Submitted to: Canadian Journal of Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 14, 2007
Publication Date: October 5, 2007
Citation: Wanner, L.A. 2007. Population studies of Streptomyces in dedicated common scab fields. Canadian Journal of Microbiology. 53(9):1062-1075.
Interpretive Summary: Field tests provide important information on plant disease resistance for breeders and growers during development and release of new plant varieties, but results of field tests often differ in different environments. This study was designed to determine why varieties of potato respond differently to common scab disease in two field sites where the pathogen is present. We found the same pathogen type in both locations, but the two locations had different populations of other soil organisms. This suggests that the presence and kinds of non-pathogenic soil organisms affects disease development differently, even when the same pathogen is present.
Variability in symptoms of potato common scab between locations and years necessitates testing potato cultivars for common scab resistance in multiple field locations with different disease pressures. Several species or strains of plant pathogenic Streptomyces that cause the disease are found in the USA. To determine whether different species or strains predominate in test locations with differing disease severities, Streptomyces isolates were collected from two fields that have been in use for common scab resistance testing for more than 25 years. Pathogenic isolates were recovered from all types of lesions, and, in significantly lesser proportions, from healthy potatoes and tubers with surface blemishes atypical of common scab. The species and the haplotype of the pathogenicity island (PAI) were determined to be the same in nearly all of the pathogenic isolates collected during 3 years in Minnesota, and 4 years in Maine. It is thus concluded that the same Streptomyces species and strain is responsible differences in disease pressure in different locations, and was responsible for all types of scab lesions. A variant PAI haplotype was recovered in Minnesota in minor amounts in two of three years. In addition, other streptomycetes were also recovered from all types of lesions in both locations. The relative proportion of pathogenic to non-pathogenic streptomycetes differed between lesion types, with significantly higher proportions of pathogenic isolates recovered from raised and pitted lesions than from unblemished potato skin, or tubers showing other skin defects not typical for common scab. A significantly lower proportion of pathogenic to non-pathogenic Streptomyces isolates was recovered from the most resistant genotypes, suggesting that a genetically heritable feature of potato cultivars influences the population dynamics of streptomycetes. Although disease severity was lower in Minnesota in all years, the proportion of pathogens to non-pathogens was higher in Minnesota, while the diversity of non-pathogenic streptomycetes recovered was higher in Maine. It is concluded that other biological factors, such as the populations of streptomycetes and other soil microbes in association (or competition) with S. scabies, are responsible for the differences in disease pressure in the two locations.