Organic amendments as a disease control strategy for soilborne diseases of high-value agricultural crops

Manures and by-products derived from the processing of plants and animals have been used for centuries as sources of fertiliser, but beneficial or detrimental effects on plant diseases were never thoroughly investigated. We found that certain organic amendments controlled a variety of soilborne diseases of potato (including common scab and verticillium wilt), various pests (including plant parasitic nematodes) and weeds at nine field locations in Ontario and Prince Edward Island, Canada. The mechanism of disease control for high-nitrogen-containing amendments is the generation of ammonia and / or nitrous acid following degradation of the amendments by microorganisms. The formation of these products to concentrations lethal to pathogens is regulated by the soil pH, organic matter content, nitrification rate, sand content and buffering capacity. Liquid swine manure reduced scab and wilt, but at only three of six locations tested. In acidic soils, swine manure killed Verticillium dahliae within a day after application, but had no effect in neutral or alkaline soils. The toxic components in the manure were identified as volatile fatty acids. Ammonium lignosulfonate reduced scab at all six sites tested. The mechanism of disease control is not yet known. Although many of these amendments reduced pathogen populations, total soil microorganism numbers increased by 10- to 1000-fold after application, indicating that not all organisms were killed. Understanding the mode of action of amendments is essential for the improvement of their effectiveness and assimilation into crop production systems. If costs can be decreased and benefits ensured, organic amendments will have a major role in reducing plant diseases. The discussion on use of these amendments is based on cost/benefit analysis as well as societal and regulatory considerations.