The oversummering and dispersal of inoculum of Puccinia striiformis.

Stripe rust (Puccinia striiformis) was found on grasses Urediospores were trapped on rod spore samplers in wheat-growing regions and in adjacent mountains of exposed in the field only after stripe rust was Oregon during late spring only after the disease was well well established on the wheat crop. The number of spores established on the wheat crop. Disease incidence on these trapped followed closely the development of the disease grasses diminished during summer, and approached zero in wheat fields near the traps. Throughout northeastern during August. Stripe rust was seen less frequently on Oregon, urediospores moved predominantly from west to grasses in the mountains than on grasses in the wheatland. east. Most plants of susceptible grass species were free from The results suggest that urediospores which infect the stripe rust in the field, but became infected when fall wheat crop come from rust that oversummers on inoculated in the greenhouse. Commercial wheat plantings residual green wheat and grasses within the wheatland. served as the sources of inoculum for natural infections of wheat in plots located up to 64 km from wheat fields. Phytopathology 63:13-17 The source of stripe rust (Puccinia striiformis established wheat plots in the mountains at various West.) inoculum for fall infection of Oregon's wheat distances from wheatland (Fig. 1). Ten wheat crop has remained undetermined since the discovery cultivars, differing in their susceptibility to stripe of the disease there in 1916 (4). The fungus has no rust, were grown at each site (Table 2). Nearly all the known alternate host, so urediospores must serve as wheat grown commercially in northeastern Oregon primary inoculum. The critical period in the disease during this study were the cultivars Gaines, Moro, and cycle is the period between ripening of the wheat Omar. Druchamp was also grown in the Willamette crop in July and the emergence of the new crop in Valley. October (12). During this interval, there is very little Two plots consisting of a 2-m row of each cultivar green wheat in Oregon because of the dry summers, were established at each location in the fall. Poultry We occasionally found stripe rust on ripening wheat wire cages (2 m X 6 m X 1.5 m), with rodent shields and residual green wheat during July and early August (12). The earliest when stripe rust was found on the fall-sown crop was in December, and then only Wheot-. Growing Area in early-sown fields. Since apparently little stripe rust oversummers on wheat in Oregon, we conducted investigations to determine whether rust oversummers on hosts other than wheat or whether inoculum moves in from some distant source. MATERIALS AND METHODS. -Wheat in northeastern Oregon is grown in three areas separated • . '" by mountains (Fig. 1). These are the Columbia Basin, . rx i . ;:a _:i. consisting of Wasco, Sherman, Gilliam, Morrow, and -. ,". •I . : Umatilla Counties; the grande Ronde Valley of '. o, . I, L-_, '. -I adjacent mountains. Observations were also made in ) i"i'" . iI the Cascades, which separate western and eastern L l i I". " ' Oregon, in the Willamette Valley, and in the Coast -"-)--I-. :. ', j_ . Mountains. Stripe rust collected from grasses or wheat was Fig. 1. Wheat-growing areas of Oregon and location of the tested in the greenhouse for pathogenicity on Mountain plots: 1 =Mary's Peak, 975 m; 2 =Tupper, 1,311 Oregon's commercial wheat cultivars and grasses. in; 3 = Dale, 1,615 m; 4 = Meachum, 1,219 in; and 5 To study long-distance spore dispersal, we Tollgate, 1,524 m.