Common scab is an important disease in potato (Solanum tuberosum) caused by Streptomyces spp. In Finland, morphological and physiological characterization (5) and comparison of the 16S rRNA gene sequences have suggested that Streptomyces scabies and S. turgidiscabies are the main causal species (2), but occurrence of S. europaeiscabiei has not been previously studied. In September 2011, potato tubers of cvs. Fambo, Melody, Puikula, Rosamunda, Victoria, and Van Gogh showing symptoms of common scab were collected from 10 fields in southern (Lammi), eastern (Liperi), and western Finland (Isokyrö, Kalajoki, Tyrnävä). Symptoms included superficial, raised, and pitted corky lesions ranging from 3 to 10 mm in diameter. Isolations were made from scab lesions on water agar. Colonies phenotypically characteristic of Streptomyces were transferred to glucose yeast malt extract agar (GYM) after 7 days. Pure cultures were obtained through subsequent transfers to fresh GYM medium, on which the strains produced golden brown colonies and white to grey spores. DNA was extracted from bacteria using the E.Z.N.A. SP Plant DNA Mini Kit (Omega Bio-Tek, Inc.). Primers developed for the 16S rRNA gene sequences (4) were used to detect S. scabies and S. turgidiscabies by PCR. Of the 14 strains recovered, nine were assigned to S. scabies, three to S. turgidiscabies, and two remained unidentified. However, S. scabies and S. europaeiscabiei cannot be distinguished by the 16S rRNA gene sequence, but the ITS1 region of the 16S operon sequence amplified by PCR is cleaved by Hpy99I in S. scabies but not in S. europaeiscabiei (1). Altogether, 18 strains were tested, including nine obtained in 2011 and seven Finnish and two Swedish strains isolated and assigned to S. scabies in the mid-1990s (2,4,5). The ITS1 sequence of S. scabies type strain, ATCC49173, was cleaved with Hpy99I, in contrast to all other strains that were consequently assigned to S. europaeiscabiei. To further confirm the identity of the Finnish strains, sequences of atpD, recA, and rpoB genes from three strains (one from 1995, two from 2011) (GenBank Accession Nos. KJ802471 to 79) were found to be 100, 99.8, and 100% identical, respectively, to corresponding S. europaeiscabiei type strain CFBP 4497 sequences (3). Pathogenicity of the S. europaeiscabiei strains isolated in 2011 was confirmed using radish seedling assay (1). All strains prevented or reduced the growth of radish seedlings (cv. French Breakfast) or caused severe necrosis in repeated experiments. No symptoms were observed on the seedlings grown on oat meal agar without bacteria. The pathogenicity of the S. europaeiscabiei strains isolated in the mid-1990s was confirmed using minituber assays (4,5). In addition, two of these strains were tested in a glasshouse experiment and two in a radish seedling assay and shown to be pathogenic. The results suggest that S. europaeiscabiei is an important cause of potato common scab in Finland. In the earlier studies, S. europaeiscabiei may have been mistaken for S. scabies, as the restriction analysis of the ITS1 region was not done.
References: (1) R. Flores-Gonzáles et al. Plant Pathol. 57:162, 2008. (2) J. F. Kreuze et al. Phytopathology 89:462, 1999. (3) D. P. Labeda. Int. J. Syst. Evol. Microbiol. 61:2525, 2011. (4) M. J. Lehtonen et al. Plant Pathol. 53:280, 2004. (5) P. Lindholm et al. Plant Dis. 81:1317, 1997.
