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TECHNICAL BULLETIN Mechanisms of biological control of plant diseases with composts Research data and observations in nurseries have shown that addition of composted organic matter to potting mixes results in suppression of Phytophthora root rots and of some other soil borne diseases. The concentration of suppressive microorganisms in compost-amended substrates is very high but greatly reduced in "worn out" soils or potting mixes at two years after the amendment. As a result, predictive disease suppression models have been developed based on the composition and concentration of microbial biomass. Data is currently being developed to correlate Beta-glucosidase activity with suppressiveness. The concentrations of lignins and of protected cellulose in the substrate predict suppressiveness. The effectiveness of composts in suppression of soil borne diseases is dependent
on heat kill, organic matter decomposition level
(fresh and pyrolized organic material have a negative effect), recolonization of
compost by suppressive microorganisms following
heat kill, and physical and chemical factors. Although previous work has focused
on soil borne diseases, current research indicates
that potting mixes containing composted organic materials which also have been
inoculated with Trichoderma hamatum 382can be
effective as a biocontrol alternative to foliar fungicides; however, the
mechanism of this systemic type of induced resistance is not Although the nursery industry has traditionally relied on aged pine bark and composted biosolids to provide the potential for disease suppression in potting mixes, research indicates that composted animal manures have the potential to replace some of these components but a consistent quantity and quality of these materials will need to be incorporated. The maturity (stability) of the composted manure and its salinity largely determine its ability to induce suppression.
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