Evaluation of St. Augustinegrass (Stenotaphrum secundatum (Walt.) Kuntze) Germplasm for Gray Leaf Spot Resistance.

Gray leaf spot (GLS) of St. Augustinegrass is caused by Magnaporthe grisea (T.T. Hebert) Yaegashi & Ugadawa and occurs throughout the southeastern United States. Under favorable conditions, disease develops rapidly and entire grass swards can be killed within a few days. Cultural management practices often do not provide adequate control due to  rapid development of the disease. Development of cultivars with genetic resistance to the fungus would be a potential management alternative. The objective of this investigation was to screen 62 St. Augustinegrass accessions for response to M. grisea in order to identify resistant genotypes for use in cultivar development.  All experiments were performed under controlled environmental conditions at the Southeastern Environment Laboratory, Raleigh, NC. The parameters evaluated were incubation period, final disease incidence, final mean lesion length, and derived parameters such as disease progress rate, lesion expansion rate, area under the disease pressure curve (AUDPC), and area under the leaf expansion curve (AULEC). Polyploid and diploid genotypes with high levels of resistance to the fungus were identified. While polyploid resistant materials have been previously identified, this is the first report of resistance to GLS being observed among diploid materials. The identification and utilization of resistance genes in diploid genotypes is of great importance for future St. Augustinegrass breeding efforts, as this germplasm pool can be more readily exploited because of the lack of reproductive barriers with most commercial cultivars and plant introductions.