Well-prepared composts exert numerous beneficial effects on soil physical characteristics and contribute to plant nutrition. During the last four decades, their potential direct benefits on plant health have also became evident, and disease suppression has been identified against fungal, oomycete and bacterial diseases, as well as against some nematodes. However, substantial variability among commercial composts has called into question the reproducibility and practicality of the use of compost as a suppressive agent against soil-borne diseases (SBD). Only when commercial producers adhered to clear professional recommendations were the results of large-scale operations similar to those of scientific experiments. The main causal agents of suppressiveness 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 largely responsible for this phenomenon. Residual abiotic activity results from fungistatic compounds present in some compost types. Several mechanisms that are described here were proposed to explain the suppressive effect of composts on SBDs. Some of these mechanisms, such as competition for nutrients and colonization sites, hyperparasitism, antibiosis, induced systemic resistance and systemic acquired resistance, are well documented and proven. The last two mechanisms also affect some above-ground (e.g. foliar) diseases. Several other proposed mechanisms were validated only occasionally. Examples of these mechanisms are induction of futile spore germination, in the absence of host, and adsorption of root signals on active surfaces, and their eventual decomposition by soil microorganisms. In some cases, a joint activity of two or more mechanisms was suggested, which strengthened the suppressiveness capacity of some composts.
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