THE INHERITANCE OF HOST-PLANT RESISTANCE AND ITS EFFECT ON THE RELATIVE INFECTION EFFICIENCY OF MAGNAPORTHE-GRISEA IN RICE CULTIVARS

The inheritance of host plant resistance and its effect on the relative infection efficiency for leaf blast was studied in the crosses 'IR36'/'CO39' (partially resistant x highly susceptible) and 'IR36'/'IR64' (both partially resistant). On the natural scale, gene action appeared multiplicative. After log transformation, additive effects described most of the genetic variation in the cross 'IR36'/'CO39', while additive and dominance effects were about equal in magnitude in the cross 'IR36'/'IR64'. Dominance was towards increased resistance. No transgressive segregation occurred in the cross 'IR36'/'CO39'. The number of genes that reduce lesion number was estimated to be zero in 'CO39' and five or more in 'IR36'. The cross 'IR36'/'IR64' showed transgressive segregation in both directions, and 'IR36' and 'IR64' each contain at least one gene that is not present in the other cultivar. The heritabilities (narrow sense) in the F-2 were low (range 0.06-0.16), while narrow sense heritabilities based on F-3 lines were much higher (range 0.41-0.68). Lesion numbers in F-3 lines were reasonably correlated with those in F-5 progenies derived from the same F-2 plant (r was +/- 0.6 in both crosses). Partial resistance can be effectively improved by selecting the most resistant plants from the most resistant F-3 lines.