Large patch, caused by Rhizoctonia solani AG-2, is a destructive disease of zoysiagrass (Zoysia japonica) worldwide. When it occurs, multiple applications of fungicides in the fall and spring are often required to minimize damage. Repeated applications are costly and increase the risk of selecting fungicide-resistant pathogen populations.  Alternatively, mustard (Brassica juncea) seed meal (MSM), a byproduct of Brassicaceae seed oil extraction processing, has been reported to suppress several soilborne pathogens, including some Rhizoctonia species. MSM contains glucosinolates, which are converted into toxic or biocidal isothiocyanates by the enzyme myrosinase in the presence of water.  Laboratory in vitro studies were conducted to examine the inhibition of MSM on growth of an isolate of R. Solani from infected zoysiagrass. Addition of MSM powder to the surface of V8 juice agar at rates of 210 and 1680 kg ha^-1 suppressed the growth of R. solani for 5 and 20 days, respectively. By comparison, R. solani grew visibly one day after inoculation on non-treated control plates and plates that receiving azoxystrobin fungicide at label rates. The residual effect of MSM was evaluated by assessing growth of R. solani on V8 juice agar inoculated 4 days after addition of MSM. It was found that fungal mycelial growth at 840 kg ha^-1 was reduced more than 50% compared to the growth rate on non-amended agar.  No growth of R. solani was observed on agar amended with MSM at 1680 kg ha^-1. When MSM was autoclaved prior to addition to growth medium, or added to the growing medium as aquatic extracts, suppression of R. solani was also observed. In greenhouse studies, MSM at all application rates provided significant control of large patch. Optimal control, recorded as the number of dead tillers at 3 weeks after R. solani inoculation, occurred on plants treated with75% autoclaved MSM powder at 1680 kg ha^-1 applied biweekly.