INHERITANCE OF FOLIAR RESISTANCE TO PHYTOPHTHORA-PALMIVORA (BUTLER) BUTLER IN CACAO (THEOBROMA-CACAO L.)

The genetic nature of resistance in leaves of cacao (Theobroma cacao L .) to Phytophthora palmivora (Butler) Butler was investigated in a 3 x 8 and a 4 x 4 mating design. Leaf resistance was assessed at the pene tration and post-penetration stages of infection. Wide variability was observed among the 32 families examined for lesion frequency (penetra tion resistance) and lesion size/spread (post-penetration resistance). Segregation within families showed a continuous distribution for both forms of resistance, suggesting that resistance is polygenic. The poo r association between resistance at the penetration stage and that at the post-penetration stage suggested that they were independently inhe rited and could be selected independently with the aim of accumulating genes for resistance. Genetic complementation between the two forms o f resistance and transgressive segregation were evident within the fam ilies studied. The additive effects were predominant for lesion size, while both additive and non-additive effects were important in the inh eritance of lesion frequency. The narrow sense heritabilities obtained from North Carolina Model II analysis were as high as 0.61 and 0.40 f or lesion size and lesion frequency respectively. Possible breeding st rategies based on these findings are discussed.