In recent years, worldwide interest has developed amongst scientists and commercial plant producers in the relationship between plant disease suppression and use of compost-amended growth substrates in horticultural production, particularly containerized ornamental plant production systems. The use of compost-amended potting mixes can result in a drastic reduction in disease symptoms despite the presence of plant pathogens (e.g., Pythium and Rhizoctonia spp.). Plant disease suppression has been found in many cases to be due to the activity of microorganisms, many of which have been identified. Some of these beneficial microorganisms or biocontrol agents are available commercially, marketed under various categories including biopesticides, organic amendments, and fertilizers. As living organisms, these biocontrol agents likely have optimal environmental conditions (temperature, moisture, pH, nutrition, etc.) under which they are effective in suppressing plant disease.
A major factor in the efficacy of beneficial microorganisms is the nutritional substrate available. Composted organic materials, already recognized as having value as potting mix amendments, can also serve as the nutrient base for beneficial microorganisms. A significant portion of my laboratory's research currently involves surveying a variety of potting mixes used commercially in California for horticultural production for naturally occurring plant disease suppressiveness. We are also testing several commercially available biocontrol agents for efficacy against plant pathogens in various compost-amended potting mix formulations. We have developed quick, short-term greenhouse bioassays for screening large numbers of container media, with and without commercial biocontrol agent fortification. These quick bioassays are also useful for determining consistency amongst batches of one type of potting mix. Longer term studies are also planned, to examine storage-life of disease suppressive effects, as well as efficacy during production of longer-term crops (e.g., 12-14 month woody crops). The goal of these disease suppression bioassays is to make information available to the horticultural industry that will offer growers new options for plant disease control as well as provide them, the compost end-users, with encouragement to use and demand high-quality compost amendments, for reasons of both organic waste management and plant disease management.