Methods To Reduce Nematode Populations

A general overview of some nematode management methods which reduce nematode populations in soil, such as crop rotation, antagonistic crops, resistant cultivars, soil solarization, biofumigation and nematicides is provided with especial reference to those that could be used in appropriate combination with biological control in an integrated nematode pest management strategy to improve the effectiviness of biological agents. Excellent books have been published that have been devoted to integrated nematode management (e.g. Barker, Pederson, & Windham, 1998; Whitehead, 1998; Luc, Sikora, & Bridge, 2005; Perry & Moens, 2006), and should be consulted for guidelines in the structuring of integrated management programmes.

2.1. Crop Rotation

Seasonal rotations of susceptible crops with non-host or poor-host crops on the same area of land remain one of the most important techniques used for nematode management worldwide. The occurrence of nematode communities containing multiple pest or polyphagous species with wide host ranges, such as some species of Meloidogyne, limits the potential of using acceptable non-host crops for rotation (Viaene, Coyne, & Kerry, 2006). Hence, it is necessary to determine the host status of individual crop cultivars for local nematode populations before a rotation scheme is recommended for a particular field. Rotations using poor hosts or tolerant crops together with highly susceptible vegetable crops have been used for control of root-knot nematodes in tropical condition (Stefanova & Fernández, 1995; Gómez & Rodríguez, 2005). However, crop rotations have economic costs for the grower. In the past 20 years in the UK, the number of farmers producing potato crops has declined by 80% to around 5,500 individuals but the cropped area has remained relatively unchanged. Those specialist growers remaining have invested heavily in chilled storage facilities and machinery and must grow potatoes intensively to obtain a return on their investment. As a consequence, potatoes are grown on average every 6 years instead of the 9 year rotation recommended and potato cyst nematodes continue to spread despite the use of nematicides. Devine, Dunne, O'Gara, and Jones (1999) first recorded the effects of microbes on the decline of potato cyst nematode populations between potato crops and estimated it at only 10% with most egg loss resulting from their spontaneous hatch.

Use of witchgrass in a peanut rotation has beneficial effects on soil, reducing parasitic nematode populations and increasing numbers of free-living nematodes, and also causing shifts in rhizosphere microbial ecology (Kokalis-Burelle, Mahaffee, Rodríguez-Kabana, Kloepper, & Bowen, 2002). Some bacteria and fungi that affect the development of nematodes are dependent on specific plants to support their endophytic development or growth in th rhizosphere and so can only be used in certain crop rotations. Similarly, rotation crops, such as beans, maize and cabbage that support extensive growth of the nematophagous fungus, Pochonia chlamydosporia in their rhizospheres but support only limited reproduction of root-knot nematodes, are used to maintain the abundance of the fungus in soil (Table 1) whilst suppressing populations of the nematode (Puertas & Hidalgo-Díaz, 2007). Hence, growing an approved crop in the rotation to maintain populations of natural enemies on roots is another alternative to improve the efficacy of nematode management programmes based on crop rotations (Fig. 1). For obligate parasites such as the bacterium Pasteuria penetrans, it is essential that it is introduced into the soil with a nematode susceptible crop, which will provide developing nematodes on which the bacterium will multiply (Oostendorp, Dickson, & Mitchell, 1991). Timper et al. (2001) demonstrated in rotations of peanuts with 2 years of bahiagrass, cotton or corn, in a field naturally infested with M. arenaria and P. penetrans that the abundance of the bacterium was related to the population densities of the nematode and were greatest under continuous peanut cropping and next most abundant under the bahiagrass-peanut rotation.

Marigolds For Biocontrol Nematode

Figure 1. Changes in abundance of Pochonia chlamydosporia in soil from September, 2003 until February, 2006 under different vegetable crops treated with two applications of the fungus in a field trial in Cuba. The fungus was applied on colonised rice or as a suspension of chlamydospores at a rate of 5,000 chlamydospores g-1 soil.

Figure 1. Changes in abundance of Pochonia chlamydosporia in soil from September, 2003 until February, 2006 under different vegetable crops treated with two applications of the fungus in a field trial in Cuba. The fungus was applied on colonised rice or as a suspension of chlamydospores at a rate of 5,000 chlamydospores g-1 soil.

2.2. Antagonistic Crops

Plants antagonistic to nematodes are those that are considered to produce toxic substances, usually, while the crops are growing or after incorporation into the soil. In practical nematode management strategies the use of this approach relies on pre-plant cover crops, intercropping or green manures.

Marigold, neem, sunn hemp, castorbean, partridge pea, asparagus, rape seed and sesame have been extensively studied and used as antagonistic crops for nematode control. Sunn hemp (Crotalaria spp.) is often cultivated as a cover crop for direct seeding, intercrops or soil amendment and is considered an antagonistic crop for most plant parasitic nematodes, especially root-knot nematodes (Wang, Sipes, & Schmitt, 2002). Population densities of M. incognita were affected by previous cover crops of C. juncea in north Florida (Wang, Mc Sorley, & Gallaher, 2004). Germani and Plenchette (2004), recommend the use of some Crotalaria spp. from Senegal as pre-crops for providing green manure while at the same time decreasing the level of root-knot nematode and increasing the level of beneficial mycorrhizal fungi.

Marigolds (Tagetes spp.) have been shown to suppress plant parasitic nematodes, such as root-lesion and root knot nematodes. Kimpinski, Arsenault, Gallant, and Sanderson (2000) demonstrated consistent reduction of Pratylenchus penetrans populations when marigolds were used as a cover crop followed by potato crops, with a significantly higher average yield. In Japan, where the continuous cropping of vegetables has led to nematodes (P. coffeae and M. incognita) becoming a major problem, a practical method using marigold has been developed, which requires only one season to incorporate these plants with only minor changes in the cropping system (Yamada, 2001). Biofumigation using fresh marigold as an amendment is used effectively in root knot management in the protected cultivation of vegetables in Morocco (Sikora, Bridge, & Starr, 2005).

Most antagonistic plants cultivated as pre-plant cover crops may be followed by soil incorporation of the biomass with a subsequent reduction of plant-parasitic nematode numbers and the enhancement of nematode antagonists (see Section 3.1). However, it should be noted that grower acceptance of new strategies using antagonistic plants are based on economic and logistical considerations, as well as efficacy. Too often the large amounts of biomass required restrict the use of the approach to cheap sources of local species/waste products. The value of these products may be enhanced by using them as media on which to culture nematophagous microbial agents either prior to or after their addition to soil. Although some empirical tests have been made, the combined use of antagonistic plants and biological control agents has been little studied.

2.3. Resistant Cultivars

Host plant resistance is currently the most effective and environmentally safe tactic for nematode management (Koenning, Barker, & Bowman, 2001; Castagnone-Sereno, 2002). When it is available in a high-yielding cultivar, it should be the foundation upon which other management measures build (Sikora et al., 2005), because resistance is highly specific, being effective against only a single species or

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