Theor Appl Genet DOI 10.1007/s00122-008-0949-2 123 ORIGINAL PAPER Use of selection with recurrent backcrossing and QTL mapping to identify loci contributing to southern leaf blight resistance in a highly resistant maize line John C. Zwonitzer · David M. Bubeck · Dinakar Bhattramakki · Major M. Goodman · Consuelo Arellano · Peter J. Balint-Kurti Received: 13 June 2008 / Accepted: 4 December 2008  Springer-Verlag 2009 Abstract B73 is a historically important maize line with excellent yield potential but high susceptibility to the foliar disease southern leaf blight (SLB). NC292 and NC330 are B73 near-isogenic lines (NILs) that are highly resistant to SLB. They were derived by repeated backcrossing of an elite source of SLB resistance (NC250P) to B73, with selection for SLB resistance among and within backcross families. The goal of this paper was to characterize the loci responsible for the increased SLB resistance of NC292 and NC330 and to determine how many of the SLB disease resistance quantitative trait loci (dQTL) were selected for in the development of NC292 and NC330. Genomic regions that diVerentiated NC292 and NC330 from B73 and which may contribute to NC292 and NC330s enhanced SLB resis- tance were identiWed. Ten NC250P-derived introgressions were identiWed in both the NC292 and NC330 genomes of which eight were shared between genomes. dQTL were mapped in two F 2:3 populations derived from lines very closely related to the original parents of NC292 and NC330—(B73rhm1 £ NC250A and NC250A £ B73). Nine SLB dQTL were mapped in the combined populations using combined SLB disease data over all locations (SLB AllLocs). Of these, four dQTL precisely colocalized with NC250P introgressions in bins 2.05–2.06, 3.03, 6.01, and 9.02 and three were identiWed near NC250P introgressions in bins 1.09, 5.05–5.06, and 10.03. Therefore the breeding program used to develop NC292 and NC330 was highly eVective in selecting for multiple SLB resistance alleles. Introduction The rapidly growing demand for ethanol as a fuel source has increased the demand for maize (Aho 2007; WesthoV et al. 2007). This increased demand heightens the impor- tance of maximizing and stabilizing yields by, among other things, increasing disease resistance. Most disease resis- tance used in maize (Zea mays L. ssp. mays) is quantitative rather than qualitative in nature. However, very little is known about the physiological or molecular genetic basis of quantitative disease resistance. Southern leaf blight (SLB), causal agent Cochliobolus heterostrophus (Drechs.) Drechs. (anamorph = Bipolaris maydis (Nisikado) Shoemaker), is a widespread disease Communicated by T. Lübbersted. Electronic supplementary material The online version of this article (doi:10.1007/s00122-008-0949-2) contains supplementary material, which is available to authorized users. J. C. Zwonitzer Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695-7616, USA D. M. Bubeck Pioneer Hi-Bred International, Inc., Johnston, IA 50131, USA D. Bhattramakki Pioneer Hi-Bred International, Inc., Dallas Center, IA 50063, USA M. M. Goodman Department of Crop Science, North Carolina State University, Raleigh, NC 27695, USA C. Arellano Department of Statistics, North Carolina State University, Raleigh, NC 27695, USA P. J. Balint-Kurti (&) U.S. Department of Agriculture-Agricultural Research Service (USDA-ARS) Plant Science Research Unit and Department of Plant Pathology, North Carolina State University, Raleigh, NC 27695-7616, USA e-mail: Peter.Balint-Kurti@ars.usda.gov