Inheritance of resistance to brown spot disease in upland rice in Uganda

Brown spot disease caused by Bipolaris oryzae [Breda de Haan (Shoem.)] is one of the most important diseases affecting rice (Oryza sativa L.) worldwide. Host plant resistance is considered an effective, cheap and environment friendly means of managing this disease. Nine rice genotypes with varying resistance levels were crossed in a full diallel mating design including reciprocals and parents. Parents, reciprocals and F2 progenies were evaluated in an alpha lattice design in the screen house and field trials at the National Crops Resources Research Institute in Uganda in 2013-2014. The objectives of the study were to determine the mode of inheritance for resistance to brown spot disease and characterize segregation patterns of specific F2 progenies. Significant (P ≤0.001) variation for brown spot resistance occurred among the tested genotypes. The general combining ability (GCA) and specific combining ability (SCA) effects of brown spot disease scores were both significantly different (P≤0.001), indicating that both additive and non-additive genetic effects were present. There was, however, a predominance of non-additive genetic effects in the genetic control of brown spot resistance as shown by low estimates of baker’s ratio (0.29) and narrow sense coefficient of genetic determination (0.24), implying that progeny performance could not be predicted from parents GCA effects as it was better only in specific crossing combinations. Segregation patterns also indicated that resistance to brown spot was controlled by one or two dominant genes. The reciprocal effects for the crosses were significantly different (P ≤ 0.05), suggesting that cytoplasmic genetic effects modified the expression of resistance. Care should, therefore, be taken when selecting female parents during hybridization. Family-based breeding programs would also be effective for improving resistance to brown spot in rice varieties adapted to Uganda.   Key words: Diallel analysis, gene action, non-additive effects, Oryza sativa, segregation patterns