Genetic control, combining ability and heritability of resistance to stem canker in cacao (Theobroma cacao L.)

Stem canker caused by Phytophthora palmivora and Phytophthora megakarya is a serious disease of cacao (Theobroma cacao L.). Studies of the genetics of resistance to this disease is important to identify an effective procedure to use in breeding resistant varieties. The objective of this study was to determine mode of inheritance, combining ability and heritability of resistance to stem canker in cacao. A 6 × 6 full diallel mating design was used to generate 36 F1 hybrids which were evaluated for resistance to stem canker with three replications under artificial inoculation with 1 mm2 agar plug taken from actively growing colonies of 10-day old cultures of P. palmivora and P. megakarya. Twelve months later, the canker lesions on outer bark of the stem of the seedlings were measured and the canker lesions inside was also measured after scraping the bark. There were significant (p < 0.05) variation for canker sizes both before and after scraping the bark of stem among the genotypes. The general combining ability (GCA) and specific combining ability effects were both significant (p < 0.05) for stem canker resistance suggesting that both additive and non-additive gene effects are involved in expression of resistance to stem canker. However, the predominance (> 50%) of GCA suggest that additive gene effects were more important than non-additive gene effects. The overall direction of results showed that using one of the following parents: P1 (Alpha B36), P4 (T17/524) and P5 (T65/238) as the female produced progenies with better resistance to stem canker disease than when they were used as the male parent. The majority reflected the resistance level of the female parent, suggesting that resistance to stem canker in cacao is influenced by maternal inheritance. Cacao genotypes Alpha B36, T17/524 and T65/238 showed significant negative GCA effects for resistance to stem canker suggesting that these genotypes will transmit the trait to their progenies and therefore recommended for cacao breeding. Narrow sense heritability for stem canker resistance was low indicating large component of dominance effects and influence of environment in inheritance of resistance to stem canker in cacao. However, the very high broad sense heritability (0.99) coupled with large variation within the cacao tree population indicate that progeny testing and within family selection would be appropriate in breeding against stem canker in cacao.