Sustaining cocoa production through genetic resistance to back pod and leaf blight

The resistance of cocoa to Phytophthora palmivora and its mechanism(s) were investigated in leaves and pods of twelve clones. Anatomical and morphological characteristics including ; cuticle thickness, stomata! frequency and pore length were assessed on leaf and correlated with foliar resistance at penetration stage. Post-penetration resistance was also determined on leaf and pod, and their relationship assessed. Results show a significant difference in leaf susceptibility between surfaces. The adaxial surface appeared less susceptible possibly due to lack of stomata and/or the presence of a thick cuticle. Leaf susceptibility differed significantly at penetration and postpenetration stages suggesting different mechanisms. Correlation analysis shows a strong relationship between the joint effect of cuticle thickness, stomata! frequency, stomata! size and leaf resistance at penetration stage. A close relationship in the rate of spread of lesion in leaf and pod indicates that a reliable assessment of clonal reaction can be based on leaf observation. The results suggest that there are two mechanisms of resistance; one at th,e point of penetration and the other at post-penetration stage. The resistance mechanism(s) at postpenetration stage seems to be common for both leaf and pod, and possibly controlled by biochemical factor(s). This suggest that pod resistance to P.palmivora can be reliably assessed at the seedling stage using leaf resistance to spread as an indicator. Both pathological and genetic implications of these findings are discussed for the improvement of cocoa resistance to P.palmivora infections.