SUPPRESSION OF RHIZOCTONIA-SOLANI IN POTTING MIXTURES AMENDED WITH COMPOST MADE FROM ORGANIC HOUSEHOLD WASTE

Compost made from organic household and garden waste was used to subst itute part of the pear in potting mixtures used for growing woody orna mental nursery stock. The effects of amendment with compost on the col onization of potting mixture by Rhizoctonia solani (AG1) were studied in greenhouse experiments. A bioassay was developed using cucumber as a sensitive herbaceous test plant as a substitute for woody ornamental cuttings. Pathogen growth in the pelting mixture was estimated by mea suring the distance over which damping-off of seedlings occurred. Comp ost from two commercial composting facilities suppressed growth of R. solani in potting mixtures with 20% of the product when the compost wa s fresh (directly after delivery) or long matured (after 5 to 7 months of additional curing). In contrast, short-matured compost (1 month of additional curing) from the same batches stimulated pathogen growth. In vitro mycelial growth of R. solani on mixtures with mature compost was inhibited by microbial antagonism. Compost-amended potting mixture s responded differentially to the addition of cellulose powder; the ef fect on suppressiveness depended on curing time and origin of the comp ost. In long-matured compost, suppressiveness to R. solani was associa ted with high population densities of cellulolytic and oligotrophic ac tinomycetes. The ratio of the population density of actinomycetes to t hat of other bacteria was around 200-fold higher in mature suppressive compost than in conducive compost.