The goal of this research was to initiate studies on compost effects on severity of common soil borne diseases and to characterize changes plant growth due to improved nutrient availability and soil microbial activity with compost application in organic production systems. Composts were initially characterized for maturity using a commercially available compost maturity test kit. Results were compared to laboratory measures for compost suppressiveness. Preliminary results suggest that this kit may be a useful tool to assess suppressive quality after further testing. Four field experiments were conducted examining different compost effects on disease incidence and plant quality. Disease pressure was very low in field trials, despite selection of sites with historic disease problems. There was some evidence of reduced disease incidence and severity in composted treatments in one snap bean experiment conducted at a research station, but this was not organically managed and the data need to be confirmed by field scale trials. Plant stands did improve in trials using dairy manure based composts, but not poultry manure based composts. The moderate rates (2 to 15 T/A) of compost used did not increase soil microbial activity over the uncomposted controls, at a midseason sampling time. Higher rates may provide this type of increase, but may become economically unfeasible. Soil microbial activity levels were higher on an organic farm compared with several neighboring conventionally managed farms, suggesting that historic management was more important than a single application of compost. Two greenhouse disease bioassays compared commercially available compost materials for effects on plant growth and disease suppression. In the first study, composts were sterilized to eliminate microbial activity and isolate nutrient effects. The results for disease suppression in the first greenhouse study were inconclusive due to poor pathogen growth. However, two of the tested materials enhanced plant emergence rates and growth, particularly if compost microbial activity was intact (non-sterilized). A subsequent greenhouse assay focused on one poultry compost that had been previously shown to suppress Rhizoctonia root rot of beet. Low rates of this compost were not effective at reducing plant losses to this disease, under controlled conditions. It is suspected that this compost provided added fertility to increase growth of vigorous seedlings and thus result in higher yields in these plots.