Microbial analysis and test of plant pathogen antagonism of municipal and farm composts

Plant disease suppression induced by compost amendment in some cases results from the activity of antagonistic microorganisms that naturally colonize compost during the curing phase of the composting process. Based on this hypothesis, extracts from seven different mature composts, two municipal and five biodynamic farm composts, were analysed qualitatively and quantitatively with respect to their microbiota involved in plant disease suppression. The role of the microorganisms colonizing these composts was investigated in vitro for their ability to suppress Verticillium longisporum, Rhizoctonia solani, Pythium ultimum and Aphanomyces euteiches. Compost microbes were characterized by some of their functional properties such as production of hydrolytic enzymes that can be of importance for their beneficial effects in plants. The composts differed from each other both qualitatively and quantitatively with respect to their microbial composition. FAME analysis of the compost extracts divided the composts into two groups with the two municipal composts in one group and two of the biodynamic farm composts in the other. Bacteria were found as the dominating group compared with the fungi. Of the total 85 representative strains isolated from different composts, most of the strains exhibited a higher degree of antagonism towards R. solani and P. ultimum than towards V. longisporum and A. euteichus. The majority of the anti-pathogenic strains originated from two types of composts, biodynamic household- and yard waste and biodynamic manure. Cellulolytic ability was positively related to antagonism against Rhizoctonia, and fluorescence was positively related to antagonism against Pythium. Significance of the results is discussed in relation to the antagonistic potential of composts as carriers of beneficial microorganisms.