Common scab is a significant disease of potato, and affects root and tuber crops worldwide. Streptomycetes, a diverse group of soil-inhabiting gram-positive bacteria, cause common scab. To better understand the basis for variability in disease symptoms seen in field situations, strep-tomycetes were isolated from scabby potato plants. Isolates differed in morphology and pigmentation. Isolates were evaluated for pathogenicity and virulence in radish. Scab lesions varied in appearance and severity. Disease symptoms also included plant stunting, wilting, necrosis, and death. Some pathogenic isolates were missing genes from the putative pathogenicity island (PAI); several lacked the nec1 gene, and one was missing the txtA gene encoding thaxtomin biosynthesis, the most reliable pathogenicity determinant. Studies of disease severity over 5 logs initial inoculum density showed that there is a threshold inoculum density for disease. Disease severity increased with inoculum density over three logs, then reached a maximum, which is characteristic of individual Streptomyces strains. Lesion severity was not correlated with presence of melanin, the nec1 gene, or whether an isolate reduced seedling emergence or plant survival. Differences in disease symptoms and severity combined with absence of known pathogenicity determinants (txtA) or factors (nec1) suggest that there may be pathogenicity factors in addition to thaxtomin.