Suppression of Pythium ultimum by biowaste composts in relation to compostmicrobial biomass, activity and content of phenolic compounds

Seventeen composts from separately collected organic household waste plus o ne bark compost and one compost from grape marc were analysed for suppressi on of Pythium ultimum, phytotoxicity, microbial biomass and activity, subst rate-induced respiration, extractible phenolic compounds and other physical and chemical parameters. Nine of the samples were mildly suppressive to P. ultimum, the others were conducive. The bark compost sample was strongly s uppressive. Therefore of the examined composts, only the bark could be used to exert an economically relevant control of P. ultimum in horticultural m edia. A large part of the compost samples was slightly phytotoxic. Microbia l biomass and SIR had only weak correlations with disease incidence. Microb ial activity and content of extractible phenolics were positively correlate d with disease incidence. None of the tested parameters were therefore suit able as a predictive test for suppression of P. ultimum With the compost sa mples used in this study.