Modified Half‐Sib and Phenotypic Recurrent Selection for Resistance to Powdery Mildew in Winter Wheat

Abstract

Specific resistance to powdery mildew of wheat (Triticum aestivum L.), incited by Erysiphe graminis f. sp. tritici, has been associated with several major genes that are relatively easy to select in segregating populations. Nonspecific resistance is more difficult to select because of genetic and nongenetic factors that lower heritability. The objectives of this study were to: (i) determine the effectiveness of recurrent selection for accumulating genes for nonspecific resistance to powdery mildew in two winter wheat populations, and (ii) examine the effect of changes in the level of resistance on other components of resistance. Two winter wheat populations (designated Early and Elite) carrying a dominant male sterile gene were subjected to two cycles of recurrent selection for powdery mildew resistance using phenotypic recurrent selection and modified half‐sib family selection. Ten‐day‐old seedlings were inoculated in the greenhouse with a spore suspension consisting of equal amounts of four isolates possessing combined virulence that overcame all known powdery mildew resistance genes except Pm3b and Pm8. Beginning 7 d after inoculation, area under the disease progress carve (AUDPC) was determined four times at 4‐d intervals and used select individuals for the next cycle. The mean AUDPC on the primary leaves of S1 families and the realized gains after two cycles of selection indicated that the Elite population averaged 46% gain over the two selection methods compared to 35% gain for the Early population. Over two cycles, half sib selection gains of 40 and 53% were realized in the Early and Elite population, respectively, while phenotypic recurrent selection reduced AUDPC 30% in the Early population and 40% in the Elite population. For both populations, selection for low AUDPC resulted in significantly decreased final disease severity ratings, colony number, and cumulative sporulation capacity. Duration of the incubation period was significantly increased only with two cycles of half‐sib selection for AUDPC. A high correlation between AUDPC and the final disease severity rating indicated that the breeder could select for nonspecific resistance using a final disease severity rating provided the test materials are all inoculated simultaneously.