Sustained Suppression of Pythium Diseases: Interactions between Compost Maturity and Nutritional Requirements of Biocontrol Agents

Several procedures were developed that predict maturity (stability) of composts prepared from municipal solid wastes (MSW). A respirometry procedure, based O2 uptake by compost, predicted (R2=0.90) the growth response of ryegrass in composts and an acceptable level of maturity. Spectroscopic methods (CPMAS13-NMR and DRIFT spectroscopy) showed that the stabilizing compost contained increasing levels of aromatic structures. All procedures predicted acceptable plant growth after approximately 110 days of composting. MSW compost suppressed diseases caused by a broad spectrum of plant pathogens including Rhizoctonia solani, Pythium aphanidermatum and Fusarium oxysporum. A strain of Pantoea agglomerans was identified that caused lysis of hyphae of R. solani. Evidence was obtained, suggesting that thermophilic biocontrol agents also might play a role in suppression. 13C-NMR spectra revealed that the longevity of the suppressive effect against Pythium root rot was determined by the concentration of readily biodegradable carbohydrate in the substrate, mostly present as cellulose. Bacterial species capable of inducing biocontrol were replaced by those not effective as suppression was lost. The rate of uptake of 14C-acetate into microbial biomass in the conducive substrate was not significantly different from that in the suppressive substrate although specific activity was higher. The suppressive composts induced systemic acquired resistance in cucumjber roots to Pythium root rot and to anthracnose in the foliage. Composts also increased peroxidase activity in plants by the conducive substrate did not have these effects. In summary, the composition of the organic fraction determined bacterial species composition and activity in the substrate, which in turn regulated plant gene expression relative to biological control.