Composts are used to enrich soil quality by providing organic matter, supplying nutrients and boosting microbial activity (Rodale 1955; Hoitink and Grebus 1994), all of which contribute to disease suppression.
Microbial composition and maturity of compost both aid in the suppression of plant disease. Other pathogens may need to be suppressed using commercially available biocontrol agents.”
Plants can be strengthened by the nutrients present in compost, which can subsequently reduce disease (Rodale 1955; Menzies and Calhoun 1976). Microbial content plays a major factor in disease suppression, as microbials compete with pathogens for nutrients, or act as antagonists, producing antibiotics that destroy pathogens (Quarles 1993; Quarles and Grossman 1995; Hoitink and Grebus 1994). Disease suppression of composts depends on the microbial composition, the composting process, stability/ maturity of the compost, quantity of available plant nutrients, loading rates, time of application to soil and other factors (de Ceuster and Hoitink 1999). The type of microbes, microbial competition and levels of antagonism are also very important factors (Dinelli 2000; Quarles and Grossman 1995). The type of microbes present in compost can influence its effectiveness in disease suppression, and is one of the indicators of compost quality.
Composted bark is a favorite for container production as a lot of research has gone into its production and it offers numerous benefits to plants. Early experiments showed that composted hardwood bark was suppressive to Phytophthora probably due to microbial activity in the compost (Hoitink et al. 1977). A number of reviews also confirmed the microbial nature of the suppression (Hoitink 1980; Hoitink and Fahy 1986; Hoitink et al. 1991; Hoitink and Grebus 1994; 1997).
Composted bark also suppresses diseases caused by Fusarium, Pythium, Phytophthora, Thielaviopsis, Verticillium and other pathogens (Hoitink 1989; Hoitink and Kuter 1985). The addition of 20% bark compost to potting soil infested with Fusarium oxysporum reduced mortality of carnations planted in the soil (Filippi and Pero 1989). Composted pine bark suppressed root rot caused by Phytophthora spp. in an ornamental planting (Spencer and Benson 1981), but composted hardwood bark was found to be more effective than pine bark (Spencer and Benson 1982).
Despite composted bark being a favorite for container production, yard waste, bio solids and other materials have also been used in composts (Quarles and Grossman 1995). A study to suppress Fusarium wilt of tomatoes grown in pots, used five different composts. These were shown to enhance microbial activity, reduce disease incidence by 44-96%, and reduce mortality by 75-100% (Harender and Kapoor 1997).
Compost matures (becoming finished or stabilized) once it is recolonized by microbials (Hoitink et al. 1997ab; Gotaas 1956; Waksman 1932). This stage occurs after the hot phase, once it has cooled. The maturity of the compost is a factor in disease suppression, as once recolonization has occurred, the compost can be used to suppress Pythium and other pathogens. Composts are most effective at suppressing disease during the window that occurs between recolonization and the development of full maturity. This is because a point will be reached where most of the available nutrients will have been used (Waksman 1932).
There are, of course, different types of compost and these can have varying impacts on disease suppression. Dairy manure is a commonly used material and can be composted by windrow or vermicomposting methods. One study, whereby cucumber flower pots were infected with Fusarium oxysporum (which leads to root and stem rot) demonstrated that disease symptoms were suppressed by compost produced by windrow composting but the vermicompost had no effect (Kannangara et al. 2000).
Composts from kitchen and garden waste can also be effective in preventing plant diseases in the garden. Due to the varying standards of compost production, experimental results are often inconsistent. However, Schueler et al. 1989 indicated that composted household waste in pot experiments effectively suppressed diseases caused by Pythium ultimum and Rhizoctonia solani in a number of plants.
Composts can indeed suppress plant disease due to the nutrients they add to the soil, (which enriches the plants,) and the microbial compositions that help combat disease. Disease suppression by composts often depends on the strains, numbers, and metabolic activity of the microbes present, as well as the compost’s stability and maturity (de Ceuster and Hoitink 1999). Different composts yield different results. The research should be used as a guide in selecting/creating the best compost for suppressing a particular pathogen or protecting a specific plant.
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