Mapping and confirmation of QTL for resistance to gray leaf spot in perennial ryegrass

Gray leaf spot (GLS) is a serious disease caused by the fungus Pyricularia grisea, recently reported on the important turfgrass species, perennial ryegrass (Lolium perenne L.). This fungus also causes blast in rice, which is usually controlled by host resistance, though resistance durability is problematic. Most perennial ryegrass cultivars are susceptible to GLS. However, greenhouse inoculations using one ryegrass isolate and one rice-infecting strain revealed partial resistance in an Italian x perennial ryegrass mapping population. A linkage map of this population was used to identify quantitative trait loci (QTL) for GLS resistance. QTL were detected on four linkage groups (LGs), and resistance to the different strains were associated with different QTL. Of three QTL detected using the ryegrass isolate, the one with strongest effect was located on LG3 of the MFB parent, explaining 20%-37% of the phenotypic variance. Another QTL was detected on LG6 of the MFA parent, explaining 5%-10% of the phenotypic variance. For resistance to the rice strain, two QTL were located on LGs MFA-2 and MFB-4, each explaining about 10% of the phenotypic variance. Further, QTL on LGs 3 and 4 appeared syntenic to blast resistance loci in rice.; To confirm that QTL from the MFAxMFB population are still detected in the same locations in the next generation, a resistant segregant from that population was crossed with an unrelated susceptible perennial clone, to form a new mapping population segregating for GLS resistance. QTL analysis was performed, using two different ryegrass field isolates and RAPD, RFLP, and SSR marker-based linkage maps for each parent. Results indicate the QTL on LG3 of the resistant parent is still significant, with LOD and percent of phenotypic variance explained ranging from 2.0 to 4.2 and 6% to 11%, respectively. Two new QTL were detected in the susceptible parent, with similar LOD and phenotypic variance explained. Although the LG6 QTL was not detected, the major QTL on LG3 was confirmed. The ultimate goal of these studies is understanding the genetic architecture of GLS resistance, facilitating its utilization in perennial ryegrass breeding via marker-assisted selection, and benefiting growers of perennial ryegrass.