Compost produced from agricultural wastes can be used as the organic component of container media. We have studied the properties of composted grape marc (CGM) and composted separated cattle manure (CSM). Media amended with these composts were found to be suppressive to soilborne plant pathogens such asRhizoctonia solani andPythium aphanidermatum. Suppression is the result of microbial activity developing during the composting process. It is suggested that suppressive compost can be used in horticulture as a means of biological control.