TAXONOMIC STUDIES ON STREPTOMYCETES CAUSING POTATO COMMON SCAB - A REVIEW

Several Streptomyces species cause common scab of potato. The predomin ant pathogenic species, S. scabies, is associated with shallow, raised , and deep-pitted scab lesions. This species displays more diversity t han one would expect in a conventional species. In spite of common mor phological and physiological traits, S. scabies isolates vary consider ably in fatty acid and protein composition. Furthermore, DNA-DNA hybri dization Studies reveal that S. scabies isolates can be divided into a t least three genetic groups. In addition to S. scabies, other species such as S. acidiscabies and a species related to S. albidoflavus can induce scab symptoms in potato. S. acidiscabies is characterized by it s ability to cause common scab in acidic environments. The species rel ated to S. albidoflavus usually induces deep-pitted lesions and is ass ociated with infected tubers grown in well-irrigated soils. Virulence factors of plant pathogenic streptomycetes are still unknown but the a bility to produce phytotoxins called thaxtomins is a physiological pro perty shared by most pathogenic strains.