Genetic control of morpho-physiological, histological and biochemical factors of resistance to black pod disease in Cocoa (Theobroma cacao L.)

Black pod disease caused by Phytophthora palmivora and Phytophthora megakarya is an important disease of cocoa of which host plant resistance is considered the most effective method of controlling the disease. However little is known about host factors conferring resistance and the type of gene action governing them. The objective of this study was to identify morpho-physiological, histological and biochemical traits conferring resistance to black pod disease in cocoa and to study their genetic controls. Twelve cocoa genotypes and their hybrids in 6x4 North Carolina Design II and 6x6 diallel design were evaluated by leaf discs and detached pod inoculations at penetration and post-penetration stages of infection and natural field infections. The cocoa parents and their hybrids were also evaluated for morpho-physiological, histological and biochemical characters. The morpho-physiological, histological and biochemical factors evaluated were correlated with resistance to black pod disease. Morpho-physiological factors such as pod husk thickness and epicuticular waxes on abaxial and adaxial surfaces of leaf and on pod surface were significantly and negatively correlated with black pod disease. Epicuticular wax on pod husk was significant and negatively correlated with penetration infection of Phytophthora species but was not significant with post-penetration infection suggesting its role in restricting penetration of the fungus. Percentage moisture content was significant and positively correlated with black pod disease. Histological factors such as width and length of vascular bundles, epicarp thickness, distance between vascular bundles and epicarp, number of cells in epicarp and number of cells in mesocarp showed a significant negative correlation with black pod disease. Number of vascular bundles and cell width were positively correlated with black pod disease. Susceptible genotypes were observed to have thinner epicarp and higher numbers of vascular bundles suggesting their porosity to Phytophthora species. The cells in epicarp and mesocarp were arranged more compactly in resistant than highly susceptible genotypes. The cell walls of resistant genotypes stained deep red with phloroglucinol, a lignin specific stain, suggesting the presence of lignin. Biochemical factors such as amount of nitrogen, protein, soluble and insoluble sugars, total polyphenol, flavonoids, tannins and lignin in leaf and pod of cocoa were significant and negatively correlated with black pod disease. This suggests that as biochemical factors increase, susceptibility to black pod disease reduces. The biochemical contents in leaf and pod increased after P. megakarya infection, suggesting their role in resistance to Phytophthora species. Principal components analysis suggested that amount of epicuticular wax on pod and leaf surfaces, endocarp thickness, percentage moisture in leaf and pod husk, number of vascular bundles, epicarp thickness and number of cells in epicarp, cell width, insoluble sugar, flavonoid, tannin and lignin were the reliable parameters to select cocoa genotypes for resistance to black pod disease. In general, additive genes were more important than non-additive genes in the inheritance of resistance to black pod disease and its related morpho-physiological, histological and biochemical traits. Recurrent selection procedures and inter-crossing aimed at general combining ability would be more suitable for these traits.

Source: KNUST