Isolation of antagonistic Streptomyces sp against a potato scab pathogen from a field cultivated with wild oat

Fungi, actinomycetes, and bacteria isolated from soil and plant samples from a potato field in which wild oat (Avena strigosa) had been pre-cultivated were screened for microorganisms that can be used as biocontrol agents for common scab of potato. Of 342 isolates assessed in initial pot trials for their suppressive effect on the severity of potato scab caused by Streptomyces turgidiscabies, 26 isolates were selected as antagonistic candidates based on their ability to reduce disease severity, then tested in a second pot trial. Of the 26, five actinomycetes, isolated from either the rhizosphere soil of wild oat or the soil adhering to potato stolons and tubers, were selected as antagonists. A comparison of partial sequences of 16S rRNA genes from the five isolates indicated that they belong to the genus Streptomyces. Of these five, WoRs-501 most strongly inhibited in vitro mycelial growth of S. turgidiscabies and was also the most effective in suppressing potato scab in a third field pot trial. In that pot trial, a 10% (v/v) mix of WoRs-501 (6.2 x 10(8) colony-forming units [CFU]/g dry mass) decreased the disease severity by 78-94% in comparison with the untreated control at 5 x 10(4) to 5 x 10(6) CFU S. turgidiscabies/g dry soil. WoRs-501 also grew well in vitro at a wide range of pH levels and temperatures. These results suggest that WoRs-501 is a promising candidate for biocontrol of potato scab.