Abstract:
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.
