Host Plant Resistance to Phytophthora Pod Rot in Cacao (Theobroma cacao L.): The Role of Epicuticular Wax on Pod and Leaf Surfaces

Black pod disease caused by P. palmivora and P. megakarya in Ghana is one of the major causes of yield loss in cocoa. Several defense mechanisms are required to counter attack the pathogen. To investigate the role of epicuticular wax in resistance of cocoa to Phytophthora pod rot, 12 cocoa genotypes with and without epicuticular wax on pod were inoculated with P. palmivora and P. megakarya. The wax layers were removed by washing the surfaces of leaf and pod in chloroform for 30 sec. The level of resistance of cocoa genotypes was higher in leaves and pods with wax layer than in chloroform washed leaves and pods (wax removed). Cocoa genotypes with higher amount of wax were more resistant than cocoa genotypes with lesser amount of wax. These results suggest that epicuticular wax layer provide an extra defense against Phytophthora species. There was a significant difference in lesion number and lesion size of cocoa genotypes after removal of their waxes suggesting that other factors are also involved in the resistance of cocoa to Phytophthora species. Cocoa genotypes with higher amount of cuticular waxes on the surfaces of their leaves and pods retained smaller amount of water and take shorter time for moisture to evaporate from their surfaces than cocoa genotypes with lesser amount of wax. Water retention and time to drying of water varied significantly among the cocoa genotypes with intact wax layers. However, there were no significant differences among cocoa genotypes for moisture retention and time to drying of water from leaf and pod surfaces after washing leaves and pods in chloroform. This indicates hydrophobicity of epicuticular waxes on leaf and pod surfaces of cocoa. The implications of these findings in breeding for black pod disease resistance are discussed.