Altering Community Balance:
Organic Amendments, Selection
Pressures, and Biocontrol
Carol E. Windels
Cultural practices, including addition of organic amendments to soil, augment naturally occurring biological control of plant pathogens ( Cook and Baker 1983). Numerous reviews have summarized the beneficial effects of organic amendments on plant growth and disease control ( Baker 1991; Baker and Snyder 1965; Bruehl 1975; Cook and Baker 1983; Hoitink and Fahy 1986; Huber and Graham 1992; Jarvis 1992; Linderman 1989; Lumsden et al. 1983b).
In the 1950s and 1960s, considerable research was devoted to establishing principles to explain how crop residues decreased or increased incidence of plant diseases ( Lumsden et al. 1983b). Attempts to evaluate amendments in the field that showed potential for reducing disease incidence under controlled conditions in the laboratory or greenhouse, however, often were disappointing. In recent years, concerns about a narrowing arsenal of pesticides, safety of pesticide use, and greater public awareness in conserving the environment have provided new impetus for exploring biological control of plant pests.
Addition of organic matter to soil is one of the most effective ways to change the soil environment and to favor an increase in populations of beneficial soil organisms. Organic materials improve physical properties of soil--aggregate stability, bulk density, porosity, organic matter content, and moisture-holding capacity--that enhance root respiration and growth ( Allison 1973). Amendments also are a source of inorganic nutrients to plants and microbes, which, in turn, contribute to soil nutrient availability and soil aggregation. Thus, alterations in physical and chemical properties of soil, as well as in composition of the biological community, directly and indirectly affect the plant and its health.
Soil-borne fungal pathogens are associated with the top few decimeters of soil and thus are potentially amenable to being affected by soil amendments. Recent ev-