One of the biggest changes to world agriculture in the last 30 years has been the development of no-till farming. Various forms of conservation tillage are now applied to many millions of hectares of cropped land, and when combined with practices such as crop rotation and cover cropping, it has resulted in farming systems that are much more profitable and sustainable than they were in the past. One of the benefits from this change will be an increase in the suppressiveness of soils to soilborne disease (Stone et al. 2004).
Given the economics of broad-scale agriculture and the cropping area involved, enhancing general suppressiveness through the farming system is probably the only realistic way of improving the level of biological control in most of the world's agricultural land. The role of farming systems in enhancing suppressiveness should therefore be a major focus of future research. We need to know how the main soil management practices available to farmers (e.g. tillage, fallowing, rotation crops, cover cropping and organic inputs from crop residues and amendments) influence the physical, chemical and biological properties of soil and in turn affect the development of suppressiveness to various pathogens, including nematodes.
A comprehensive review by Wardle (1995) demonstrates that tillage practices have a major impact on the detritus food web and could therefore be expected to affect the processes that regulate populations of plant-parasitic nematodes. The quantity of C and N (the resource base for the detritus food web) is usually lower under conventional tillage than no-tillage, microbial biomass and the ratio of micro-bial biomass to organic C tends to decline when soil is tilled and bacteria tend to be favoured over fungi. The larger soil organisms (predatory and omnivorous nema-todes, springtails and mites) are particularly vulnerable to tillage and all tend to respond positively when tillage is reduced. Given that fungi, predatory nematodes and microarthropods are the main predators of nematodes and tillage is detrimental to all of them, a move from conventional to minimum tillage could be expected to enhance the general suppressiveness of soil to plant-parasitic nematodes. Observations on cereals and sugarcane in Australia (Rovira 1990; Stirling 2008) and results from long-term tillage experiments with soybean in the USA (Westphal et al. 2008; Seyb et al. 2008) indicate that populations of several plant-parasitic nematodes are lower in soils under minimum tillage than in cultivated soils. Although this effect is not necessarily due entirely to enhanced suppressiveness, detailed ecological studies of these and other no-till systems are clearly warranted.
In addition to reducing the frequency and intensity of tillage, practices such as crop rotation, cover cropping, more careful residue management and greater organic inputs from amendments can also be used by farmers to improve levels of soil organic matter and thereby influence the biological status of soil and its general suppressiveness to nematodes. The role of organic matter in enhancing suppressive-ness is discussed in the following section, but from a farmer's perspective, the challenge is to integrate these practices into a farming system that is profitable and sustainable. The way this is done will depend on factors such as climate, soil type and the principal crops involved, but results of a recent research program in Australia provide an example of what is achievable.
In the early 1990s, the Australian sugar industry was facing an uncertain future because productivity was declining due to a problem known as yield decline. At that time, sugarcane was grown on beds 1.5 m apart, machinery wheel spacings did not match crop row spacings and the crop residues remaining after harvest were often burnt rather than retained. After a plant and 2-4 ratoon crops, an expensive program of ripping and cultivation was required to remove the old crop, alleviate compaction caused by farm machinery and then replant the field to sugarcane. A multidisciplinary research team was established to develop solutions to the problem and its initial studies showed that soils under long-term sugarcane monoculture were physically and chemically degraded, while large yield responses to soil fumigation and nematicides indicated that biological constraints were also limiting productivity. A 12-year research program (summarised by Garside et al. 2005; Stirling 2008) resulted in the development of a new farming system based on residue retention, minimum tillage, a leguminous rotation crop and controlled traffic using global positioning system guidance. This system is now being adopted by growers because it increases sugar yields, reduces costs, improves soil health and provides additional income from rotation crops such as soybean and peanut. From a nematological perspective, losses from P. zeae and M. javanica have been reduced because (1) the introduction of a rotation crop has reduced nematode population densities at planting, (2) damage thresholds have increased as soil health has improved and (3) suppressive mechanisms of biological control are now operating more effectively.
Economic pressures and the entrenched attitude of some growers will always make it difficult to make major changes to a farming system. However, the fact that the Australian sugar industry was able to make such a change and in the process overcome obstacles that were initially perceived as insurmountable, indicates that the task is achievable. Reducing losses from nematodes and other soil-borne pathogens may not be the primary reason for embarking on such a process, but is likely to be one of the outcomes.
Globally, the farming system that is perhaps in most need of urgent attention from a farming systems perspective is the plasticulture system commonly used for vegetable production. In many countries, vegetable crops are grown intensively on beds mulched with plastic film; water, nutrients and pesticides are delivered to soil via trickle irrigation tubing; double or multiple cropping is common; soil is bare-fallowed between crops; there is limited crop rotation; organic inputs from cover crops and amendments are rare; and soil is routinely fumigated. This farming system treats the soil as an inert medium to support the plant, and in the absence of any biological buffering, it is not surprising that root-knot nematode and other soil-borne pathogens re-establish following fumigation and quickly build up to high population densities (Desaeger and Csinos 2006). It is therefore disappointing that over the last decade or so, much of the money allocated to finding alternatives to methyl bromide was spent on testing alternative fumigants rather than on developing more sustainable vegetable farming systems. There are production systems that warrant further testing (e.g. Stirling 2008; Stirling and Eden 2008; Bhan et al. 2010), but until the vegetable industry is prepared to take a long-term view, invest in research on alternative farming systems and then persist with those alternatives for 5-10 years, the status quo will remain.
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