Streptomyces turgidiscabies and S. acidiscabies: two new causal agents of common scab of potato (Solanum tuberosum) in the UK

During a survey of Streptomyces infecting potato Solanum tuberosum in the UK, two strains were isolated which were distinct from S. scabiei, which is already known to occur in the UK . Amplification with species-specific PCR primers (Wanner, 2006) identified these strains as S. turgidiscabies (Miyajima et al., 1998) and S. acidiscabies (Lambert & Loria, 1989), and the same PCR assay indicated presence of these species in several fields in England and Scotland . Representative strains were deposited in the UK National Collection of Plant Pathogenic Bacteria as NCPPB4444 and NCPPB4445 respectively. Strain NCPPB4444 was isolated from cv. Desiree tubers, grown in South Yorkshire, England, with large erumpent lesions (Fig. 1). A majority of tubers from the same seed stock in the same field exhibited similar symptoms. Strain NCPPB4445 was isolated from cv. Maris Piper, grown in Aberdeenshire, Scotland , on soil of pH 5.4, where disease incidence was high and dry flaky scab symptoms typically covered almost 100% of the tuber surface (Fig. 2). Both strains were isolated on NPPC medium (Schaad et al., 2001), producing small white sporulating colonies.

Inoculation of radish seedlings with both strains resulted in severe necrosis within two weeks. Potato minitubers (cv. Maris Piper) grown in pots containing sterilised compost were inoculated with 250 ml suspensions of Streptomyces (approx. 10⁷ cfu/ml) when leaves first emerged. After 12 weeks, lesions were observed on tubers in each of five replicate pots. Uninoculated controls were free from symptoms. Isolates recovered from lesions were confirmed by PCR to be the same species as the inocula. To confirm species identity, a portion of the 16S rDNA of each original isolate was sequenced. The sequence from strain NCPPB4444 (GenBank Accession No. FJ817424) was 100% identical to that of S. turgidiscabies type strain ATCC700248. The sequence from strain NCPPB4445 (FJ804480) was 100% identical to that of the S. acidiscabies type strain ATCC49003. Although several Streptomyces spp. are known to cause common scab of potato, the species identity is of significance as different diagnosis and control methods may apply. This is the first report in the UK of common scab of potato caused by S. turgidiscabies or S. acidiscabies.