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Near‐isogenic lines differing in resistance genes were developed to study the effects of genes in barley ( Hordeum vulgare L.) giving specific resistance to powdery mildew ( Erysiphe graminis DC f.sp. hordei Em. Marchal). This paper describes breeding techniques, degree of similarity between isolines, and resistance genes present in the isolines. Twenty‐four donors of resistance genes were crossed and backcrossed five times to the spring barley cv. ‘Pallas’, and homozygous resistant BC 5 F 2 plants were selected. Resistant lines were evaluated and selected for phenotypic similarity with the recurrent parent in the BC 5 F 3 , through BC 5 F 7 generations. To select for genetic similarity in the region of the resistance locus Ml‐a, two linked hordein loci, Horl and Hor2 , were studied in the BC 5 F 4 generation. The resulting 24 near‐isogenic lines originated from single BC 5 Fs 4 plants in each of 24 different progenies. With a few exceptions, the near‐isogenic lines were similar to the recurrent parent in morphological and physiological characteristics. Tests with the hordein genes indicated that double crossing‐over occurred very close to the resistance locus Ml‐a in five of the isogenic lines. With 22 powdery mildew isolates, 22 lines were similar to their donors in resistance reaction, while two lines gave reactions that indicated that they have lost a second resistance gene. The 24 near‐isogenic lines comprised 14 lines with one resistance gene each, and 10 lines with two or more resistance genes. All the isogenic lines differ in resistance. The degree of genetic similarity between the 24 isogenic lines and the recurrent parent Pallas is high according to the morphological and physiological observations and may be even higher than theoretically expected in lines with double recombination in the region of locus Ml‐a. These near‐isogenic barley lines are recommended for both practical and theoretical research work in the barley‐powdery mildew system.
Kølster et al. (Mon,) studied this question.