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 × 8 and a 4 × 4 mating design. Leaf resistance was assessed at the penetration and post-penetration stages of infection. Wide variability was observed among the 32 families examined for lesion frequency (penetration 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 poor association between resistance at the penetration stage and that at the post-penetration stage suggested that they were independently inherited and could be selected independently with the aim of accumulating genes for resistance. Genetic complementation between the two forms of resistance and transgressive segregation were evident within the families studied. The additive effects were predominant for lesion size, while both additive and non-additive effects were important in the inheritance of lesion frequency. The narrow sense heritabilities obtained from North Carolina Model II analysis were as high as 0.61 and 0.40 for lesion size and lesion frequency respectively. Possible breeding strategies based on these findings are discussed.