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Survie et dissémination du nématode Radopholus similis (Cobb) Thorne dans les sols bruns-rouilles à halloysites (nitisols) : effets de l'état hydrique et des flux hydriques

Abstract : The burrowing nematode Radopholus similis is a major pest of bananas and plantains all over the world. Introduced from the XVIth century in Africa and America, this nematode causes important losses in the traditional banana plantations. Recent development of new cropping systems reduced its impact; but additional knowledge is required to understand its persistence in soil and to prevent its dispersal. Usually considered as marginal, the soil phase of Radopholus similis has not been extensively studied because the concentrations of R. similis are not very important in the soil (almost 1/1,000 of the concentration in roots). While taking into account both soil volume and nematode numbers, the soil phase contains 30 to 50 % of R. similis individuals. After considering the characteristics of the West Indian volcanic soils, especially classes of capillaries which are likely to contain R. similis (capillaries from 20 to 300 µm in diameter), we evaluated the various methods of extraction of nematodes: Baermann filtration, elutriation, centrifugation-flotation, aspersion, maceration in hydrogen peroxide. The extraction efficiency of each method, their advantages and disadvantages were discussed. This work was completed by the evaluation of a method of vital coloration to differentiate the dead nematodes from the living ones. Then, the capacity of survival of R. similis in soils was evaluated. We placed a R. similis suspension in 30 mL containers filled up with soils of different types at different water potential, but always without food source. These containers were stored for up to 180 days in conditions of optimal temperature for R. similis. The observed survival durations were slightly higher than those usually found in the bibliography: water saturated soils set apart, we recovered after six months from 1.7 to 9.3 % of the initial population in the nitisol and from 9.5 to 11.9 % in the andosol. Considering the adults, the decreasing curves of R. similis populations could generally be adjusted either with an exponential model or with a Teissier's model. These curves depended on the soil nature, the water potential and the nematode sex. We thus observed that males had a remarkable survival capacity; in the nitisol, their average survival rates were close to twice those of females. This survival capacity is likely to be linked with a lesser daily energy expense in males than in females: it would seem that males, whose digestive system is atrophied, did not forage and did not use reserves for the development of eggs. Apart from this, in the sterilized soil, survival was optimal when soils were close to the field capacity; in the natural soils, survival was optimal in the driest conditions. These contradictory results may be explained by the interactions with other soil organisms which, by decreasing the capacity of survival, may have an antagonistic effect. A complementary study was carried out in tubes filled with water or with soil solutions, extracted from a nitisol by centrifugation. After 35 days, 7 to 8 % of the initial populations of R. similis of each sex remained alive. This species can thereby remain long enough in running waters to be dissipated by runoff. We thus studied the dispersion of R. similis by water flows; on soil surface with rainfall simulators and along soil depth with soil cylinders on which we sprinkled simulated rainfalls which totalized up to 3.7 times the pore volume of the cylinder (total of 540 mm rainfall). These studies were completed by a field trial which led to successive maps of nematode recontaminations in subplots isolated or not from upstream water by ditches. The passive dissemination seems very weak inside the soil and affects only a marginal fraction of the populations. It seems that R. similis resorts to a behaviour of fleeing from leaching (plasticity reflex?) to resist water flows. On soil surface, rainfall simulation showed that dissemination by surface waters required very particular conditions: soils close to water saturation, unearthed roots or toppled-over plants on soil surface. Nevertheless, passive water dissemination occurs mostly on soil surface, but this phenomenon is restricted to "disaster" events at the scale of the nematode: destruction of its environment by tropical showers with heavy runoff. Actually, R. similis leaching by runoff water is sufficient to contaminate a banana plot in less than two years. All these studies showed that the behaviour of R. similis is an essential knowledge to understand its survival and dispersal. Its biology allows good adaptation to the long-lasting cultures with vegetative reproduction, such as banana plants. When these are cultivated in monoculture, R. similis damages can be considerable.
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Submitted on : Wednesday, May 16, 2012 - 4:22:02 AM
Last modification on : Friday, October 8, 2021 - 4:26:15 PM
Long-term archiving on: : Friday, August 17, 2012 - 2:20:57 AM

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Christian Chabrier. Survie et dissémination du nématode Radopholus similis (Cobb) Thorne dans les sols bruns-rouilles à halloysites (nitisols) : effets de l'état hydrique et des flux hydriques. Phytopathologie et phytopharmacie. Université des Antilles-Guyane, 2008. Français. ⟨tel-00697213⟩

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