based on a questionnaire scoring method to determine the weed inducing potential  of introduced  organisms.  Risk assessment is only one tool of many, and will likely have limited utility given that the number of potentially invasive species far outstrips the ability to assess the risk of each one, and high-income countries are better equipped to conduct risk assessments than low-income ones. Full eradication is generally quite difficult to achieve, and requires a significant commitment of resources. Therefore prioritization of IAS management by potential impacts, such as to those that alter fundamental ecosystem processes, and to value of habitats is an important starting point.

6.1.3 Plant root health
The ability to address yield stagnation and declining factor productivity in long-term cropping systems will depend on efforts to better manage root pests and diseases primarily caused by plant-parasitic nematodes and plant-pathogenic fungi (Luc et al., 2005; McDonald and Nicol, 2005). Soil-borne pests and diseases are often difficult to control be­cause symptoms can be hard to diagnose and management options are limited, such as with plant-parasitic nematodes. Nematodes prevent good root system establishment and function, and their damage can diminish crop tolerance to abiotic stress such as seasonal dry spells and heat waves, and competitiveness to weeds (Abawi and Chen, 1998; Ni­col and Ortiz-Monasterio, 2004). With future temperature increase, crops that are grown near their upper thermal limit in areas with high nematode pressure, such as in some cereal systems of South and Central Asia (Padgham et al., 2004; McDonald and Nicol, 2005), could become increas­ingly susceptible to yield loss from nematodes. Approaches for managing soil-borne pests and diseases are changing due to increasing pressure (commercial and environmental) for farmers to move away from conventional broad-spectrum soil fumigants, and greater recognition of the potential to achieve biological root disease suppression through prac­tices that improve overall soil health.

6.1.3.1 Low input options
Soil solarization, heating the surface 5-10 cm of soil by ap­plying a tightly sealed plastic cover, can be a highly effective means of improving root health through killing or immobi­lizing soilborne pests, enhancing subsequent crop root colo­nization by plant-growth promoting bacteria, and increas­ing plant-available nitrogen (Chen et al., 1991). Biofumiga-tion of soils is achieved by the generation of isothiocynate compounds, which are secondary metabolites released from the degradation of fresh Brassica residues in soil. They have a similar mode of action as metamsodium, a common syn­thetic replacement of methyl bromide, and have been used to control a range of soilborne fungal pathogens including Rhizoctonia, Sclerotinia, and Verticillium (Matthiessen and Kirkegaard, 2006). For many plant parasitic nematodes, significant control is often achieved when solarization is combined with biofumigation (Guerrero et al., 2006).
     Soil solarization is an environmentally sustainable alter­native to soil fumigation, though its application is limited to high value crops in hot sunny environments (Stapleton et al., 2000), Soil solarization of nursery seedbeds is an important but underutilized application of this technology, particularly

for transplanted crops in the developing world, where farm­ers contend with high densities of soilborne pests and have few if any control measures. Solarization of rice seedbed soil, which is commonly infested with plant parasitic nema­todes, can improve rice productivity in underperforming rice-wheat rotation areas of South Asia (Banu et al., 2005; Duxbury and Lauren, 2006). This technique has potential for broader application, such as in transplanted vegetable crops in resource-poor settings. Biofumigation using isothi-ocynate-producing Brassicas has reasonably good potential for replacing synthetic soil fumigants, especially when com­bined with solarization. Commercial use of biofumigation is occurring on a limited scale. However, there are signifi­cant hurdles to the broad-scale adoption of Brassica green manures for biofumigation related to its highly variable biological activity under field conditions compared with in vitro tests, and to the logistical considerations involved with fitting Brassicas into different cropping systems and grow­ing environments (Matthiessen and Kirkegaard, 2006). The repeated use of chemical replacements for methyl bromide and biofumigation can lead to a shift in soil microbial com­munities. This shift can result in enhanced microbial biodeg-radation of the control agent, diminishing its effectiveness (Matthiessen and Kirkegaard, 2006).

6.1.3.2 Research needs and options

Biological control. Future nematode biocontrol could be made more effective through shifting the focus from con­trolling the parasite in soil to one of targeting parasite life stages in the host. This could be accomplished through the use of biological enhancement of seeds and transplants with arbuscular mycorrhiza, endophytic bacteria and fungi, and plant-health promoting rhizobacteria, combined with im­proved delivery systems using liquid and solid-state fermen­tation (Sikora and Fernandez, 2005; Sikora et al., 2005). Better biocontrol potential for both nematodes and fungi could also be achieved through linking biocontrol research with molecular biology to understand how colonization by beneficial mutualists affects gene signaling pathways related to induced systemic resistance in the host (Pieterse et al., 2001).

Disease suppression. Understanding the link between cul­tural practices that enhance soil health (crop rotation, con­servation tillage, etc.) and the phenomena of soil disease suppressiveness would aid in developing alternative ap­proaches to chemical soil fumigation, and could enhance appreciation of local and traditional approaches to man­aging soilborne diseases. Soil health indicators are needed that are specifically associated with soilborne disease sup­pression (van Bruggen and Termorshuizen, 2003; Janvier et al., 2007). Given the complex nature of soils, this would necessitate using a holistic, systems approach to develop in­dicators that could be tested across different soil types and cropping systems. Advances in genomics and molecular bi­ology could aid in developing such indicators. Advances in the application of polymerase chain reaction (PCR)-based molecular methods of soil DNA may enable greater under­standing of functional diversity, and relationships between soil microbial communities and root disease suppression