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Disease Management

Cultural Controls

The use of cultural control methods to manage root-knot nematodes is the most environmentally sustainable and potentially most successful method for limiting root-knot nematode damage. However, because root-knot nematodes have very large host ranges, cultural control methods require careful planning. Vegetable fields infested with M. hapla can potentially be planted to a nonhost crop such as corn, but the grower's short-term economic return could be diminished. In addition, the grower may have to invest in new equipment or subcontract for some labor. If the grower can identify an alternative nonhost crop with high economic return, crop rotation can be very successful. In contrast, M. incognita on cotton, as discussed earlier (see Symptoms), can usually be managed effectively with crop rotation

Another cultural control strategy is the use of cover crops. Cover crops can be grown outside of the normal agricultural growing season, and some are antagonistic to nematodes. Cover crops such as sudangrass and marigolds actually produce chemicals that are toxic to nematodes. Cover crops have the added benefits of stabilizing topsoil and improving soil quality. As with crop rotation, however, specialized equipment may be required to deal with different cover crops. Other techniques, including flooding and solarization of fields, have controlled nematodes, but only in warm climates and when a particular field can be removed from cultivation for long periods during the treatment. While cultural control methods are extremely valuable tools, they require extensive consideration, planning and economic investment before successful implementation can be achieved.

Chemical Control

Root-knot nematodes are very difficult to manage because they are soilborne pathogens with a wide host range. Because root-knot nematodes live in the soil, chemical control requires applications of large amounts of chemicals with specialized equipment. Fumigants (such as 1,3-dichloropropene, methyl bromide and dazomet) are commonly applied as pre-plant treatments to reduce nematode numbers, but they must thoroughly penetrate large soil volumes to be effective. Some fumigants volatilize very quickly, so treated soil must have a cover or tarp to maintain the fumigant in the soil long enough to be effective.

In addition to broad-spectrum fumigants, nervous system toxins (including oxamyl and fenamiphos) have been shown to be extremely effective for controlling root-knot nematodes (Figure 24). Because both nematodes and humans have nervous systems, chemical treatments that target nematode nerves also are a potential danger to human nerves. These chemicals (carbamates and organophosphates) are extremely toxic to humans and other nontarget organisms. Currently, these chemicals are the most economically feasible control method for root-knot nematodes. Because they are not toxic to plants, they are the only chemical options for established plants.

Figure 24

Plant Resistance

In certain crops, root-knot nematodes are effectively controlled by resistance genes (Figure 4). In tomato, genetic resistance to root-knot nematodes is conferred by the Mi gene which was obtained from Lycopersicon peruvianum, a wild relative of the common tomato. When resistance genes are transferred into susceptible germplasm, the genetically altered plants become resistant to infection by certain species of root-knot nematode. However, populations of root-knot nematodes that can circumvent root-knot resistance have been identified in both the greenhouse and agricultural fields, suggesting the potential for the eventual failure of root-knot resistance. Many other resistance genes have also been identified that are effective against species of Meloidogyne. These include the Mi2 through Mi8 genes (all from Lycopersicon) and the Me and N genes from pepper. In many cases, however, these genes become ineffective at higher temperatures. Besides these genes, there are a number of genes that have not yet been named, and new sources of genetic resistance to root-knot nematodes are frequently being identified.

Figure 4

Biological Control

Control measures employing organisms antagonistic to root-knot nematodes have been attempted by many researchers. The most commonly used biological control agents are fungi and bacteria. There are many kinds of nematophagous (nematode-feeding) fungi, and most use mycelia traps or sticky spores to capture their nematode prey (Figures 25, 26). Bacterial antagonists include species of Pasteuria. These bacteria attach to the cuticle of a juvenile nematode, produce penetration structures that enter the nematode, and slowly consume it. Although various biological controls have been shown to be effective in both greenhouse and field experiments, there have been a number of problems in making these strategies agronomically feasible. The most significant problem in developing effective biological control agents is the inability to economically generate the large amounts of biological material necessary for application over large areas.


Figure 25	Figure 26

Integrated Management

The most successful approaches to nematode control rely on integrated pest management strategies (IPM). IPM combines management options to maintain nematode densities below economic threshold levels. IPM techniques can still be difficult to implement against a pathogen as aggressive and resilient as root-knot nematodes. Nevertheless, a combination of management tactics/tools, including cultural practices (rotations with nonhost crops and cover crops that favor the build-up of nematode antagonists), resistant cultivars, and chemical soil treatments, if necessary, generally provide acceptable control of root-knot nematodes. The extent of this success, however, is dependent upon having accurate damage threshold densities and available and readily acceptable resistant cultivars.