A large proportion of the food production in many tropical and subtropical countries comes from smallholders farming relatively small parcels of land. Crops may be grown together in mixed cultivations often following a system that has been long established and generally successful.
In the era of "progress" signalled by the development of monoculture with new varieties, crop protection aids and inorganic fertilizers, it is regrettable that in hindsight the knowledge of yesterday contained in the practices and methods of these smallholders has not been given more attention.
The trend towards urbanization has generated the need for regular supplies of food to be produced from farms and smallholdings close to cities, or with reasonable access to a road network. Intensification of food-crop production to meet this consumer demand has caused various problems for agronomists, soil scientists and crop protectionists, particularly if the land is in continuous cultivation.
The traditional system of agriculture relied on the use of long periods of fallow between crops. During this time the natural vegetation regenerated, soil fertility was restored and populations of pests and diseases declined. Traditional farming systems are usually of mixed crops (even including trees or woody perennials) that provide an ecosystem in which total crop loss due to pests and diseases is rare. Nevertheless, subsistence farmers will usually grow enough to cover the possibility of losing up to 40 percent of production before or after harvest (Hussey, 1990). The problems of a rapidly increasing human population for land use has led to the abandonment of traditional systems, and these are now practised only in sparsely populated countries.
Crop losses due to pests and diseases appear to be more frequent as improved crop varieties are grown in monoculture on the same land with the aid of pesticides and inorganic fertilizers. In many instances, crop damage has increased as a result of the inadvertent introduction of alien pests on crop plants. The distribution of several nematode species has been increased in this way, e.g. Radopholus similis and Pratylenchus spp. on bananas, Tylenchulus semipenetrans on citrus and Globodera spp. on potatoes.
In considering potential strategies for controlling nematodes there are certain practices that can be implemented once the life history and the behaviour of the pest is known. Plant-parasitic nematodes occur in greatest abundance in the rooting zones of crops, mostly in the top 30 cm of soil; nematodes may also penetrate to deeper soil layers particularly in unfavourable conditions (Prot, 1980) only to return to the surface layers when crop growth resumes. The free-living stages of some species survive the unfavourable season through physiological or structural adaptation (Evans, 1987; Gaur and Perry, 1991a), or survive for several seasons as eggs, as in the case of the potato-cyst nematode.
Nematodes are usually at their most vulnerable at the time when they are actively searching for host roots and when surviving unfavourable growing seasons. The survival of free-living stages in the absence of hosts can be longer than originally thought, and fallow periods of less than 12 months may be insufficient to control those species that reproduce rapidly. The main abiotic factors that influence free-living nematodes and eggs are heat, desiccation and anaerobis. Plant-parasitic nematodes will not survive long periods at temperatures above 40°C. These temperatures may be reached on bare soil in some countries during the hot season, but heat penetration may not extend below 10 cm.
The technique of increasing soil temperatures by solarization can be of value in controlling nematodes (FAO, 1991; Gaur and Perry, 1991b), but this is only possible where there are long periods of uninterrupted sunshine. Solarization is also practised for controlling soil-borne fungal pathogens (FAO, 1991). Nematodes are aquatic animals that inhabit the films of water that surround soil particles. As soils dry, the volume of soil water decreases and the ability of nematodes to move is impaired (Wallace, 1963), eventually free-living nematodes may die as the soil moisture decreases. The extent to which species have adaptations to survive desiccation is still a matter for further study (Duncan, 1986; Evans, 1987). By regulating the metabolism in times of stress (Womersly, 1987) some species are capable of anhydrobiotic survival. The contrivance of bare fallows that deprive nematodes of a host may not always be a worthwhile control strategy especially as fallowing in hot, dry areas can deplete soil organic matter and the levels of beneficial soil biota. In such situations for specific nematode pests, a better alternative might be cover crops with non-host leguminous trees or shrubs such as Crotalaria spp.
The control of nematodes by flooding has been advocated in certain locations (Gowen and Quénéhervé, 1990). When flooded all the soil pore spaces are filled and the oxygen supply for the soil microflora and fauna becomes limiting. Many plant-parasitic nematodes are intolerant of oxygen starvation and soon die. Similar effects are observed when nematodes are stored in deep water in the laboratory, but often they can be revived by aeration. To be effective in the field therefore, the anaerobiosis has to be of sufficient duration to kill the nematodes. In Côte d'Ivoire banana fields are flooded for at least five weeks. Flooding is not a technique that can be widely used for nematode control and whether it is equally efficacious against all plant-parasitic species is not known.
The development of strategies for nematode control requires a thorough understanding of the growth of the host plant and the biology of the nematode. In some climates it is possible to avoid (or contain) damage by planting the crop at a time when nematode invasion will be slow, or when plant growth will be faster than that of the parasite. Damage to transplanted short-cycle crops can be limited if seedlings are produced in nematode-free nurseries or in soil-less potting media. In general, protection from invasion during the early growth of a crop will enable the plant to produce a yield acceptable T most farmers even if it is planted in nematode-infested soil, particularly if the crop receives adequate supplies of water and nutrients. This will not prevent the development of large nematode populations on the root systems and in the soil that will influence the subsequent crop. Brown (1987) showed that treating wheat with low nematicide doses at seeding gave a substantial yield improvement, resulting from protection from the invasion of cereal-cyst nematodes during the establishment of the crop.
The concept of growing companion plants with nematicidal root exudates alongside susceptible crops is attractive, but the actual benefits may be questionable or at least difficult to quantify. Species of Tagetes, Crotalaria and castor bean (Ricinus communis) have been widely used (Hackney and Dickerson, 1975; Huang, 1984; Naganathan et al., 1988). Where Tagetes was grown as a companion to tomato in a rotation trial in the United Republic of Tanzania, yields were significantly less than with other treatments, indicating that such a cropping practice was not likely to be adopted by farmers (Madulu and Trudgill, 1993).
The use of organic mulches for managing nematodes has been widely studied and there are various examples of plant debris having beneficial effects on plant growth (Muller and Gooch, 1982; Sikora, 1992). In small garden plots that receive heavy mulches of organic waste, nematodes are unlikely to be a major problem. Subsistence gardens are therefore well suited to the needs of householders, who, as Hussey (1990) reported, will usually grow sufficient quantities to meet immediate needs and allow for crop losses due to pests and diseases. Regular mulching is realistic for subsistence gardeners but becomes less feasible if the area of cultivation is large. To be effective, the recommended mulches may have to be used at application rates of 5 to 10 tonnes per hectare. The acquisition and distribution of such volumes is not likely to be practised by farmers growing crops on a large scale.
If chemical treatments are unavailable or inappropriate, the management of plant-parasitic nematodes will have to depend on alternative strategies.
Those farmers in the Near East region who routinely use methyl bromide or non-fumigant nematicides may in future have to seek alternative treatments. To help them do so, government-supported research will be necessary, particularly for control strategies not requiring the regular purchase of proprietary products. This type of research is unlikely to be funded by manufacturing companies.
New research will require multidisciplinary collaboration to enable the integration of traditional practices with new ideas created by the scientists. Through more research on nematode biology and the environment, nematologists must consider several possibilities:
· How nematode life cycles may be interrupted.
· How microbial activity at the root-soil interface, which could lessen nematode invasion, can be promoted.
· Identification of compounds that interfere with reception of stimuli or repel nematodes.
· The selection or breeding of resistant varieties, or varieties that tolerate nematode infestation, and that can produce acceptable yields.
In conducting field experiments that evaluate the effect of cropping systems on nematode populations, scientists should plan to suit the needs of the farmer. Many research station trials are designed to produce results for ready analysis and presentation in journals; in consequence, treatments may be selected to suit the needs of the scientist. Most farmers grow short-cycle, annual and perennial crops in close proximity. Such mixed cropping practices are rarely undertaken in field experimentation by nematologists.
Any new cultural, microbial or management approaches that are developed must be within the capabilities of the farmer and meet the necessary environmental and economic requirements. New strategies will be considered as successful if they lower nematode damage thresholds. The farmer will be satisfied if these treatments are reliable, practicable and economically justified; his customer, the consumer, will be content if the product has the desired quality, contains no toxic residues, and is fairly priced.
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