Quantitative trait loci (QTLs) for resistance to gray leaf spot and common rust diseases of maize

Gray leaf spot and common rust diseases can greatly reduce grain yield of maize in susceptible genotypes by between 10 and 70% on average. Control of these diseases through conventional measures has been quite ineffective and difficult to sustain. The most feasible way to control them is by breeding and deploying resistant maize genotypes. This study was carried out to evaluate germplasm for QTLs associated with resistance to GLS and common rust diseases by use of microsatellite markers and artificial inoculation with the two pathogens. A total of 41 genotypes comprising 23 recombinant inbred lines, 14 parental inbred lines and 4 hybrid checks were screened using 28 SSR markers and disease pressure by artificial inoculation (Tables 1 and 2). Out of the 14 parental inbred lines, only 4 were found to carry the QTL associating positively for the two diseases, and 10 out of the 23 recombinant inbred lines with possible lineage from any of the 14 parents, were positively associated with the traits and seven of the markers used (Table 3). GLS QTLs were significant for two markers; bnlg1258 with a LOD score of 16.0 and umc2019 with a LOD score of 17.9 from regions 2.06 and 2.08 of chromosome 2, respectively. Significant QTLs for common rust resistance were identified in three regions of chromosome 10, corresponding to markers phi054 with a LOD score of 14.0 at bin 10.00, umc1319 with a LOD score of 4.0 at bin 10.02 and bnlg1451 with a LOD score of 14.3 at bin 10.03. The effects of these QTLs were different from genotype to genotype. The disease severity scores (scale of 1-5) of artificial inoculation ranged from 1.5 to 2.5 for gray leaf spot with a mean of 1.88 and a range of 1.5 to 3.0 with a mean of 1.74 for common rust. All the inbred lines scored better for the two diseases than the four hybrid checks which scored 3.5. The most resistant genotypes showed a score of 1.5 for gray leaf spot and common rust. All parents showed a score of less than 2.5 for GLS and common rust. Parental genotypes MAL40, MAL9, MAL13, MAL41, MAL11, MAL19, MAL23-2, MAL24, and MAL19-1, carried QTLs associated with resistance to grey leaf spot and common rust and thus were identified as sources of resistance conferred to the inbred lines. The selected lines are being used to make single hybrids, double crosses, three way hybrids and synthetics resistant to diseases. The marker data was also used to analyze the diversity of the genotypes studied, with relevance to immune/resistant, tolerant or susceptible to the two diseases. Using the GLS genotypic data, 13 genotypes clustered into 11 groups, and using the common rust data, the 13 genotypes clustered into 12 clusters. This indicates that almost each of these genotypes was grouped in a cluster that contained lines that did not have positive association of marker and trait data. These results indicated that the putative QTLs for GLS are associated with the 13 genotypes and two markers in chromosome 2 (bnlg1258 and umc2019), whereas those of common rust are associated with the 13 genotypes and three markers on chromosome 10 (phi054, umc1319, and bnlg1451).