Suppression of soil-borne plant diseases with composts: A review

Numerous container-based studies in greenhouses or growth rooms have consistently demonstrated a suppressive effect of composts on soil-borne diseases such as damping-off and root rots (Pythium ultimum, Rhizoctonia solani, Phytophthora spp.), and wilts (Fusarium oxysporum and Verticillium dahliae). Composts have also been shown to suppress several diseases in the field, although the effects have been generally smaller and more variable than in container experiments. Several diseases of turf grass were suppressed by top-dressing with compost. These diseases are Fusarium patch (Microdochium nivale), red thread (Laetisaria fuciformis), damping-off (Pythium graminicola), brown patch (Rhizoctonia solani), dollar spot (Sclerotinia homoeocarpa) and snow mould (Typhula ishikariensis). The disease suppressive effect of compost generally increased with rate of application. Compost inclusion rates of at least 20% (v/v) were normally required to consistently obtain a disease suppressive effect, particularly in peat-based media, but significant disease suppression has been found at lower inclusion rates in soil. Reported levels of disease suppression were variable, even using apparently similar composted materials at the same rates. Sterilisation of composted materials generally resulted in a loss in disease suppressiveness, indicating that the mechanism was often or predominantly biological, although chemical and physical factors have also been implicated. The mechanisms and antagonistic micro-organisms involved in disease suppression are not fully understood. Physiological profiling and the use of DNA-based techniques such as denaturing gradient gel electrophoresis (DGGE) may lead to an improved understanding of the changes in microbial communities associated with disease control resulting from compost amendment of soil, sand or peat. The inoculation of composts with biological control agents may improve the efficacy and reliability of disease control obtained.