GENETIC ANALYSIS OF RESISTANCE TO SOYBEAN RUST DISEASE

Soybean rust (Phakopsora pachyrhizi Sydow.) causes the most damage of all the pathogens known to attack soybean (Glycine max. Merril). A study was conducted in Uganda to estimate the magnitude of genetic parameters controlling soybean rust resistance and to estimate narrow sense heritability of the resistance. Soybean crosses were made and progenies analysed according to the North Carolina II mating design with three resistant parents acting as males namely Maksoy 1N, UG5 and GC00138-29; and three susceptible parents acting as females namely Nam 1, Kabanyolo 1 and Wondersoya. F1s and F2s were planted in the field during two rainy seasons (2004 - 2005). Rust severity was scored using a scale of 0 – 10. Genetic parameter estimates were VE = 0.86186, VD = 0.3 0145, VA = 0.4616, VP = 1.6248 and VG = 0.763. Resistance gene for rust expressed complete dominance with checkmark4VD/ 2VA = 1.1. Broad sense heritability (hb²) was 0.5, while narrow sense heritability, hn², derived from parameter estimates as a ratio of Additive Variation to Phenotypic variation (VA/VP), as 0.3. Environmental variation contributed most (53%) to total variation (VP) due to high “within families component of variance, VE” at F2 generation, which accounts for the low heritability estimates. General Combining Ability (GCA) and Specific Combining Ability (SCA) were significant with the parent GC00138-29 having the lowest GCA value (-0.488) making it the best combiner that can be used in breeding programs for resistance to soybean rust. The cross GC00138-29 x Wondersoya had the lowest SCA value (-0.18) suggesting that it would produce the most highly resistant hybrid offsprings.