Scab-causing Streptomyces spp. Streptomyces scabies is the major scab-causing pathogen on several important root and tuber crops, such as potato, carrot, raddish, beet and peanut, and is responsible for considerable annual losses in the USA. Scab symptoms are also provoked by Streptomyces acidiscabies, S. turgidiscabies, S. europaeiscabiei, S. stelliscabiei, S. aureofaciens, S. reticuliscabiei, S. luridiscabiei, S. bottropensis, S. puniciscabiei and S. niveiscabiei, whereas S. ipomoea is the causative agent of soil rot or pox, a widespread and destructive disease of sweet potato (Loria et al., 2006). The genomic sequence of S. scabies strain 87–22 has been determined (Seipke and Loria, 2008). Based on DNA–DNA homology and rRNA sequences, the phytopathogenic streptomycetes show great diversity, but the virulence-associated genes are conserved. Divergent non-pathogenic saprophytic species have probably acquired the ability to infect and colonize plant tissues through horizontal gene transfer of a mobile pathogenicity island (Loria et al., 2006). The pathogenicity island of S. scabies, S. acidiscabies and S. turgidiscabies contains a tomA homologue (Seipke and Loria, 2008) and other pathogenicity genes, such as nec1, a non-ribosomal peptide synthetase, a P450 monooxygenase and a nitric acid synthase involved in thaxtomin production (Loria et al., 2008). Thaxtomins, modified dipeptide phytotoxins that inhibit cellulose synthesis in expanding plant cells, are the main pathogenicity determinants. Full virulence also requires the conserved nec1 gene, which codes for an unknown protein. Bioassays revealed that secreted Nec1 causes necrosis on potato tuber tissue, suggesting a function in colonization of the root meristem or a role in host defence suppression (Joshi and Loria, 2007).
The pathogenicity island of S. turgidiscabies carries an operon that is related to the fasciation (fas) operon of the Gram-positive phytopathogen Rhodococcus fascians and is involved in cytokinin biosynthesis. When inoculated on tobacco or Arabidopsis, S. turgidiscabies induces leafy galls indistinguishable from those induced by R. fascians. Currently, the role of cytokinins in scab disease remains to be determined (Joshi and Loria, 2007).