Occurrence of Turnip Scab Caused by Phytotoxin-Producing Streptomyces spp

Turnips (Brassica rapa L var. rapifera) purchased from a New Brunswick supermarket in the winter of 2003 contained discernable corky scab-like lesions on their surfaces. The turnips were surface-disinfested with a warm-water rinse, and microorganisms from excised lesions were isolated with methodologies developed for the specific isolation of Streptomyces spp. (1). Nine isolates morphologically characteristic of Streptomyces scabies strains (2) were subsequently examined for pathogenicity on potato mini-tubers and production of the scab phytotoxin, thaxtomin A (1). Four of the nine isolates proved pathogenic on potato mini-tubers. When grown on an oatmeal broth medium, only the pathogenic isolates generated thin-layer chromatographic detectable quantities of the requisite phytotoxin, thaxtomin A (1). Previous greenhouse studies have demonstrated the ability of pathogenic Streptomyces spp. isolated from scab lesions on potatoes to infect turnips (2). To our knowledge, this is the first investigation to confirm the presence of thaxtomin A producing isolates on field-grown turnips. References: (1) R. R. King et al. Am. Potato J. 68:675, 1991. (2) R. Loria et al. Plant Dis. 81:836, 1997.