Agric. Res. Inst. Hung. Acad. Sci.

Combining ability for resistance to stalk rot, ear rot, common smut and head smut diseases --M. Odiemah and I. Kovacs Six inbred lines of maize were evaluated in a diallel set of 15 single crosses to study the inheritance of resistance to stalk rot, ear rot, common smut and head smut diseases by estimation of GCA and SCA effects. The entries were evaluated under artificial-natural infection at Martonvasar. Ratings for stalk rot and ear rot were recorded during harvest on individual plants per plot according to a scale from 1 to 9, while data of common and head smuts were based on the percentage of infected plants of fully grown plants for each plot. These entries were grown in an experiment of a randomized complete block design with four replications of two-row plots. Each row was 6m long and contained 10 hills (plants). The inter-row and intra-row distances were 70 and 30 cms, respectively. The analysis of variance was performed on plot means over two seasons. Partitioning of F1 hybrids into GCA and SCA effects was important for resistance to all diseases with the exception of common and head smuts where the SCA effects were not significant, GCA mean squares were substantially larger than SCA mean squares.

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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