran

Abstract

Bacterial-like symptoms, such as chlorosis, necrosis, and rot, have recently been observed on lettuce leaves in Khuzestan province, located in southwest Iran. To identify and specify the bacterial causal agents responsible for this outbreak, bacterial strains were purified from samples collected from lettuce fields in Dezful, Shushtar, and Shousha Daniel regions. The pathogenic bacterial strains were identified through hypersensitivity response (HR) and pathogenicity tests. By comparing the sequences of 16S rDNA of pathogenic strains, and phylogeny analysis, it was clarified that the pathogenic strains belonged to Erwinia persicina and genus Sphingomonas. Phenotypic study showed that Sphingomonas strains were resistant to streptomycin and gentamicin, and produced yellow pigment, and E. persicina strains fermented glucose, and produced pink soluble pigment. Additionally, the E. persicina strains were sensitive to streptomycin, gentamicin, levofloxacin, and erythromycin. The pathogenicity test showed that E. persicina strains induced necrosis, yellowing, and rot in the leaves of Romaine type of lettuce. Similarly, Sphingomonas strains caused necrosis, accompanied by a large yellow halo, in the leaves of Romaine type. Furthermore, the E. persicina and Sphingomonas strains induced necrosis of the veins of Romaine type, as well as brown spotting in the leaves of Iceberg type. In addition, the study of the host range of E. persicina and Sphingomonas strains showed that they infect the leaves of different crops (including cucumber, tomato, pepper, celery, and eggplant) and mallow weed. These crops are grown in rotation or simultaneously with lettuce in Khuzestan province. The diverse host range of mentioned pathogens facilitates their survival during the year in this area. This is the first report of the genus Sphingomonas in lettuce and the first report of E. persicina from Iran.