Investigating Pathogenicity and the Virulence Genes txtAB, nec1, and tomA in Streptomyces Isolates from Wisconsin Potato Systems

Potato common scab (CS) is a disease caused by a complex of Gram-positive, filamentous bacteria belonging to the genus Streptomyces. Most plant pathogenic strains of Streptomyces cause disease by producing thaxtomin A from the txtAB gene, along with two additional virulence genes, nec1 and tomA, which may be present. CS lesion phenotypes have been categorized as superficial-, raised-, or erumpent-lesions, pitted scabs, and fissure. The present study focused on characterizing potato-pathogenic Streptomycete isolates from potato growing regions during 2012–2013. Isolates from CS-infected tubers sampled from primary potato-growing regions in Northern and Central Wisconsin were analyzed for phenotypic attributes and phylogenetic features based on 16 S rRNA genetic regions. In total, 46 isolates were recovered from CS-symptomatic tubers expressing 3 categories of lesion types (superficial, raised, and pitted scab) from 6 potato cultivars from 7 Wisconsin counties. Forty-three out of 46 isolates were determined to be pathogenic based on radish seedling inoculation assays, and all of the pathogenic isolates contained the txtAB gene. The topological arrangement of the phylogenetic tree constructed with 16 S rRNA sequences of this isolate collection reflected homology with the most commonly reported species, S. scabiei, S. bottropensis and others. The collection revealed thirteen Streptomyces ribotypes isolated from scab lesions. These findings suggested a change in the profile of the soilborne pathogen populations from 2006 to 2012–2013. We determined correlations between thaxtomin A production and the presence of virulence factors nec1 and tomA. Our nec1-positive Streptomyces strains produced significantly more thaxtomin A than the nec1-negative strains, and tomA-containing strains produced more thaxtomin A than tomA-negative strains.