Southern Leaf Blight (SLB) and Gray Leaf Spot (GLS) are important foliar pathogens of maize. The commonly used stiff stalk line B73 is susceptible to SLB, while the sister lines NC250 and NC250A, lines derived from crosses between temperate and tropical germplasm, show extremely high resistance. Two highly SLB-resistant B73 near-isogenic sister lines, NC292 and NC330, were derived from a B73 x NC250 cross followed by several backcrosses to B73 under recurrent selection for SLB resistance. A whole genome scan was undertaken to identify NC250 introgressions in NC292 and NC330 which are ³90% B73-like. Nine NC250 introgressions were identified, with some corresponding to regions where QTL were identified in the B73 x NC250A F_2:3 population. Determining the role each of the identified introgressions plays in SLB resistance is important. Lines with single or double NC250 introgressions were generated via additional backcrossing to B73. Near-isogenic lines (NILs) homozygous for single or double NC250 introgressions were selected through detailed genotyping in the BC₁F₁ and BC₁F₂ generations, and BC₁F₃ lines were phenotyped in a SLB field nursery. BC₁F₃ lines containing a NC250 introgression within the region encompassed by SSR markers phi099 and umc1025 on chromosome 3 were often among some of the most SLB resistant lines. A SLB QTL was identified in this region in this and other QTL studies. NC250 introgressions located within markers umc2246 and umc2363 on chromosome 2 and markers bnlg2305 and umc2307 on chromosome 5 were often associated with the more resistant lines. Additionally, a set of 89 BC₃F_2:3 NILs derived from a B73 x Tx303 cross, carrying assorted Tx303 introgressions in a B73 background, were selected and screened for GLS resistance. Results indicate that a Tx303 introgression on the distal end of the short arm of chromosome 1 is highly correlated with increased GLS susceptibility.