Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: March 5, 2007
Publication Date: March 5, 2007
Citation: Wanner, L.A. 2007. Distribution of scab-causing Streptomyces species in potato-growing regions in the United States. Meeting Abstract. Technical Abstract: Common scab of potato is caused by gram-positive bacteria in the genus Streptomyces, a genus best known as a soil saprophyte and the source of numerous medically important antibiotics. Streptomycetes are abundant in soil, although few species cause plant disease. At least eleven common scab-causing species have been described, based on 16s ribosomal DNA sequences, whole genome sequence similarity and biochemical characteristics. These plant pathogenic species share all or parts of a large block of genes including at least one group of genes encoding a pathogenicity determinant for common scab, the phytotoxic compound thaxtomin. The block of genes resembles a Pathogenicity Island (PAI) and has apparently been transferred among certain species of Streptomyces, creating new pathogenic species or strains. Recently, extensive data on the species of pathogenic Streptomyces found in the USA have been collected from multiple field locations in 19 US states and one Canadian province. Four species were identified among more than 700 isolates determined to be likely pathogens, on the basis that they contained genes encoding the pathogenicity determinant thaxtomin. In almost all cases, only a single species was found in a field location, although different species were frequently found in near-by locations. The most common species was S. scabies, the first-described and best known plant pathogenic species. Others included a new species or strain not previously described and found only in the western states. Genetic variability in the PAI was found, primarily between but sometimes within locations. Additional genetic variation among species sharing PAI markers and in the same location was identified. An interesting finding was that apparently non-pathogenic Streptomyces species that inhabit potato tuber skins and common scab lesions group into three recognizable groups or clades, phylogenetically separate from the pathogenic species. Although these species are found together with pathogenic species in scab lesions, they have not acquired a PAI, suggesting some natural limitations or barriers to exchange of the mobilizable pathogenicity region. Although both pathogenic and non-pathogenic strains could be isolated from common scab lesions, greater numbers of non-pathogenic species were associated with disease-free potato skin or other types of surface lesions, and with resistant potato varieties, while greater numbers of pathogenic isolates were found in typical common scab lesions and more susceptible varieties. The role of differences in Streptomyces flora on potato skins in common scab disease deserves further investigation. The significance of different scab-causing species and genetic variation within species for the severity and management of the disease is currently not known, although examples of differences in symptom severity and cultivar susceptibility between scab-causing species have been recently confirmed in Europe.