Stem rust in perennial ryegrass seed crops: epidemiological and genetic research at USDA-ARS

In production of perennial ryegrass seed in the Northwest USA, stem rust is the primary disease constraint in terms of management cost and potential yield loss. The USDA-ARS research program on this problem is focused on improved decision making for fungicide use, and tools for genetic improvement. Data from 10 years of field and greenhouse research was used to construct an epidemiological model. This model is based on the effects of weather (temperature, leaf wetness) on plant growth and pathogen activity, and includes the effects of two major fungicide classes (triazoles and strobilurins) on disease development, particularly on the spread of disease within a plant. The model is implemented as a decision aid on a publicly-accessible internet site. Users enter scouting data and select a location for weather data to be loaded into the model. The website displays the model results, including the relationship of modeled disease development to an action threshold. The decision aid has performed well in large-scale field demonstration plots across 10 location-years, providing an average economic advantage of $39 per acre (range: $0 to $140) compared to standard fungicide programs. In genetic research, a perennial ryegrass mapping population was constructed from a cross between resistant and susceptible parents. The progeny (193 individuals) were tested by inoculating with two different pathotypes of the stem rust fungus, in separate experiments. Using a genetic map of these parents constructed with SSR and RAD markers, QTL analyses were conducted. The goal of this genetic research is to produce genetic markers for marker-assisted selection of stem rust resistance, as well as to provide information about genomic organization of disease resistance in perennial ryegrass. Stem rust on perennial ryegrass and tall fescue in the Pacific Northwest USA In perennial ryegrass (Lolium perenne) and tall fescue grown for seed, the economically most important disease in major seed production areas of the United States is stem rust caused by Puccinia graminis subsp. graminicola (Welty & Azevedo, 1984) Perennial ryegrass seed yield losses up to 98% due to stem rust damage have been recorded (Pfender, 2009a). The disease is also a significant production constraint in New Zealand, where yield losses of 35% were documented (Hampton, 1986). Since the 1980s, there has been a research program at the USDAARS Forage Seed Research Laboratory in Corvallis, OR, USA. Starting in the late 1990s the research focused on developing an epidemiological model that could be used in a decision