The role of organic matter in enhancing suppression of soilborne diseases caused by fungi, Oomycetes, bacteria and nematodes has been known for many years and there are now well-documented examples in many quite different agricultural systems. These include suppression of Pythium in Mexican fields following the application of large quantities of organic matter over many years (Lumsden et al. 1987); broad-spectrum control of Pythium, Phytophthora and Rhizoctonia in peat and compost-based soilless container media (Hoitink and Boehm 1999); the use of cover crops, organic amendments and mulches to suppress Phytophthora root rot of avocado in Australia (Broadbent and Baker 1974; Malajczuk 1983; You and Sivasithamparan 1994, 1995); suppression of the same disease with eucalyptus mulch in California, USA (Downer et al. 2001); the management of a fungal, bacterial and nematode-induced root disease complex of potato in Canada with chicken, swine and cattle manures (Conn and Lazarovits 1999; Lazarovits et al. 1999, 2001), and the use of crop residues, animal manures and organic waste materials to reduce damage caused by plant-parasitic nematodes (reviewed by Muller and Gooch 1982; Stirling 1991; Akhtar and Malik 2000; Oka 2010).
It is obvious from the above examples that a wide range of types and sources of organic matter can be used to enhance suppressiveness and that they are effective in many different situations. However, studies (summarised by Hoitink and Boehm
1999 and Stone et al. 2004) in relatively simple nursery potting media have given us a much better understanding of the mechanisms involved. Suppression is generated soon after an amendment is added to soil and is associated with the activity of indigenous microorganisms that colonise organic material during the decomposition process. Development of suppression is associated with high levels of micro-bial activity, with many studies showing that the rate of hydrolysis of fluorescein diacetate (FDA) is a relatively good indicator of suppressiveness. Since microbial activity must remain high to maintain suppressiveness, the quantity and quality of the organic inputs have a major impact on the duration of suppressiveness. The labile constituents of organic matter (e.g. sugars, proteins and hemicelluloses) are degraded relatively quickly and suppression is then sustained by the subsequent decomposition of more recalcitrant materials in the coarse and mid-sized particulate fraction (Stone et al. 2001).
Perhaps the most important feature of organic-matter mediated general suppression is its capacity to act against most, if not all, major soilborne pathogens of food and fibre crops. Since root disease problems in the field rarely involve a single pathogen, enhancing the suppressive potential of a soil with organic matter is one of the only non-chemical techniques available to control a suite of pathogens. This does not mean that manipulating organic matter to manage several pathogens is a simple matter. When pathogens which are good primary saprophytes but poor competitors are involved (e.g. Pythium and Fusarium), the fact that they may multiply on fresh organic matter before being suppressed must be taken into account when designing application strategies. In the case of Rhizoctonia, which has a high competitive saprophytic ability due to its capacity to degrade cellulose as well as simple sugars, organic-matter mediated general suppression is often insufficient to achieve control and specific antagonists may also be required (Stone et al. 2004).
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