A genetic study of the relationship between height, time of ear emergence and resistance to Septoria nodorum in wheat

Winter wheat cultivars of varying height were intercrossed and the progeny were studied in field trials to determine whether the greater resistance to S. nodorum of the taller cultivars was fortuitous or the result of genetic association. Variation in height, in resistance and in time of ear emergence was continuous, providing no evidence of the existence of individual genes of major effect. In F 2 and backcross generations, the inheritance of all three characters was principally additive. Among random F 3 and F 4 families there was a clear tendency for tall straw and resistance to segregate together, proving that the association between these characters was not fortuitous and providing evidence of linkage or pleiotropy in their genetic control. There was a similar though less consistent tendency for late ear emergence and resistance to segregate together. There was substantial residual variation in resistance, not associated with variation in height or time of ear emergence, but its heritability was difficult to prove by comparisons between F 3 and F 4 generations, perhaps partly because of genotype x environment interaction. It is proposed that resistance is under polygenic control and pleiotropy rather than linkage is suggested as the explanation of its genetic association with height and with time of ear emergence. The existence of genes affecting resistance independently of height and time of ear emergence is also inferred. It is suggested that they permit the breeding of resistant cultivars of any height, though with increasing difficulty as height is reduced.