Resistance to Gray Leaf Spot in St. Augustinegrass: Characterization and QTL Mapping.

Gray leaf spot [GLS, causal agent; Magnaporthe grisea (T.T. Hebert) Yaegashi & Ugadawa] is a major disease of St. Augustinegrass [Stenotaphrum secundatum (Walt.) Kuntze]. Plant introduction 410353 was identified as a source of resistance to the fungus. Incorporation of this resistance into commercial cultivars could be expedited through the use of closely linked molecular markers. In the present study, genetic linkage maps of St. Augustinegrass were constructed for quantitative trait loci (QTL) analysis of GLS resistance using a total of 224 AFLP fragments and 125 SSRs on a pseudo F₂ population of 77 progeny. Fourteen linkage groups were identified, covering 1662.6 cM. Four linkage groups were ascribed to the susceptible maternal cultivar, ‘Raleigh’, three linkage groups to the resistant paternal accession, PI410353, and seven groups were found to be homologous.  The Raleigh (64 markers) and PI410353 (21 markers) maps covered 369.2 cM and 267.2 cM, respectively, while the homologous linkage groups consisted of 264 markers covering 1026.2 cM.  The average marker density across all groups was 4.76 cM/marker.  Four putative QTL, two associated with both AUDPC and AULEC and two with AUDPC only, were identified through interval and composite interval mapping.  These results present the first linkage map produced for St. Augustinegrass, providing a template for further genetic mapping. Additionally, markers linked to the QTL identified in this study can be used by breeders to transfer resistance to gray leaf spot from unadapted germplasm to elite breeding lines and cultivars.