Marker-assisted breeding of downy mildew resistant corn for the Philippines

Corn is the second most important(staple) crop in the Philippines. As a standard cultural management,seed-materials are pre-treated with metalaxyl fungicide prior to souring. Without fungicide seed treatment, and follow-up foliar spray, disease outbreak particularly caused by downy mildew(DM)fungus, Peronosclerospora philippinensis Weston (Shaw), can reduce corn produce to as high as 80-100% while the availability of metalaxyl-fungicide preparation is being constrained for large-scale application, the Philippines DM has been reported to develop resistance against this pesticides. DM resistance is available in corn germplasm. However,breeding for DM resistant and outstanding corn variety has not been successful through pure conventional means.The uses of biotechnology particularly DNA markers has proven efficient to breed for complex trait in major crops including corn e.g. drought tolerance and enhanced protein.The authors report the successful application of DNA marker technology to breed for DM resistant outstanding yellow corn variety for the Philippines. Corn microsatellite(SSR and EST-SSR)and resistance gene analog (RGA) markers wore successfully used to (a) purify parent lines, (b)re-establish genetic structure of recombinant inbred line (IRL) population for genetic mapping, (c)fine-map DM resistance loci(QTL),(d) develops DM resistance gene-specific markers, (e)approximate combining ability based on genetic distance, and (f)in combined marker-assisted selection breeding(MAB)schemes.Resistance QTL were validated in multilocation DM screening involving the major corn growing regions in the Philippines. Multi-location field resistance, resistance under controlled DM-nursery assay and foreground/background DNA markers were the main basis in selecting the final corn lines and test hybrids for yield trials. Best performing DM resistant line/hybrid will be released as a registered new corn variety and/or genetic stock. MAB scheme to introgress DM resistance into the white corn germplasm is proposed; as well as MAB pyramiding with the maize quality protein gene.