Inheritance and general combining ability studies of detached pod, leaf disc and natural field resistance to Phytophthora palmivora and Phytophthora megakarya in cacao (Theobroma cacao L.)

Black pod is a major disease of cocoa. Knowledge of the inheritance of resistance to black pod would be important in devising strategies to breed resistant varieties. The mode of inheritance of resistance to black pod disease was investigated using leaf disc, detached pod tests and natural field observation in a 6?×?4 factorial and 6?×?6 diallel mating designs analyzed using North Carolina design-II approach and Griffing??s method II model I approach, respectively. The dominant component was smaller than the additive component for pod lesion size, leaf disc scores and natural field infection. Both general combining ability and specific combining ability (SCA) effects influenced the inheritance of pod lesion number and leaf disc scores suggesting that both additive and non-additive (dominance or epistatic) effects influence the inheritance of resistance to black pod disease. However, SCA effect was not significant for pod lesion size and natural field infection suggesting the importance of additive genes in the inheritance of these traits. The higher heritability of leaf disc scores, pod lesion numbers and pod lesion sizes than the heritability of natural field infection which was almost zero, shows the influence of environment on natural field infection and suggests that ranking of resistance of cocoa genotypes need to be done under controlled environmental conditions to observe true resistance levels. Good general combiners, in combination with each other, did not necessarily yield good specific combinations suggesting that parents?? performance cannot be used for selecting superior progenies. The non-significance of reciprocal effects indicates absence of maternal effects or cytoplasmic inheritance in resistance to black pod disease. Recurrent selection procedures and inter-crossing aimed at general combining ability would be more suitable for improvement of black pod disease resistance. Cocoa genotypes with good combining ability for resistance to black pod disease such as T60/887, Pa 150, Sca 6, and Pa7/808, identified in this study could be important parents to form base population for such recurrent selection procedures.