Efficacy of screening methods used in breeding for black pod disease resistance varieties in cocoa

Abstract

Black pod is an important fungal disease of cocoa (Theobroma cacao L.) that causes severe yield losses. The disease affects the pods, beans, flower cushions, leaves, stems and roots. The objective of this study was to evaluate the efficacy of resistance screening methods used in breeding black pod disease resistance in cocoa. The resistance of cocoa leaves and pods to Phytophthora palmivora was investigated in twenty five international genotypes of cocoa in five laboratory experiments and a field observation at the Cocoa Research Institute of
Ghana. A significant clonal difference for leaf and pod resistance was observed at penetration and post-penetration stages of infection. Correlation between resistance of leaves and pods was positive and significant at both penetration and post-penetration stages of infection. The occurrence of such a characteristic of cocoa leaf suggests the value of use of leaves of cocoa seedlings for the prediction of pod resistance to black pod disease. The reliability of the tests was evaluated by correlating results of the inoculation tests with the level of field infection. These correlations were generally positive and significant, both for detached pod and leaf tests. The high positive correlation between detached leaves and pods and natural Phytophthora pod rot infection in the field showed that detached organs can be used for prediction of resistance in the field. Overall, for the convenience of
operations and reliability of results, leaf disc test was the best screening method and was also time saving. Clones SCA 6, T85/799, LAF1, ICS 1 and GU 225V were noted to be promising for breeding against black pod disease. The susceptible clones were MO 20, T79/501, VENC 4-4, PA 120 and MOCORONGO.