Levels of resistance to Phytophthora pod rot in cocoa accessions selected on-farm in Côte d'Ivoire

Resistance to Phytophthora pod rot (Ppr) is becoming an increasingly important criterion for selection of new cocoa cultivars in Côte d'Ivoire. The predominant species of the pathogen, Phytophthora palmivora, causes pod losses of 10-15% but the more aggressive Phytophthora megakarya present in the eastern part of the country causes losses of 40-60%. The latter species is expected to continue its spread to the main cocoa belt in Côte d'Ivoire over the next decade. Since 2000, the Centre National de Recherche Agronomique has been selecting new cocoa cultivars with direct involvement of farmers. More than 250 farmers were visited and their knowledge is being used to collect accessions with high yield potential, with low pest (mirids) infestation or with low Ppr incidence. Farmers were capable of identifying mother trees with low Ppr incidence only in regions with high disease pressure (Abengourou and Aboisso). Open-pollinated seedling progenies obtained from 226 promising trees in farmers' fields were screened for resistance to Ppr by inoculating leaf discs from nursery plants with spores of a P. palmivora isolate. Three clones (SCA6, PA150 and NA79) and three recommended hybrids were used as control cultivars. The results of the leaf disc test confirmed the known variation of resistance of the control cultivars. The relative level of resistance of the farm accessions varied mainly between moderately resistant and susceptible, but several accessions could be considered as resistant, in relation to the control cultivars. The major part of the 15% most resistant farm accessions came from the Abengourou and Aboisso regions in the eastern part of the country. Progenies from mother trees that farmers had selected as showing low Ppr incidence in their fields also appeared to be more resistant in the leaf disc test. This suggests that the resistance identified to P. palmivora in the leaf disc test may be efficient in the field in areas infected with P. megakarya. It is recommended that the genetic diversity identified through participatory selection of promising mother trees in farmers' fields be further exploited in breeding to obtain new hybrid or clonal cocoa cultivars with low incidence of Ppr, good yield and low mirid damage.