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 × 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 F2 were low (range 0.06-0.16), while narrow sense heritabilities based on F3 lines were much higher (range 0.41-0.68). Lesion numbers in F3 lines were reasonably correlated with those in F5 progenies derived from the same F2 plant (r was±0.6 in both crosses). Partial resistance can be effectively improved by selecting the most resistant plants from the most resistant F3 lines.