Management of soil-borne plant pathogens with organic soil amendments: a disease control strategy salvaged from the past

The utilization of organic amendments for control of soil-borne plant pathogens has often been considered at best variable, but more often as a snake-oil remedy. Understanding the mode of action of these products, however, provides insight as to where and how to exploit these underutilized energy sources for the benefit of plant and soil health. Organic amendments containing high nitrogen, such as poultry manure, meat and bone meal, and soymeal, significantly reduced populations of a wide spectrum of soil-borne plant pathogens. Pathogen control was shown to arise from the ammonia and (or) nitrous acid generated, the concentrations of which are controlled by pH, organic matter content, soil buffering capacity, and nitrification rate. Swine manure can reduce pathogen populations by both these mechanisms as well as by an additional process involving volatile fatty acids. Volatile fatty acids are active only at pH conditions below 6.0 and were present in most, but not all liquid manures tested. Disease control with organic amendment occurs when soil and biological factors are conducive to activating the processes that reduce pathogen survival. Understanding the mechanisms allows prediction of efficacy based on analysis of the soil and the organic amendment. For some products, such as ammonium lignosulfonate, a by-product of the pulp and paper industry, disease control was demonstrated but the mode of action is as yet unclear. Although organic amendments reduce populations of plant pathogens, overall they lead to an increase in soil microorganisms populations by up to 1000-fold following application. Thus, the displacement of pathogens is selective and can persist in fields for several years after single application. As we unravel the potential benefits of organic treatments, there is indication that they may return as a valuable tool in disease management strategies.