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Garoua Boulaï, Cameroon

Snoeck D.,CIRAD - Agricultural Research for Development | Abolo D.,IRAD | Jagoret P.,CIRAD
Agroforestry Systems | Year: 2010

In Cameroon, cocoa trees are mostly grown in forests and without fertilization. Our aim was to learn more about the temporal dynamics of soils in cocoa agroforests by comparing young (1-4years old) and old (over 25years old) cocoa agroforests. Short fallow and secondary forest were used as treeless and forest references. The numbers and diversities of soil vesicular arbuscular mycorrhizal (VAM) fungi on 60 cocoa producing farms in the Central province of Cameroon were assessed based on the classical morphotyping of spore morphology. We also observed the soil organic matter, nitrogen and major soil nutrients. VAM spore density was significantly lower in the young cocoa agroforests (16 sporesg-1 dry soil) than in the old cocoa agroforests (36 sporesg-1 dry soil). Levels in the nearby secondary forest (46 sporesg-1 dry soil) were not significantly different from old cocoa. The spore density was significantly highest in the short fallow (98 sporesg-1 dry soil). The Shannon-Weaver index also showed significantly lower biodiversity in young cocoa (0.39) than in old cocoa agroforests (0.48), secondary forest (0.49) and short fallow (0.47). These observations were supported by significant differences in the C:N ratio, Ca, Mg, and cation exchange capacity between young and old cocoa agroforests. We concluded that unfertilized cocoa agroforests could be sustainable, despite a decrease in some soil characteristics at a young stage, due to traditional land-conversion practices based on selective clearing and burning of secondary forest. © Springer Science+Business Media B.V. 2009.

Saj S.,CIRAD | Jagoret P.,Montpellier SupAgro | Todem Ngogue H.,IRAD
Agroforestry Systems | Year: 2013

In Central Cameroon cocoa is mainly produced by household farming systems based on complex associations between cocoa and companion trees. Setup either on native/remnant forest or savannah, these agroforestry systems (AFS) are managed according their geographical position and local pedoclimatic conditions. In this paper, we investigated the effects of local management strategies on carbon (C) storage of live trees in three different cocoa production zones of Central Cameroon. In the 58 fields studied, 8,996 cocoa trees and 1,258 companions were surveyed. Tree sampling was non-destructive and to estimate C storage we used allometric models for above- and belowground biomasses. We measured abundance, height, diameter at breast height and determined species of companion trees. We distinguished between four cocoa plantation age categories (immature, young, mature and senescent) and three preceding systems (forest, forest gallery and savannah). We surveyed farmers' use of each associated tree, allocated it to a functional category and asked if it had been introduced or conserved. Total C content of live trees was on average close to 70 t ha-1. We found that it mostly relied on associated trees-cocoa trees contribution being ac. 2-12 % of live trees total C. The level of contribution to C storage of companions from different use categories differed between sites-trees producing food had contributed most in Bokito and Obala while trees used for shading or fertility contributed most in Ngomedzap. Dynamics of C storage in live trees was found to be independent from cocoa trees growth and age. When aging, AFS continuously lost companion trees and especially conserved ones putatively because of farmers' selective logging. Yet, AFS apparently maintained equivalent C storage abilities with time. Hence, even if cocoa do not contribute significantly to C storage in our study, the systems into which they are included are able to significantly store C and may also contribute to other ecological services such as conservation. © 2013 Springer Science+Business Media Dordrecht.

Brevault T.,CIRAD - Agricultural Research for Development | Brevault T.,University of Arizona | Nibouche S.,CIRAD | Achaleke J.,IRAD | Carriere Y.,University of Arizona
Evolutionary Applications | Year: 2012

Non-cotton host plants without Bacillus thuringiensis (Bt) toxins can provide refuges that delay resistance to Bt cotton in polyphagous insect pests. It has proven difficult, however, to determine the effective contribution of such refuges and their role in delaying resistance evolution. Here, we used biogeochemical markers to quantify movement of Helicoverpa armigera moths from non-cotton hosts to cotton fields in three agricultural landscapes of the West African cotton belt (Cameroon) where Bt cotton was absent. We show that the contribution of non-cotton hosts as a source of moths was spatially and temporally variable, but at least equivalent to a 7.5% sprayed refuge of non-Bt cotton. Simulation models incorporating H. armigera biological parameters, however, indicate that planting non-Bt cotton refuges may be needed to significantly delay resistance to cotton producing the toxins Cry1Ac and Cry2Ab. Specifically, when the concentration of one toxin (here Cry1Ac) declined seasonally, resistance to Bt cotton often occurred rapidly in simulations where refuges of non-Bt cotton were rare and resistance to Cry2Ab was non-recessive, because resistance was essentially driven by one toxin (here Cry2Ab). The use of biogeochemical markers to quantify insect movement can provide a valuable tool to evaluate the role of non-cotton refuges in delaying the evolution of H. armigera resistance to Bt cotton. © 2011 Blackwell Publishing Ltd.

Achaleke J.,IRAD | Brevault T.,CIRAD - Agricultural Research for Development | Brevault T.,University of Arizona
Pest Management Science | Year: 2010

Background: The cotton bollworm Helicoverpa armigera (Hübner) has developed esterase-mediated resistance to pyrethroids in Central Africa. To gain a better understanding of how quickly this resistance can evolve, its genetic basis and stability were examined in a field-derived strain of H. armigera (474-fold resistant to cypermethrin). Results: Genetic crosses between susceptible, resistant and F1 hybrids indicated that resistance was inherited as a dominant trait (DLD = 0.86) and conferred by a single autosomal gene. The dominance of resistance decreased as the cypermethrin dose increased, so that resistance was incompletely dominant (DML = 0.73) at the highest dose tested. Resistance (DL50) of the F1 hybrid progeny significantly decreased over five generations in the absence of pyrethroid exposure. Conclusion: Rapid selection of resistance alleles due to dominance supports the ability of H. armigera to develop resistance to pyrethroids in Central Africa. However, associated fitness costs provide useful information for managing the evolution of resistance. © 2009 Society of Chemical Industry.

Mouen Bedimo J.A.,IRAD | Bieysse D.,CIRAD - Agricultural Research for Development | Notteghem J.L.,Montpellier SupAgro | Cilas C.,CIRAD - Agricultural Research for Development
Plant Pathology | Year: 2010

The development of coffee berry disease (CBD) epidemics (caused by Colletotrichum kahawae) in Cameroon was monitored over two successive years (2004 and 2005) on coffee trees protected from rainfall by transparent plastic sheets and on unprotected control trees. This work was done to assess how rain affected disease development when it did not fall directly onto the coffee trees and to determine the influence of primary inoculum on the severity of CBD. Weekly observations over the 2 years showed that there were 1·1% diseased berries on coffee trees completely protected from rainfall, compared with 45% diseased berries on unprotected coffee trees. Disease severity on unprotected trees during the 2 years of the experiment was estimated at 53% diseased berries, compared with 27% on trees only protected in the first year. These results confirmed rainfall as one of the key physical factors in the development of Arabica CBD. They also provided evidence of a subsequent effect of protecting coffee trees from rainfall in 2004 on the severity of CBD in 2005. This suggested some practices that might lead to very effective cultural control of CBD in regions where severe epidemics of the disease occur. Journal compilation © 2010 British Society for Plant Pathology.

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