Progress obtained in Côte d¿Ivoire on mirid resistance studies

The development and use of resistant cocoa varieties is no doubt a major component of the integrated management of cocoa pests and diseases. However, breeding cocoa for resistance to cocoa pests, especially mirids, has been confronted to the lack of appropriate screening methods to identify sources of resistance. During the past five years, the implementation of the CFC/ICCO/Bioversity project on Cocoa Productivity and Quality improvement: a Participatory Approach has enabled to develop a new screening method for cocoa tolerance to mirids. This method is based on the reaction of the plant to the fungi associated to dieback due to mirid feeding lesions, after identification of the fungus species involved. The fungi species associated to mirid feeding lesions were isolated from cocoa twigs and pods naturally attacked in the field and were identified, the most predominant being Lasiodiplodia theobromae. The tolerance test consists in the evaluation of the level of natural infection of the cocoa genotypes to be evaluated by Lasiodiplodia after mechanical injury in the field or in the budwood garden. Results have shown that susceptibility of the cocoa tree to Lasiodiplodia varies from one genotype to another. Hence, the size of the lesions caused by Lasiodiplodia could be a good indicator of resistance to that pathogen and therefore an indicator of tolerance to mirid. However, correlation between two series of tests performed using the same clones was positive (r=0.43, P=0.23) but not significant, suggesting that the method need to be refined. The CFC/ICCO/Bioversity project also allowed investigating various components of cocoa resistance to mirids using laboratory and field screening methods. Results have revealed significant positive correlation (r=0.97, P=0.005) between field damage and antixenosis in the laboratory, indicating that antixenosis is involved in the level of damage observed in the field. Moreover, selected cocoa clones and hybrids have been screened for resistance/tolerance to Sahlbergella singularis, using the available screening methods. The major achievements are presented here. (Resume d'auteur)