Mechanisms
of biological control of plant diseases with composts
Dr. Harry Hoitink
The Ohio State University
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 predicts 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 yet understood.
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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