Isolation and characterization of plant-pathogenic Streptomyces species associated with potato common scab disease in Turkiye

Potato common scab (PCS), a significant potato disease, negatively impacts tuber quality. The present study was conducted to isolate and characterize pathogenic Streptomyces species associated with PCS disease in Turkiye based on their morphological, physiological and molecular characteristics. Field-grown potatoes that exhibited scab lesions were collected from four provinces in 2020-2021, and 200 bacterial isolates were obtained from netted, superficial and deep-pitted common scab lesions. Pathogenicity assays, including in vitro tuber slice and in planta radish seedling bioassays, identified 150 pathogenic isolates. Morphological and physiological characterization of 92 selected isolates revealed several Streptomyces species that exhibited diverse mycelium colours, sporulation patterns and pigmentation. Molecular analysis using 16S rRNA sequencing, species-specific primers, PCR-RFLP of the 16S-23S (ITS) region with Hpy99I restriction enzyme, and multilocus sequence analysis (MLSA) based on atpD, recA, rpoB and trpB genes revealed that S. scabiei was the dominant species, followed by S. europaeiscabiei, S. caniscabiei, S. bottropensis, S. stelliscabiei and S. turgidiscabies. PCR analysis revealed the presence of the thaxtomin synthetase genes (txtAB) in all tested samples, while the necrogenic protein (nec1) and tomatinase (tomA)-encoding genes were absent from three and two isolates, respectively. Phylogenetic analysis of 32 representative isolates conducted with sequences from 16S rRNA, species-specific PCR and MLSA confirmed their morphological identification and clustered them with reference strains. This study contributes to the understanding of distribution of Streptomyces species associated with PCS, and to our knowledge, is the first molecular confirmation of S. caniscabiei and S. turgidiscabies causing potato scab in Turkiye.