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Disease Management

Management of R. similis in the tropics and subtropics is complicated by several factors. First, the nematode’s most important agricultural hosts are perennial crops, making eradication or reduction of nematode populations difficult and expensive. In addition, subsistence agriculture is practiced in most developing countries in these latitudes so fallowing, crop rotation, clean planting material, or nematicidal chemicals may not be practical, affordable, or attainable. Finally, economic thresholds (nematode population at or above which crop loss would cost more than management efforts) for many of these crops have not been established. As a result, crop damage and high nematode populations may occur before the problem is addressed. Nevertheless, there are actions available to exclude, eradicate and avoid R. similis, or to protect crops growing in infested soils.

Exclusion

Efforts between governments to exclude plant pathogens by quarantine are essential, but expensive and often defeated. Radopholus similis is present internationally in most areas where its host plants are grown, though some races are not widespread. For example, rooted citrus from Florida must currently be certified free of burrowing nematode (citrus race) before it can be accepted for import and distribution within the European Union (EU) or other citrus-growing states like California. However, the citrus race is not morphologically or genetically distinct, so all R. similis races are quarantined by the EU. Local contamination of clean fields and greenhouses can be prevented by using nematode-free planting material (Figure 17) and controlling the movement of infested soil into new areas on tools, machinery, humans, or in soil or surface waters. Nurseries must use nematode-free planting material and maintain strict sanitary practices, including disinfesting pots and benches (Figure 18).

Figure 17

Figure 18

Eradication

Destroying all burrowing nematodes in an area is theoretically possible, but practically difficult. Eradication requires removal of all belowground parts of the host crop, volunteers, alternative hosts, and all weed hosts: R. similis does not form survival structures and will eventually starve in the soil. The “push and treat” method for nematode control in citrus begins with bulldozing and removal of trees, roots and all, followed by nematicide and herbicide applications to reduce or eliminate host plant and nematode population growth. A restriction on the use of nematicides and their degradation by soil microorganisms, however, has reduced the effectiveness of this approach. Host removal is followed by either a clean fallow of 12 months, rotation with non-host crops, or planting a nematode-suppressive cover crop (e.g. Crotalaria). An efficient method of crop destruction in banana plantations is to inject pseudostems with the herbicide glyphosate. This causes roots and rhizomes to die and rot without the need to dig them up. Flooding the field for 8 weeks is effective but expensive and seldom practiced.

Other nematode species such as root-knot (Meloidogyne), lesion (Pratylenchus), citrus (Tylenchulus semipenetrans), or spiral (Helicotylenchus) nematode often occur with R. similis and their populations may increase if R. similis is eliminated. See other disease lessons on nematodes.

Avoidance

Damage by R. similis can be avoided by planting disease-free crops in uninfested soil or a clean, soilless potting medium. Injury may be minimized in previously infested fields by planting after the eradication procedures mentioned above (e.g. fallow, crop rotation) have reduced nematode populations in the soil.

Protection

Cultural practices that maintain plant health and vigor may improve yield by allowing plants to rapidly replace roots damaged by R. similis. Incorporating organic material into the soil provides nutrients and improves its cation-exchange and water-holding capacity, which may also increase the number of beneficial microorganisms and nematode antagonists in the root zone. Some cover crops, like sunnhemp (Crotalaria) or marigold (Tagetes), produce chemicals that are toxic to nematodes (allelopathy) and may reduce burrowing nematode populations in the soil.

Soil fumigation with methyl bromide, 1,3 dichloropropene, or chloropicrin can provide good control of burrowing nematodes but these fumigants must be applied pre-plant. Nonfumigant nematicides such as aldicarb and phenamiphos can be used post-plant and usually give systemic plant protection and reasonable control of burrowing nematodes. Chemical control of nematodes in general is decreasing in the U.S., however, due to regulatory restrictions, costs, and environmental concerns. Use of the important pre-plant fumigant methyl bromide, for example, has been restricted to a limited number of crops due to its effect on the ozone layer. Most nonfumigant nematicides are neurotoxic and can also affect humans and useful microorganisms. Some of these concerns may not be regulated in other growing regions of the world where burrowing nematodes exist and nematicide use in these locales can be relatively high. For example, nonfumigant nematicides are routinely applied in commercial banana plantations worldwide, increasing production up to 250%. Nematicides will not kill all parasitic nematodes in the soil and roots, so must be reapplied periodically. With repeated use, however, soil microorganisms become more efficient at degrading nematicides, making them less effective. The high cost of nematicides also limits their use by small growers.

Biological controls of plant-parasitic nematodes target different stages of nematode development using unique modes of action. The fungi Paecilomyces lilacinus and Trichoderma atroviride destroy nematode eggs, while toxic metabolites from non-pathogenic strains of Fusarium oxysporum inhibit or kill juveniles. Mycorrhizal fungi appear to compete with plant-parasitic nematodes for nutrients, such as phosphorus. The bacterium Pseudomonas kills juveniles and adults by producing lethal hydrogen cyanide. Conversely, Pasteuria spp. are bacterial hyperparasites (organisms that parasitize other parasites) that kill nematodes after attaching endospores to their outer cuticle. Despite their potential, the use of biological controls for plant-parasitic nematodes is quite limited at present.

Resistant varieties, when available, are central to any integrated nematode management strategy. Resistance to R. similis is present in some banana hybrids, but their commercial qualities are still not equal to standard cultivated varieties. There are no black pepper varieties resistant to burrowing nematode, so favored cultivars are sometimes grafted onto the nematode-resistant rootstock of Piper colubrinum, a wild relative. Citrus and tea cultivars of high quality and yield are also grafted onto resistant rootstocks. The economic and environmental benefits of developing plant resistance to nematodes cannot be overemphasized and are a compelling area of research.

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