Composting of agricultural wastes for their use as container media: effect of heat treatments on suppression of Pythium aphanidermatum and microbial activities in substrates containing compost

Composted separated manure (CSM) and composted grape marc (CGM) were found to suppress Pythium damping off. This effect was negated by autoclaving the medium, but was restored when compost that had not been autoclaved was mixed with the sterile one. Heat treatment (55°C for 2 h) did not affect suppression even after two growth cycles: in fact, suppression of Pythium in compost and peat mixtures was improved by heating. It is suggested that microorganisms in CSM are more thermostable than peat microflora. Fungal and bacterial counts as well as measurements of biological activities were performed. The CSM microflora was more active than the peat population in terms of fluorescein diacetate hydrolysis and CO₂ evolution. Both activities were less sensitive to 55°C heat treatments in CSM than in peat, but as sensitive as in peat to 80 and 100°C heat treatments. The effect of incubation on the activities was studied and different patterns were observed for CSM and peat. The role of microbial activity in suppression of Pythium is discussed.