Suppressive Effects of Composted Yard Wastes against Soil Borne Plant Diseases in Organic Horticulture

摘要

The suppressive effects of yard waste-(YWC), biogenic waste-(BWC) and cattle manure compost (CMC)-amended growing media against Pythium and Phytophthora root rots were compared against peat mixes. Inoculation of sterilised sand or natural YWC- and BWC-amended growing media with Pythium ultimum resulted in significantly increased fresh weight yields and a reduced disease incidence of 30 to 50 % on several hosts. CMC showed such suppressive effects only occasionally. For these reasons, YWC produced in commercial composting plants according to a fixed system was amended up to 40 % (v/v) with peat of german or baltic origin by organic horticulturists involved in the project. Suppression of several pathogens (P. ultimum, Phytophthora spp.) by YWC-amended media was observed in both standard bioassays and under commercial conditions on Euphorbia pulcherima, Pelargonium peltatum and Chamaecyparis lawsonianae. Reduction of disease incidence ranged from 30 to 80 % when the disease incidence in the controlpeat mixes was 70 to 90 %. The suppressive effects of the composts were mainly related to biological factors as demonstrated by gammairradiation of composts, higher counts of 1) antagonistic bacteria, 2) cellulose degrading fungi in YWC and BWC than in CMC and 3) fungistatic effects produced by YWC and BWC. There was no correlation between the suppression of P. ultimum and microbial activity on the three composts. However, specific activity (unit microbial activity per unit microbial biomass) did correlate to suppressive effects (r = 0,742). YWC and BWC had high specific activities whereas the specific activity of CMC was low. Microbial communities in YWC and BWC seemed to have a greater competitive potential for easily available substrates, i.e. rootexudates than those in CMC. In conclusion these composts provided biocontrol of these diseases in growing media used in organic horticulture.