Suppression of Pythium aphanidermatum damping-off in container media containing composted liquorice roots

Pythium damping-off is a major problem in nurseries using peat in container media. Composted liquorice (Glycyrrhiza glabra) roots (CLR) was used as a peat substitute in container media and was suppressive to P. aphanidermatum. CLR and peat media were inoculated with the pathogen and planted with cucumber. The inoculum level was low, so that no seedlings were killed in the first planting. Damping-off occurred in the peat medium during the second and third cycle, but not in the CLR medium. Disease development in the peat medium was highest after three plantings, when 85% disease was recorded, compared with 10% in the CLR medium. CLR therefore was effective in preventing disease build-up. Compost taken from a heap during the thermophilic stage was not suppressive and sterilized compost lost its ability to suppress Pythium damping-off. Apparently the suppression of plant pathogens in compost is due to a biological mechanism developed during the final stage of composting, i.e. during maturation.