Technical Abstract: Common scab is the fourth most important potato disease, and affects root and tuber crops world-wide. Scab is caused by streptomycetes, a diverse group of soil-inhabiting gram-positive bacteria. Most are not plant pathogens. To better understand the basis for plant pathogenesis, and the variability in disease symptoms seen in laboratory and field situations, we are investigating pathogenicity determinants and virulence factors among plant pathogenic streptomycetes. Streptomycetes that cause scab are phylogenetically diverse, but production of the toxin thaxtomin is considered to be a pathogenicity determinant. Pathogenicity is also associated with a proposed pathogenicity island (PAI) containing a gene for a pathogenicity factor (nec1) within a conserved region that may be horizontally transferred into distantly related streptomycetes to produce new plant pathogenic strains. We isolated streptomycetes from scabby potatoes from several regions of the USA and assessed their pathogenicity in radish and potato. Disease symptoms varied in severity and appearance. Many pathogenic isolates do not produce the pigment melanin, and a few do not contain the nec1 gene. No isolate missing the nec1 gene was pathogenic, although some isolates not producing melanin were pathogenic. We have further characterised the putative PAI regions of the isolates using PCR, with the nec1 gene and a second linked gene as anchor points. In contrast to our expectations for structural conservation in this region, we find large variation among plant pathogenic strains. We plan to identify additional molecular and biochemical/physiological characteristics of streptomycetes contributing to variability in scab disease symptom development.