Freezing Temperature Effect on Survival of Puccinia graminis subsp. graminicola in Festuca arundinacea and Lolium perenne.

The Willamette Valley in Oregon is a major seed production area for cool-season grasses. Puccinia graminis subsp. graminicola survives over winter on its hosts as uredinial infections and causes epidemics of stem rust, the area's major disease on perennial ryegrass and tall fescue. To determine the possible importance of freezing temperature on rust survival, infected plants taken from the field were subjected to controlled freezing across a range of temperatures representative of those that can occur in the region. After treatment, plants were placed in a warm greenhouse, and the number of actively sporulating pustules was recorded at 3-day intervals for 21 days. The pathogen responded similarly to freezing treatments whether in perennial ryegrass or tall fescue. Compared with the nontreated standard, there was no significant reduction in pustule number after exposure to -3 or -6°C. Exposure of infected plants to -10°C caused a 75 to 90% reduction in rust survival, and exposure to -13°C killed all rust infections in tall fescue and over 99% in perennial ryegrass. The decline in rust survival with temperature was slightly steeper for perennial ryegrass than for tall fescue. A higher absolute number of infections in perennial ryegrass than in tall fescue resulted in higher numbers of surviving infections on perennial ryegrass. Survival of rust infections appeared to be primarily a function of host tissue survival. Between 1960 and 1997, years with winter temperatures as low as -10 or -13°C have occurred in the Willamette Valley with frequencies of approximately 39 and 8%, respectively. We conclude that year-to-year variation in winter temperature could have a significant effect on the survival of the grass stem rust pathogen.