Suppressing Soil-Borne Diseases of Container-Grown Plants Using Composts

Although most substrates are initially pathogen-free, infestations by root pathogens during the course of the growing cycle are common. If not deliberately treated, peat moss, an important organic medium constituent, is conducive to spread of several pathogens. Contrary to peat, some compost types suppress a wide variety of soil-borne pathogens. Compost is a term describing organic matter that has undergone long, thermophilic, aerobic decomposition a.k.a. composting. In many, but not all cases, disease suppressiveness is associated with compost's degree of maturity. The main suppressiveness causal agents are consortia of microbial and fungal populations, which colonize the compost mainly during the curing stage. Sterilization largely negates compost suppressiveness, strengthening the notion that biological activity is responsible for most of this phenomenon. Residual non-biological activity is probably related to fungistatic compounds, present in some compost types. The use of composts as disease suppressive components of growing media is discussed in relation to the nature of the raw materials, methods of composting and effective application rates. The main constraints for compost use in growing media are its salinity, pH and physical characteristics. Examples of compost suppressiveness against a wide variety of microorganisms are described and putative mechanisms are critically discussed. Required future research is highlighted.