Introduction

Stripe rust, caused by Puccinia striiformis West., is the most
important disease of wheat in western North America, and it has become
more important in the South-Central United States since 1984. The
disease was first recognized in North America in 1915 by F. Kolpin
Ravn, a visiting scientist from Denmark, who was traveling in the West-
ern United States with a U.S. Department of Agriculture crop survey
team (Carleton 1915). Once recognized, the disease was detected
throughout the Western United States in 1915 and in Canada within a few
years (Humphrey et al. 1924, Johnson and Newton 1928). Later, H.B.
Humphrey (Humphrey et al. 1924), after examining herbarium specimens,
determined that P. striiformis had been present in western Washington
for at least 23 years before it was recognized. Early plant
pathologists were aware of the destructiveness of stripe rust in Europe
and Asia and were concerned about its possible spread to the major
wheat-growing regions in the United States and central Canada
(Hungerford 1923, Humphrey et al. 1924, Newton and Johnson 1936).
Consequently, several major studies were conducted during the 1920's.
However, the only reports on the pathogenicity of P. striiformis in
North America were on its host range and on the resistance of local
cultivars. Hungerford and Owens (1923) reported that Bromus sterilis
was resistant to stripe rust collections from Hordeum jubatum but not
to those from Elymus glaucs and Bromus marginatus. That report
indicated the existence of races, or strains, of P. striiformis.

Newton et al. in 1932 (Newton et al. 1933) and Bever in 1933 (Bever
1934) each isolated two physiologic forms (races) of P. striiformis
using differential wheat cultivars. Newton and Johnson (Newton and
Johnson 1936) reported that their two races were similar to the race
from Montana and the race from Idaho reported by Bever. The race from
Montana was virulent on Chinese 166 and Red Russian and avirulent on
Heines Kolben. The collection from Moscow, Idaho, was virulent on
Heines Kolben and avirulent on the other two cultivars.

No reports were published on the virulence of P. striiformis in North
America for the next 25 years. Concern about stripe rust diminished,
the disease was no longer considered important in North America, and
research projects on stripe rust were eliminated. But the situation
changed drastically in the late 1950's and early 1960's, when severe
epidemics caused massive losses in California and the Pacific Northwest
(Shaner and Powelson 1971, Tollenaar and Houston 1967). These
devastating effects of stripe rust led to an increased emphasis on
breeding for resistance to the rust. Purdy and Allan (Purdy and Allan
1963) observed that at least three stripe rust races existed in
Washington. Additional races were subsequently identified in the
Western United States (Purdy and Allan 1966; Beaver and Powelson 1969;
Line 1972, 1976, 1980, 1984; Volin and Sharp 1973).

The races detected in the early 1930's were identified on a set of
differential cultivars that had been used in Europe (Gassner and Straib
1932). But that set did not effectively differentiate the predominant
races of P. striiformis in North America. Therefore, a set of
differential cultivars for the United States was selected in 1969, and
a uniform system of describing and naming the races was agreed on by
those working on stripe rust in the United States (Line et al. 1970).
Since then, the system, with some modification, has been used to
monitor the virulence and distribution of pathotypes (races) in North
America. Additional differential cultivars were added as new races
appeared (Line 1972, 1976, 1980). This report summarizes the results of
more than 20 years of research on the differentiation, virulence, and
aggressiveness of races of P. striiformis in North America; detection
of the races; geographic distribution of the races; interrelationships
of the races; and movement of inoculum from region to region.