Agric. Res. Inst. Hung. Acad. Sci.
It means that additive gene action conditioning resistance to all diseases is of major importance in this set of diallel crosses. Generally, the present study indicated that both additive and nonadditive genetic effects are important in resistance of such diseases. Furthermore, the importance of additive genetic variation and the absence or relatively small magnitude of the nonadditive genetic effects for resistance to numerous diseases have been reported by several investigators. This indicates that most quantitative genetic resistance to maize diseases may have similar additive gene actions.
Estimates of general (GCA) and specific (SCA) combining ability effects are shown in Table 1. The GCA effects were significant for most inbred lines with all disease ratings. Resistance figure is of negative (-) direction because resistant reactions in entries were indicated by lower ratings or extremely resistant, whereas the higher ratings of positive (+) direction belonged to severe infections. Table 1 revealed that parental line 6 had the largest negative GCA effect for stalk rot followed in order by 5 and 4. By contrast, inbreds 1 and 2 had the largest positive GCA effects on susceptibility while inbred 3 had the least. Parental lines 3 and 4 appeared to have a negative GCA effect for ear rot, whereas parental lines 1, 4, 5 and 6 contributed positive effects. Parental lines 3 and 4 contributed negative GCA effects for common smut whereas the other parental lines contributed positive GCA effects. In head smut, parental lines 2, 3, 5 and 6 also contributed negative GCA effects, while lines 1 and 4 contributed positive effects.
Table 1. General GCA and specific SCA combining ability effects for the resistance to major diseases of six parental lines in maize averaged over two seasons.
The majority of the significant SCA effects for stalk rot and ear rot were negative; the exceptions were Sc12, Sc23, Sc24, SC46 and Sc56. The SCA effects were nonsignificant in the analysis of variance for common and head smuts, although a negative effect was indicated in some single crosses.
From the standpoint of breeding, each
parental line with negative GCA effect would be conducive to increasing
resistance to disease directly. It may be possible to find recombinants
of these genotypes with resistance to most diseases in a large population.
Once such a composite population is established, it can then be improved
for disease resistance by any suitable recurrent selection method.
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