Extensive chromatography of Streptomyces scabies infected potato tissue yielded five compounds which on aseptically cultured minitubers individually reproduced symptoms typical of the common scab disease. Spectroscopic examination of the minor components elicited sufficient information to identify them as related analogues of the major constituents thaxtomin A and thaxtomin B, which had been previously characterized as unique 4-nitroindol-3-yl-containing 2,5-dioxopiperazinea. Subsequently, a number of synthetic variants were prepared to further understanding of the structural requirements for phytoactivity in this class of compounds.