Biological diversity suppresses but does not eliminate disease.

National Sustainable Agriculture Information Service
attra.ncat.org/attra-pub/lateblight.html
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Sustainable Management of Soil-Borne Plant Diseases
attra.ncat.org/attra-pub/summaries/soilborne.html
Soil-borne diseases result from a reduction of biodiversity of soil organisms. Restoring beneficial organisms that attack, repel, or otherwise antagonize disease-causing pathogens will render a soil disease-suppressive. Plants growing in disease-suppressive soil resist diseases much better than in soils low in biological diversity. Beneficial organisms can be added directly, or the soil environment made more favorable for them through use of compost and other organic amendments. Compost quality determines its effectiveness at suppressing soil-borne plant diseases...
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