Streptomyces ipomoeae is the causal agent of an economically important disease of sweetpotato, and a similar pathogen Streptomyces scabies causes common scab on potato. Previously, we isolated and characterized the phytotoxins thaxtomin A, B and their analogs from S. scabies-infected potato tissue. In this study, chromatographic investigations of S. ipomoeae-infected potato and sweetpotato tissues yielded three compounds that individually reproduced symptoms typical of common scab on aseptically cultured minitubers. Spectroscopic examination identified the two minor components as analogs of the major constituent, which was characterized as 12-N-methylcyclo-(~-4-nitrotryptophyl-~-phenylalanyl) (thaxtomin C). Subsequent investigations also identified a possible toxin precursor substance as 12-N-methylcyclo-(~-tryptophyl-~-phenylalanyl). We describe the production and characterization of these thaxtomin analogs, investigations regarding the generality of the pathogen-toxin relationship, and the identification of the phytotoxin precursor.