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Activities carried out to evaluate cocoa resistance to mirids (Sahlbergella singularis) at IRAD, Cameroon

Babin R., Mpe J.M., Dibog L., Amang a Mbang J., Nyassé S., Eskes A.. 2006. In : Eskes Albertus (ed.), Efron Yoel (ed.). Global approaches to cocoa germplasm utilization and conservation : Final report of the CDC/CCO/IPGRI project on "Cocoa germplasm utilization and conservation: A global approach" (1998-2004). Amsterdam : CFC, p. 162-169. (CFC Technical paper, 50).

Field and laboratory tests were conducted for different components of the resistance of cocoa to mirids (Sahlbergella singularis) at the Nkoemvone and Nkolbisson research stations of IRAD as part of the CFC/ICCO/IPGRI project activities in Cameroon. Almost 40 local or international genotypes were tested for one or for several components of resistance. Laboratory micro-tests were used to assess attractiveness. Antixenosis was measured by enclosing mirid nymphs in sleeve cages on young flushes. Attractiveness and antixenosis were estimated in the same set-up by counting the feeding points after 24 hours. Ability to recover from mirid damage (tolerance) was assessed by observing degradation of twigs with time. Finally, antibiosis tests were conducted with the aphid Toxoptera aurantii, with the aim of evaluating whether this aphid could be used as an indicator for cocoa antibiosis towards mirids. Statistical analysis generally revealed a strong effect of genotypes on the different components of the resistance. However, there was no significant difference between the genetic groups studied (Upper Amazon and Lower Amazon Forasteros, Trinitarios and hybrids), which appears to be in contrast with field observations on mirid damage in Côte d'Ivoire. We also failed to identify genotypes that combine favourable responses to several resistance components. As an important result, the activities have made it possible to improve and standardize the evaluation methods. Mechanisms involved in attractiveness and antixenosis appeared to be similar, therefore one test should be enough to evaluate these components. We propose to standardize the methods for antixenosis (under no-choice conditions) and tolerance. We suggest that a new assessment method be developed for the effect of genotype on antibiosis (female mirid fecundity and nymph survival), using cocoa pods on the tree as the feeding and egg-laying environment. Finally, we propose to include in our future activities studies on resistance of cocoa to fungal pathogens that may be associated with mirid damage, with the aim of achieving a better understanding of tolerance and dieback. (Résumé d'auteur)

Thématique : Génétique et amélioration des plantes; Ravageurs des plantes; Méthodes de recherche

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