Geographic Distribution of Races
The seven geographic regions of North America (fig. 4) are determined
based on geographic barriers, prevailing winds and other weather, crop
management, and occurrence and virulence of races of Puccinia striiformis.
Region 1 has the greatest number of races (fig. 4) and the races with the
greatest range of virulence, i.e., virulence on the greatest number of
differential cultivars in North America (table 4). Of the 39 races
identified in North America, 31 occur in region 1; of those 31, 26 were
first detected in the region. Of the five races that were first
detected in other regions, CDL-8, CDL-16, and CDL-18 were detected in
region 5, CDL-25 was detected in region 4, and CDL-6 was detected in
region 6. Based on the characteristics of the regions, it is deduced
that CDL-6, CDL-8, CDL-16, and CDL-18 probably evolved independently in
region 1 and were not introduced from other regions. CDL-25 could have
been introduced from region 4, but all available evidence indicates
that it evolved independently in region 1. Because region 1 is bordered
on the west and east by mountains and the prevailing winds are from the
southwest, the probability of the introduction of spores from other
regions is low.
Region 1 is an important wheat-growing region with an environment that
is favorable for rust survival in winter and rust development in
spring. During the last 30 years, 3 out of every 4 years have been
favorable for severe rust epidemics in fields of susceptible cultivars
in region 1. In addition, cropping practices favor the survival and
development of rust in region 1. All but one of the major classes of
wheat are grown in region 1. Soft white winter wheat and soft white
spring wheat are grown primarily in the eastern third of the region
(which has the region's highest rainfall) and in irrigated fields
throughout the region. Hard red winter wheat is grown primarily in the
lower-rainfall areas in the central part of the region. Hard red spring
wheat is grown in both the low-rainfall areas and under irrigation in
the central and western parts of region 1. Club wheats are grown
primarily in intermediate-rainfall areas bordered on the west by the
hard red winter wheat and on the east by the soft white
winter wheat. Spring durum wheat is grown by a few farmers, usually
under contract. Under drier conditions, winter wheat is planted in
August and September; under wetter conditions, it is planted in October
and November. Spring wheat is planted in March or April. Thus
management of these wheats provides a continuous green host for the
survival and development of rust in region 1.
The distribution and prevalence of the races of P. striiformis in
region 1 are primarily related to susceptibility of the cultivars. The
races that are virulent on Moro, Paha, Lee, and Tyee have been
consistently most prevalent in the low-to-intermediate rainfall areas
where susceptible club wheats and hard red winter wheats are grown.
Races virulent on Fielder, Druchamp, Produra, and Stephens have been
consistently most prevalent in the southern half of region 1, where
Stephens is most commonly grown. Races virulent on Fielder but not
Druchamp, Produra, and Stephens are prevalent in the northern half of
the region, especially where Daws is grown. Races virulent on Lemhi and
Heines VII remain common throughout the region. Races virulent on
Yamhill are occasionally detected, and races virulent on Chinese 166
are very rare.
Table 4. Virulence and avirulence of races of Puccinia striiformis from
seven regions of North America on differential wheat cultivars.
_____________________________________________________________________________
North Differential wheat cultivar number
America
_____________________________________________________________________
region1 1 2 3 4 5 6 7 8 9 10 11 12 13
_____________________________________________________________________________
1 V3 V V V V V A3 V V V V V V
2 V V V V A V A V V V A V A
3 V A V A A V A V A V A V A
4 V V V V A V A V V V V V A
5 V V V V A V A V V V V V A
6 V V V A A V A V A V A V A
7 V A V A A A A A V A A A A
_____________________________________________________________________________
1 See fig. 4 for map of regions.
2 1 = Lemhi, 2 = Chinese 166, 3 = Heines VII, 4 = Paha, 6 = Druchamp,
7 = Riebesel 47-51, 8 = Produra, 9 = Yamhill, 10 = Stephens,
11 = Lee, 12 = Fielder, 13 = Tyee.
3 V = virulent, A = avirulent.
Regions 2 and 3 are somewhat isolated from other regions to the west,
and generally their winters are usually unfavorable for the survival of
stripe rust. Consequently, severe epidemics are less frequent in those
regions. All races of P. striiformis identified from 1968 to 1987 in
regions 2 and 3 were first detected in region 1. Races virulent on
Lemhi, Heines VII, Druchamp, Produra, Stephens, and Fielder (table 4)
are prevalent in both regions. Virulences on Chinese 166, Moro, and
Yamhill are present in region 2 but not region 3. CDL-5 was prevalent
in region 2 in the late 1960's and early 1970's, but its prevalence
decreased when Crest wheat was no longer grown. CDL-5 or other races
that are virulent on Moro have never been detected in region 3 even
though many of the hard red winter wheat cultivars in that region are
susceptible to them. All data on the races indicate that the races in
regions 2 and 3 originated in region 1. When rust is severe in region
1, rust epidemics often occur later in the season in region 2, and the
races that are detected are the same as those that occurred earlier in
the eastern and northern sections of region 1. Because of the late
development of epidemics, spring wheat cultivars, such as Fielder,
Fieldwin, and Sterling (all susceptible to the same races), are the
most vulnerable to stripe rust, especially in Alberta, Canada. Races
that are virulent on Fielder are common in that area.
The geographic barriers, direction of prevailing winds, favorable
temperature and precipitation, and virulence of the races of P.
striiformis all provide evidence that stripe rust races in region 4
probably originate locally and that the introduction of inoculum from
other regions, except by humans, seldom (if ever) occurs. Races
virulent on Yamhill and Stephens, the major cultivars grown in region
4, are most prevalent in the region. All races in the region are
virulent on Lemhi, and virulence on Heines VII, Druchamp, Produra, and
Fielder is common. In the early 1970's, experimental plots were
inoculated with races virulent on Chinese 166 and Moro, but those races
did not spread very far from the inoculation site, probably because
there was and is no selective pressure for the virulences.
Because of mountain barriers and prevailing winds, stripe rust is
indigenous to region 5 and rust inoculum does not enter from other
regions. Further, because of mild winters and cool springs and summers,
stripe rust in region 5 has been severe annually for the last 16 years.
Eighteen of the thirty-nine races have been detected in region 5, and
nine of the races were first detected there. Seven races (CDL-8,
CDL-10, CDL16, CDL-17, CDL-18, CDL-23, CDL-37) are virulent on Yamhill,
and three races (CDL15, CDL-23, CDL-37) are virulent on Stephens.
Yamhill and Stephens are the two major cultivars grown in region 5. One
race (CDL-11) is virulent on only Lemhi. The most prevalent races have
been and currently are those virulent on Heines VII, Yamhill, Druchamp,
and Stephens. Races virulent on Yamhill are also virulent on
Heines VII, and races virulent on Stephens are also virulent on
Druchamp. Virulence on Druchamp was probably present in the region for
years. Druchamp has been grown in the region and was still grown in the
1970's on Lopez Island in region 5. Races virulent on Yamhill were
prevalent in the 1970's, when Yamhill made up more than 95% of the
wheat grown in region 5. Most farmers in the region changed to Stephens
in 1978. Since then, more than 95% of the wheat has been Stephens, and
races virulent on Stephens have been most prevalent. Virulence on
Heines VII and Druchamp may be explained by their common association
with virulence on Yamhill or Stephens. However, the occurrence of some
of the other races cannot be explained by the susceptibility of local
cultivars to those races. The presence of races that are virulent on
Chinese 166, Moro, Lee, Produra, or Fielder is not related to cultivars
grown in the region. Region 5 is a center of major rust diversity, and
stripe rust has existed there for more than 100 years (Humphrey et al.
1924). Stripe rust also occurs on several species of wild grasses and
on cultivated rye, and those hosts may contribute to the diversity of
the races. However, predominant rust collections from those species
have been identical to collections from adjacent wheat fields in the
region.
Region 6, which is also separated from the other regions, has a
favorable environment for rust during the growing season (October to
June) but a less favorable environment during summer, when wheat and
triticale are not grown as crops. However, those hosts are present as
volunteer plants in other crops, along the roadsides, and on irrigation
dikes (Line 1976). Eleven races have been detected in region 6, of
which four were first detected in the region. Virulences on Lemhi,
Heines VII, Druchamp, Produra, Stephens, and Fielder are common (table
4), and virulence on Chinese 166 is more common in region 6 than in
other regions. CDL-21 and CDL-35 have been collected from only region
6; CDL-11, which occurs only in regions 5 and 6, is most
common in region 6. Each of these three races has a narrow range of
virulence (table 3). CDL-21 and CDL-35 appear to have a closer
association with triticale than to wheat. The rust occurs on various
grasses, especially Hordeum sp., and these additional hosts may
contribute to the uniqueness of the races in region 6.
Region 7 includes all wheat-producing areas east of the Rocky
Mountains. The winter weather in the south is favorable for the
survival and increase of stripe rust, but the hot spring and summer
weather in the south and hot summers in the north often limit the
development of severe epidemics. However, stripe rust has become more
prevalent in region 7 since 1984, especially in the South-Central
States. Virulence of the races is consistent throughout the region,
probably because of prevailing winds and the absence of geographic
barriers. Isolates collected from that region and northern Mexico
during the period before 1985 were all identified as CDL-3. Since then,
a closely related race, CDL-8, has been found in region 7. There may be
other virulences that cannot be detected by use of the current
differentials. However, of the many cultivars from region 7 that have
been tested, none have detected virulences that have not been detected
by the current differentials.