Understanding the mode of action of organic soil amendments provides the way for improved management of soilborne plant pathogens

Organic amendments such as meat and bone or soybean meal, poultry or liquid swine manure, and fish emulsion significantly reduced disease in some soils tested, but had no effect in others, and sometimes increased disease severity. To understand this apparent variability we investigated the modes of action of such amendments. Depending upon the type and amount of amendment used and the characteristics of the soil, the amendments killed microsclerotia of Verticillium dahliae by ammonia, nitrous acid, and/or volatile fatty acid toxicity; and/or reduced microsclerotia survival or disease by unknown mechanisms, possibly by stimulating biological control. The most important factor affecting disease control efficacy was soil pH as the toxic products responsible for killing pathogens exist only when the soil pH was below 6 or above 8. From an understanding of the modes of action, amendments can be formulated to improve efficacy and reduce variability. This includes basing application rates upon soil properties, manipulating amendment and/or soil pH, altering the rates of nitrification and organic matter contents, screening of amendments for appropriate chemistries, improving storage conditions, altering their microbiology, and improving the timing and mode of application.