favorable conditions for disease development, can negate beneficial effects of amendments. Also, there are trade-offs: Papavizas ( 1966) found that cruciferous amendments reduced Aphanomyces root rot but increased Rhizoctonia root rot on pea.
Soil and environmental factors profoundly affect decomposition and effectiveness of organic amendments in disease control ( Lumsden et al. 1983b). Soil types differ in chemical and physical properties and in biological composition. Among a multitude of factors, temperature, moisture, and aeration critically affect the decomposition process, microbial activity, and the by-products that are formed.
Organic amendments consist of plant residues, green manure crops, constituents of plant or animal residues, animal manures, composts, and formulations. Organic amendments affect pathogens as a direct food source, stimulate antagonistic organisms, and improve the physical properties of soil and nutrient status of the crop, which can enhance disease resistance. The succession of organisms that utilize a substrate during decomposition affect the activity, survival, and population of pathogens and disease suppression through antibiosis and lysis, competition, parasitism, or predation. Populations of soil bacteria (including actinomycetes) and fungi are affected by the type and maturity of the amendment, kinds and amounts of decomposition products, proportion of available nutrients in relation to resistant components, carbon to nitrogen ratio of soil, and the physical and chemical environment.
Considerable evidence has accumulated to show that application of organic amendments has potential, and is a feasible approach, to control plant diseases. However, the effects are usually indirect. That is, amendments affect plants and soil microorganisms that in turn affect pathogens and disease. Moreover, certain combinations of host and pathogen behave differently in response to organic amendments and must be studied individually to arrive at practical disease control measures. Part of the difficulty in making recommendations is that more information is needed to elucidate factors that affect disease development in specific host-pathogen combinations in relation to organic amendments, as well as factors that affect the ecology of soil microorganisms. Current information suggests that complete control of any disease depends on a combination of environmental and biological inputs, of which organic amendments is but one. It is imperative to understand how organic amendments alter the physical, chemical, and biological composition of soil so that optimal plant productivity and consistent, effective control of soil-borne plant pathogens can be attained.
Adams, P. B. 1971. "Effect of soil temperature and soil amendments on Thielaviopsis root rot of sesame". Phytopathology 61:93-97.