Abstract
Northern Leaf Blight (NLB), caused by Exserohilum turcicum, is one of the most important diseases affecting maize production worldwide. Bin 8.06 in the maize genome is known to be associated with resistance to NLB and several other diseases. Two qualitative resistance loci and several quantitative trait loci (QTL) for NLB resistance have been localized to this region. Evaluation of the nested association mapping (NAM) population, consisting of 5000 recombinant inbred lines developed from 25 diverse maize lines, identified a QTL in bin 8.06 (designated qEtS.06) as the largest-effect QTL conditioning increased incubation period and decreased disease severity of NLB. In response to a recurrent selection (RS) program for NLB, significant changes in allele frequencies also showed evidence of selection acting at markers in bin 8.06. Consistent detection of qEt8.06 in diverse mapping populations indicates that it is widespread in maize germplasm, and that it plays a substantial role in host-pathogen interactions. To dissect the genomic complex of qEt8.06, near-isogenic line (NIL) pairs contrasting for this locus were developed from heterogeneous inbred families derived from S11 x DK888. To characterize the resistance spectrum of qEt8.06 sub(DK888), NIL pairs were challenged with different races of E. turcicum, as well as a range of maize pathogens. Preliminary results suggest that qEt8.06 sub(DK888) confers racespecific quantitative resistance to NLB, but not to other diseases. Trait-marker association in a population of 6200 F sub(9) recombinants has delimited the resistance locus to a region of 65 Mb. High marker density in the NAM population also allowed mapping of qEt8.06 to a 7 Mb region, overlapping and narrowing the region to 63 Mb. Future work will focus on isolating the gene(s) underlying this major QTL.