Genetic Mapping of Gray Leaf Spot Disease Resistance Loci and the Elucidation of Mechanisms Underlying these Loci

Gray leaf spot is a foliar disease of maize caused by Cercospora zeae-maydis and Cercospora zeina.  Quantitative resistance to GLS is important for crop production.  The nested association mapping (NAM) maize population, consisting of 25 populations of recombinant inbred lines (RIL), was used to identify quantitative trait loci for GLS resistance.  Trials were conducted in Blacksburg, VA, on a field with high natural incidence of GLS.  Lines were evaluated for disease severity three times at seven day intervals for each of three years.  Seven of the 16 significant QTL displayed allelic series with significant effects above and below the B73 allele.  Additionally, nearly isogenic lines (NIL) were developed and tested to confirm and further fine-map selected QTL.  NIL analysis confirmed three of the loci as contributing significantly to disease resistance; the confirmed loci are in bins 1.04, 2.09 and 4.05.  The resistant alleles in bin 1.04 and 2.09 conferred reductions in disease of 12% and 23%, respectively.  The susceptible allele in bin 4.05 conferred an increase of 8.4%.  The 4.05 locus is also associated with distance between major veins. This disease-related venation trait was confirmed using the 4.05 NIL lines. Genome-wide association studies revealed candidate genes related to the production of carotenoids and anthocyanin, strong antioxidant compounds that may play a role in cercosporin detoxification.   Confirmation and further fine-mapping of quantitative disease resistance (QDR) loci provides breeders with valuable information that may enhance their ability to use molecular markers as a means to rapidly introgress loci providing quantitative disease resistance.