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THE BIODIVERSITY OF FALLOW LAND:A FACTOR USEFUL FOR CONTROLLINGPLANT PARASITIC NEMATODES

September 13, 1999

Fallowing is a common practice for restoring soil fertility and structure in the tropics : it favours improvement in its physicochemical properties and the build-up of stores of organic matter, which are essential for the development of the telluric microfauna and microflora that inhabit soils. Researchers from the Laboratory of Biopedology of IRD (Institut de Recherche pour le Développement, formerly ORSTOM) at Dakar have studied this impact of fallowing on the development of nematodes in a Sudano-Sahelian ecosystem.
The Nematodes, tiny worms (less than 1 mm long), are parasitic on plants. They destroy the root cells, which prevents the plants from extracting and assimilating from the soil the nutrient elements they need for growth. In Sudano-Sahelian regions, considerable damage is caused by such nematodes. Their eradication by application of nematicides on food crops (millet, maize, groundnuts, sorghum and so on) improves yield two- or threefold. However, these treatments are expensive, polluting and hazardous for human health. They are little used in poor areas and are anyway steadily being excluded from nematode-control programmes.
In the course of a comparative study of cultivated fields and others left fallow conducted in the groundnut producing plains of Senegal, bordering Gambia, the scientists first observed that the longer the fallow period lasted, the more the abundance and diversity of nematode species increased. The two main crop-ravaging species disappear and are replaced by several others which are apparently more prolific.
Research then conducted in the laboratory has shown that paradoxically this nematode proliferation could be useful!
When millet is pot-cultivated in soil taken from a fallow field, plant growth is better than that observed if it is pot-grown in soil from cultivated fields, where there are nevertheless fewer nematodes and species. However, this observation is not surprising, seeing that the fallow soil is richer than that from fields exhausted by incessant successive crop cycles.
More remarkable still are the results obtained from experiments conducted in the course of several cultivation cycles on fallow soil and soil that had been cultivated over a very long period. In the latter, the nematode community reduces the millet's root mass, but this is not the case in the fallow soil which, however, is more abundant in both parasites and species. In these experimental conditions, it can even happen that the millet cultivated in the infested fallow soil develops better than in a soil completely devoid of nematodes (by sterilization). How can such a paradox be explained? In nematode-infested fallow soil, the plant benefits from a more extensive root system which results from the standard reaction (production of new roots) of a plant to parasites which are not particularly pathogenic. When the worm community is pathogenic, as it is in the cultivated soil, this same reaction occurs, but the new roots are in their turn destroyed by the nematodes.
These results therefore indicate that it is not always necessary physically to eliminate the nematodes in order to combat them, because it is not so much the number of parasites which seems to determine their pathogenic action, as the species which make up the community. The investigation conducted by the Dakar biopedology laboratory moreover brings evidence that the more different species there are in a nematode community, in balanced proportions, the lower is its pathogenicity.
On the basis of these findings, clearly the means are needed not to eradicate these parasites, but quite the opposite: protection strategies and measures to conserve them in the agricultural ecosystem! This is a question of a concrete case of making the most of the value of biodiversity as a way of controlling nematodes. Although it does not give such spectacular results as chemical treatments, this ecological management strategy for controlling nematode populations is an interesting alternative to achieve a durable attenuation of the impact of parasitic nematodes on tropical crop plants.

FOR FURTHER INFORMATION

Contact : Patrice Cadet, Institut de Recherche pour le Développement (IRD), Laboratoire de Bio-Pédologie, BP 1386, Dakar Senegal, Tél : (221) 849 33 12 - Fax : (221) 832 16 75, e-mail : patrice.cadet@dakar.ird.sn

Bibliographie
P. Cadet, J.F Bois, E.Pate, N'Deye N'Diaye-Faye et Ch. Floret "Diversité des nématodes parasites et durabilité du syste'me de culture de jache're au Sénégal ", communication présentée dans le cadre du séminaire international " La jache're en Afrique tropicale " (avril 1999, Dakar). A para'®tre.
P. Cadet "Gestion écologique des peuplements de nématodes phytoparasites tropicaux : importance des facteurs édaphiques et ruissellement. Gestion écologique des nématodes phytoparasites", Cahiers Agricultures, 1998.



Institut de Recherche pour le Développement, Paris (IRD)