Assessing the Genetic Diversity of Resistance to Crown Rust in Perennial Ryegrass.

Crown rust (Puccinia coronata f.sp. lolii) is a common pathogen on perennial ryegrass (Lolium perenne L.), a perennial cool-season turf and forage grass. Infection with crown rust results in turf stands that are not only aesthetically unappealing, but are also more highly susceptible to other biotic and abiotic stresses. In grass seed production, infection by crown rust may result in a reduction in yield if the affected area is not treated with fungicide. The most effective method of controlling rust is through durable genetic resistance; however, in order to breed for resistance to the pathogen, resistance in breeding material as well as wild germplasm must first be evaluated. Additionally, understanding differences in rust response between plants in seed production and turfgrass systems would assist breeders in developing cultivars that perform well in both situations.

This study evaluated germplasm from the USDA National Plant Germplasm System (NPGS) and the University of Minnesota perennial ryegrass breeding program for rust resistance in a field setting at two locations (Becker and St. Paul, Minnesota). The study was a split plot with the whole-plot factor (mowed vs. unmowed spaced plants) in a completely randomized design with four replications. Mowed and unmowed treatments represented a turfgrass and seed production system, respectively.  Forty-eight accessions (sub-plot) comprised of current breeding material, released cultivars, and collections from the USDA NPGS were included in the study. Rust infection was severe at both sites in 2009 and 2010. Results from 2009 showed differences in rust severity across accessions. Differences in mowing treatment were observed during August and September when the disease was most severe. During these months, rust severity was reduced in the mowed treatments. Both the most resistant and most susceptible accessions performed similarly regardless of mowing treatment. Improved germplasm showed varying levels of resistance from highly resistant to moderately susceptible. Of the 10 most resistant accessions, five were originally collected in Romania.