The major effort in developing pathogenic fungi into potential mycoherbicides is aimed at increasing fungal virulence to weeds without affecting crop selectivity. Specific suppression of biosynthesis of a phytoalexin derived from the shikimate pathway in Cassia obtusifolia L. by a sublethal dose (50 micromolar) of glyphosate increased susceptibility to the mycoherbicide Atternaria cassiae Jurair & Khan. Glyphosate applied with conidia suppressed phytoalexin synthesis beginning at 12 hours, but not an earlier period 8 to 10 hours after inoculation. The phytoalexin synthesis elicited by fungal inoculation was also suppressed by darkness. The magnitudes of virulence of the mycoherbicide in the dark or with glyphosate in the light were both higher than after inoculation in the light with the same concentration of conidia in the absence of glyphosate. Five times less inoculum was needed to cause disease symptoms when applied with glyphosate than without. Glyphosate did not render A. cassiae virulent on soybean (Glycine max), a crop related to the host. These results suggest that a specific inhibition of a weed's elicited defense response can be a safe way to enhance virulence and improve the efficacy of the mycoherbicide.