Evaluation of the efficacy of resistance screening methods used in the breeding of black pod disease resistant varieties in cacao

Abstract

The resistance of cacao (Theobroma cacao L.) leaves and pods to Phytophthora palmivora (Butler) Butler was investigated in 25 international genotypes of cacao in five laboratory experiments and a field trial at Cocoa Research Institute of Ghana at Tafo. Tissue-paper-mount and punch- inoculation methods were used to distinguish between resistance at the penetration and post-penetration levels based on lesion frequency and size, respectively. A significant clonal difference for leaf and pod resistance at the two stages of infection was observed. 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 cacao leaf suggests the possibility of the use of leaves of cocoa seedlings for the prediction of pod resistance to the black pod disease. The linear correlation between the detached pod tests and leaf disc test was significant. A highly significant difference was observed among levels of infection of distal, proximal and mid regions of pods inoculated with Phytophthora palmivora. 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. SCA 6, T85/799, LAFl, ICS 1 and GU 225V were noticed to be promising clones for breeding against black pod disease. The susceptible clones were MO 20, T79/501, VENC 4-4, PA 120 and MOCORONGO. The results suggest that routine application of the detached leaf and pod tests if carried out under uniform conditions and in standardized manner should both be sufficient in selection of new varieties with high levels of field resistance to black pod.