Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 16, 2000
Publication Date: January 31, 2001
Interpretive Summary: For U.S. seed production of perennial ryegrass and tall fescue, stem rust is the most important disease threat. Rust disease is managed with multiple applications of fungicides, commonly used at intervals of about 21 days and at considerable cost. Because severity of stem rust epidemics varies among years, some of these pesticide applications prove to be unnecessary. A weather-based disease advisory system therefore would be useful for managing stem rust on grass seed crops. The length of the latent period, that is the time between infection and release of a new generation of spores from that infection, is very important in determining how quickly a rust epidemic increases and spreads. Latent period length depends primarily on temperature. Experiments were conducted to determine the relationship between temperature and latent period for perennial ryegrass and tall fescue. A mathematical model for the relationship was developed, and it gave good estimates of latent period duration when tested in field experiments. The model has potential for use in a weather-based disease advisory system for stem rust management.
Technical Abstract: A temperature-response curve for latent period duration in stem rust (caused by Puccinia graminis subsp. graminicola) on perennial ryegrass and tall fescue was developed from constant-temperature experiments with inoculated plants, and was evaluated in field experiments. Under constant-temperature conditions, time from infection to 50% of pustules erupted (LP50) for perennial ryegrass ranged from 54 days at 3.5C to 5.9 days at 26.5C. LP50's for tall fescue were 69 and 8.5 days at these respective temperatures. The dependence of latent period completion rate on temperature was best described as a linear increase in rate with temperature up to approximately 26C, then an exponential decline with temperature up to the maximum (lethal) temperature of approximately 35.5C. LP20, the time required for 20% of open pustules to appear, was used as an estimator of latent period for field observations. Proportion of one latent period completed per half-hour, i.e. half-hourly rate, for perennial ryegrass was modeled as: (0.0156 T - 0.0206) (1 - exp (0.497(T - 35.5))). For tall fescue the modeled rate was: (0.0109 T - 0.00214) (1 - exp (0.417(T - 35.5))). Latent periods modeled by these equations were compared with observed latent periods in field experiments where half-hourly temperatures were measured. Linear regressions of modeled vs. observed latent periods had adjusted r2 values of 0.96 for perennial ryegrass and 0.93 for tall fescue. The latent period equations could be used as components of a weather-based disease advisory model to optimize fungicide use in stem rust management on these crops.