Until the fifties, the principal methods to control soilborne diseases were through the use of organic soil amendments derived from animal and green manures, crop rotation, and, for some crops, steam sterilisation (Baker and Cook, 1974). For the last 50 years, synthetic pesticides have been used to control soilborne diseases. Due to environmental regulations, the use of many of these common pesticides has been prohibited or severely restricted during the last two decades. For several minor crops, resistant varieties are not always available (De Ceuster and Hoitink, 1999).
The last decade a serious part of the waste problems has been solved through composting of source separated wastes. These composts have been used as soil conditioners and fertilisers in agriculture and horticulture and can partly replace peat in container media.
Several composts seem to be disease-suppressive. In preliminary experiments suppression of the soilborne disease Pythium ultimum and to a lesser degree of Rhizoctonia solani was observed after the addition of biowaste and garden waste compost to the potting soil (Ryckeboer et al., 1998).
Disease suppressiveness of compost is a result of a combination of several mechanisms: (1) competition between beneficial organisms and pathogens for nutrients, etc. (Elad and Chet, 1987); (2) antibiosis; (3) parasitism of pathogens by beneficial organisms; and (4) activation of disease-resistance genes in plants. Pathogens such as Pythium spp. and Phytophthora spp. are suppressed as a result of microbiostasis, also called general suppression. During the curing phase of the composting hundreds of microbial species colonise the compost. These organisms metabolise nutrients needed for the development of the pathogens (Boehm et al., 1993). Rhizoctonia spp. are not controlled by general suppression and require the presence of a specific microbial antagonist in the compost. This phenomenon is called specific suppression (Kuter et al., 1983).