First Report of Streptomyces turgidiscabies Causing Potato Common Scab in Spain

Potato common scab (CS) is an important crop problem worldwide. Previously, Streptomyces europaeiscabies has been considered as a predominant pathogen in the west European region. Potato (Solanum tuberosum) samples having visible CS symptoms were collected from Galicia, Spain, during 2015. Potato samples were heavily infected with CS symptoms from superficial netted scab to pitted lesions. Samples were washed with tap water before and after disinfecting with 2.8% sodium hypochlorite for 1 min. Scab lesions on the surface of infected potatoes were used to isolate Streptomyces colonies (Wanner 2006). During the survey, more than 25 Streptomyces isolates were identified as pathogenic to potatoes from different regions of Spain (data not shown). Streptomyces isolate strain AC-51 was identified after morphological and physiological characterization as S. turgidiscabies (Bouchek-Mechiche et al. 2006). Pure colonies of Streptomyces were cultured in tryptic soy broth (TSB). A Streptomyces pellet was used for genomic DNA extraction by Fast DNA kit (MP Biomedicals, U.S.A.). Purified DNA was amplified for 16S rRNA gene by using pH/pA primers (Edwards et al. 1989). The resultant 16S rDNA sequences were submitted to GenBank. NCBI BLAST analysis of S. turgidiscabies strain AC-51 (LN908786) showed 97% similarity with S. turgidiscabies (FJ546740). Streptomyces spp. was further characterized by performing a positive PCR reaction with S. turgidiscabiei-specific primers Tur-R/F (Tagawa et al. 2008) but had negative reaction with individual species-specific primers from S. scabies, S. europaeiscabies, S. stelliscabies, S. bottropensis, and S. acidiscabies (Wanner 2006). A pathogenicity island consisting of txtAB, nec1, and tomA genes was confirmed in S. turgidiscabies strain AC-51 by using different PCR primers (Bukhalid et al. 1998; Wanner 2006). The pathogenicity of strain AC-51 was tested on potatoes by pot assay in a greenhouse facility. Potato tubers (cv. Kennebec) were planted in potting soil (Compo Sana, Germany) with an average temperature maintained between 25 and 28°C. Cell suspension of strain AC-51 was prepared at concentration of 106CFU/ml. Three-week-old potato plants were drenched with 100 ml of prepared inoculum in duplicate experiments. To validate the experimental results, nonpathogenic Streptomyces sp. (KU560918) and pathogenic S. scabies RL-34 (kindly provided by Dr. J. Leiminger, DSMZ41658) were used in the same concentration as negative and positive controls, respectively. Three-month-old plants were harvested and CS symptoms on potato surface were recorded. Disease symptoms were observed from moderate to high pitted scab lesions among the pots infected with S. turgidiscabies strain AC-51 and S. scabies (Wanner 2006), while no CS symptoms were observed with nonpathogenic Streptomyces spp. Disease severity was recorded as mean surface area covered by CS symptoms as 32% for strain AC-51, 38% for S. scabies, and 0% for nonpathogenic Streptomyces spp. Pathogenic S. turgidiscabies strain AC-51 was again isolated from infected tubers in greenhouse study to confirm Koch’s postulates. To our knowledge, this is the first report of S. turgidiscabies in Galicia, Spain. This study will provide a basic knowledge about the pathogens which will helpful to develop preventive strategies against this disease. © 2017, American Phytopathological Society. All rights reserved.