Compost is a controlled aerobic process that degrades organic waste to stable material, with the resident
microbial community mediating the biodegradation and conversion processes. Many composting feedstocks carry
microorganisms that are pathogenic to plants and/or humans. Fortunately, pathogens and weed seeds are destroyed by
the high temperatures achieved during the sanitation stage of composting. Plant disease suppression is the result of
beneficial organisms with biological control mechanisms such as competition, parasitism, antagonism, and/or induced
plant resistance. The assembly of microbial communities (consortium) are organized and influenced by recipe, choice
of post-thermophilic process, and duration of curing (maturation) of composts. By intentionally designing recipes and
curing methods, compost can become a tool to manipulate or deliver a natural consortium of microorganisms in soil,
onto seeds, and planting materials. Composts of varied feedstocks can control Oomycota pathogens, Phytopthora and
Pythium. These pathogens are relatively weak competitors. In contrast, fungal pathogens that are members of the
Rhizoctonia solani complex live as saprophytes on plant debris and detritus. Their control requires mature composts
containing complex carbohydrates (e.g., lignin, tannins) such as woody materials. Until the 1990s, the knowledge of
compost microbiology was limited to organisms that would grow in petri dish culture. This era identified the strains
commonly seen on the market including species of Pseudomonas, Bacillus, Streptomyces and Trichoderma. Some
generalizable patterns are emerging as the use of molecular techniques increases. For example, the assembly of
bacterial and fungal communities that colonize compost during curing and maturation phases depend on whether the
carbon source was hay, straw, softwood (e.g., pine, cedar) or hardwood (e.g., birch). The consortium of
microorganisms that can suppress disease are able to metabolize and degrade complex matrices better than the
community found in conducive compost. Members of this ‘suppression’ consortium include an abundance of bacteria
in the phyla Proteobacteria, Bacteroidetes, Actinobacteria and Deinococcus-Thermus and relatively more fungi in the
phylum Basidiomycota than Ascomycota. One may argue that control of different pathogens on different crops
requires a different combination of microorganisms and/or mechanisms. Therefore, it is quite likely that we need to
develop “designer composts” based on defined recipes and post-thermophilic practices best for specific diseases.