Latent-Period Responses of Stem Rust in Tall Fescue Incubated at Four Temperatures

Stem rust, caused by Puccinia graminis Pers.:Pers. subsp, graminicola Z. Urban, has recently been found on tall rescue (Festuca arundinacea L.) grown for seed in western Oregon. Development of stem rust in the field is affected, to a large extent, by environmental conditions. We conducted controlled environment studies to determine the effect of incubation temperature on development of uredinia in five tall rescue cultivars. Differences in latent period (time from inoculation to urediniospore production) were observed for stem rust development in tall rescue incubated at 10, 18, 25, and 27 °C. Maximum number of erumpent pustules occurred 15, 12, 9, and 8 d after inoculation at 10, 18, 25, and 27 °C, respectively. A latent-period index (LPI) was developed to compare latent periods of stem rust among temperatures and cultivars. Average LPI was 17.4 at 10 °C 11.8 at 18 °C, 10.5 at 25 °C, and 10 at 27 °C. Values at 10 and 18 °C differed significantly from 25 and 27 °C, and from each other (P = 0.05, LSD 0.05 = 1.17). Average LPI was 11.9 for ‘Mesa’, 12.4 for ‘Fawn’, 12.6 for ‘Bonanza’, 12.6 for ‘Kentucky 31’, and 12.7 for ‘Adventure’. The LPI for cultivars did not differ significantly from each other (P = 0.05), and all cultivars were rated susceptible to stem rust. A significant cultivar × temperature interaction occurred, with some cultivars developing rust faster than others. For example, Fawn and Kentucky 31 had more plants developing pustules at earlier dates than did the other three cultivars. Conversely, Mesa usually had fewer plants with pustules than the other cultivars and usually required more time to develop pustules. Selection within tall rescue cultivars or sources of germplasm for resistance to stem rust should be done between 18 and 27 °C, to avoid falsely identifying plants as resistant when low temperature induces a delayed latent period. Selections for stem rust-resistant plants can be accomplished in less time if postinoculation incubation is done at high temperature. Joint contribution of the USDA-ARS, the Dept. of Botany and Plant Pathology, and the Dept. of Crop and Soil Science, Oregon State Univ. Contribution no. 9514 by the Oregon Agric. Exp. Stn., Corvallis.