Municipal waste and manures will be increasingly used as a source of composting material and have the added potential benefit of suppressing plant diseases. Work at Teagasc Research Centre, Kinsealy, Ireland, in collaboration with University College Dublin, has been looking at ways of producing consistently disease-suppressive composts.
The National Strategy on Biodegradable Waste identifies a large gap between projected biodegradable municipal waste production and the maximum amount of landfill permitted for this waste under the EU Landfill Directive.
Member States have to make use of alternative treatment methods - with the aim of producing usable products from organic waste. Composting is recognised as one of the main methods of reducing the amount of biodegradable municipal waste directed to landfill.
According to Teagasc researcher Michael Gaffney, “Composting organic waste has many benefits over landfill, namely, a reduction in the volume of waste; composting is an aerobic process and therefore produces carbon dioxide, whereas landfill is mainly an anaerobic process producing methane; and, compost has the potential to be used as both a fertiliser and a horticultural growing media.”
However, with the increase in compost production, finding markets for these products is now critical. The potential of composted waste to suppress plant pathogenic diseases has been identified as an area that may encourage the use of these products in horticulture.
Composted organic materials can suppress certain soil borne diseases. For example, oomycete pathogens such as Pythium and Phytophthora are more reliably suppressed than true fungal species such as Fusarium, Rhizoctonia, etc., which are common horticultural diseases. Developing consistent disease-suppressive compost could help growers reduce their costs through a reduction in pesticide application. Also, with the advent of the Sustainable Use of Pesticides Directive, alternative ways of protecting plants from disease is becoming increasingly important.
The disease suppression is due to the presence of bacteria and fungi in the compost feedstock that act as a biocontrol against plant pathogens. They do this by colonising the root surface forming a commensal relationship with the plant and become a ‘barrier’ to pathogen colonisation required for infecting the plant root. However, initial studies found that due to the variability in the feedstocks for the compost (greenwaste, household waste, catering waste, animal waste, etc.), the level of disease suppression was variable.
Further research found that an organism identified as Trichoderma harzianum, possesses the most promising characteristics as a biocontrol agent for inoculating composted waste streams for engineering disease-suppressive composts. Its high temperature tolerance allows this isolate to begin colonising the substrate before other mesophilic species become active. The combination of environmental tolerance and all round suppression against oomycete and fungal pathogens gives this isolate the necessary qualities required for colonising compost of various waste streams. If adopted, this will help to increase the use of composted waste in horticulture, decreasing the quantities of plant protection products and fertilisers needed.