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The development of resistant lines and hybrids is an economical way to control disease and improve yield stability. The objectives of this study were (i) to investigate if differences in resistance to gray leaf spot (GLS, caused by Cercospora zeina) exist among recombinant inbred lines (rILs) with and without the quantitative trait locus encompassing the resistance-carrying GZ204/IDp5 DNA segment (rDNAS) and to determine its effect on grain yield, and (ii) to determine general combining ability and specific combining ability effects for grain yield and GLS scores (GLSS). Four rILs (three with rDNAS [rL1_1, rL1_2, and rL2_1] and one without rDNAS [rL2_2]) were developed via marker-assisted selection from a cross between YML32 and Q11—an elite line susceptible to GLS. The four rILs and the susceptible parent (Q11) were crossed as testers with 13 maize (Zea mays L.) lines of known heterotic groups (Suwan1, reid, and non-reid). The three rDNAS-carrying rILs showed reduced GLSS and improved grain yield stability, but grain yield itself was not significantly increased. These three rILs also showed negative general combining ability effects for GLSS. rL2_1 was the best line for improving GLS resistance. The rILs possessing the rDNAS in crosses with lines from the Suwan1 heterotic group had lower GLSS than those from reid and non-reid heterotic groups, suggesting that resistance genes or quantitative trait loci, in addition to rDNAS, might be present in Suwan1. Z.W. Li, Yunnan Agricultural Univ., Kunming, Kunming 650201, Yunnan Province, China; L. Liu, Y.D. Zhang, and X.M. Fan, Institute of Food Crops, Yunnan Academy of Agricultural Sciences, Kunming 650205, Yunnan Province, China; D.P. Jeffers, CIMMYT Yunnan Office/Institute of Food Crops, Yunnan Academy of Agricultural Sciences; M.S. Kang, Dep. of Plant Pathology, Kansas State Univ., Manhattan, KS 66506-5502. Received 11 Apr. 2017. Accepted 14 Jan. 2018. *Corresponding author ([email protected]). Assigned to Associate Editor Leah Mchale. Abbreviations: GCA, general combining ability; GLS, gray leaf spot; GLSS, gray leaf spot scores; GY, grain yield; MAS, marker-assisted selection; NCII, North Carolina Design II; PCR, polymerase chain reaction; QTL, quantitative trait locus/loci; RDNAS, resistancecarrying GZ204/IDP5 DNA segment; RIL, recombinant inbred line; SCA, specific combining ability; SS, sum of squares. Published in Crop Sci. 58:1070–1081 (2018). doi: 10.2135/cropsci2017.04.0228 © Crop Science Society of America | 5585 Guilford Rd., Madison, WI 53711 USA This is an open access article distributed under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Published April 5, 2018