Successes in breeding for and managing durable resistance to wheat rusts

Abstract

Stripe rust, leaf rust, and stem rust-caused by Puccinia striiforrnis Westend., Puccinia recondita Roberge ex Desmaz., and Puccinia graminis Pers., respec-tively-have been considered to be the most widely destructive diseases of wheat (Triticum aestivum L.) in the world. Rust epidemics have been common throughout history and have frequently caused severe yield losses. During the first half of the twentieth century, rust resistance was short-lived. Within a few years after the release of a new resistant cultivar, new, virulent races (pathotypes) of the rust pathogens would appear and severely damage the previously resistant cultivar. Within the last 30 to 40 years, major progress has been made in developing cultivars with superior, more sustainable resistance to the rusts, and in the applica-tion of improved methods of managing that resistance. Consequently, rust epidem-ics have been infrequent, and when they have occurred, damage caused by the rusts has been less severe and less extensive. The following is a brief discussion of some of the successes and a few of the failures in controlling the rusts. The in-formation is based on firsthand knowledge of the rusts and corroborative information from many other rust specialists. Emphasis will be on control of the diseases in North America. Wheat production and rust develop-ment in North America. The environ-mental conditions that affect wheat pro-duction and rust epidemics have a major impact on the strategies and methods of breeding for rust resistance and the man-agement of that resistance. Wheat is grown in many regions of North America under a wide range of environmental conditions. It is the environment within those regions that determines the wheat types and mar-ket classes that are grown, the importance of each of the three rusts, and the effec- Accepted for publication 10 September 1995 This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. The American Phytopathological Society, 1995. tiveness of different types of rust resis-tance (10). It also is the diversity of envi-ronment, market classes, and management conditions that make breeding for rust resistance difficult. Newly developed cul-tivars of each wheat class and type must be adapted to the environment and manage-ment systems of each specific region and must have resistance to other diseases and pests in that region as well as to the rusts. Since rust spores can be wind dissemi-nated long distances, cultivars developed for some regions must also be resistant to races from other regions. All three rusts are obligate parasites, re-quire free water on the foliage for devel-dpment of epidemics, and have the ability to increase from a few primary uredia (pustules) early in the growing season to high disease intensities later in the grow-ing season. Each rust species has a unique, optimum temperature range for disease development; stripe rust is a cool tempera-ture rust, leaf rust is a moderate tempera-ture rust, and stem rust is a warm tempera-ture rust. Because of these environmental requirements, stripe rust is most destruc-tive in the West, especially the cool Pacific Northwest, and is sometimes destructive in the south central United States. Stem rust is most destructive in the north central United States and adjacent Canada, can be destructive in the Pacific Northwest, and is infrequently destructive in the southern United States. Leaf rust epidemics can occur in all of the wheat growing regions of North America. In the Pacific Northwest, stripe rust and leaf rust start in the fall, increase slowly during the winter, and increase rapidly in the spring (10). Stripe rust develops most rapidly during the cooler temperatures of early spring. Leaf rust develops most rap-idly during the warmer temperatures of late spring. Stem rust epidemics, which start from inoculum produced on barberry leaves, develop most rapidly when the weather is hottest during the late spring and summer. In the southern United States, all three rusts usually survive during the winter, but the spring temperatures are often too high for development of severe stripe rust epidemics, and the crop usually matures before severe stem rust epidemics develop. In the north central and north-eastern United States and adjacent Canada, winter survival of the rusts is rare; the primary inoculum usually originates from wheat in the southern United States and arrives in the region in late spring when temperatures are too high for stripe rust epidemics. Resistance to stripe rust. In the late 1950s and early 1960s, stripe rust caused losses in excess of 70% in the Pacific Northwest (12). Those epidemics were the impetus for a major effort in breeding for stripe rust resistance. Of the various types of resistance that have been identified, seedling resistance and high-temperature, adult-plant (HTAP) resistance are the most important (3-6,11,14,15,17). Seedling re-sistance is characterized by race specificity and low infection types at all stages of plant growth and a wide range of tempera-tures. When used extensively over time and space, new races usually circumvent the seedling resistance within 3 to 4 years after the release of cultivars with that resis-tance (9,ll). Managing the seedling resis-tance has extended the life of the cultivars. Use of the race-specific seedling resistance in a multiline cultivar has provided pro-tection for more than 10 years. Use of cultivar mixtures also has extended the duration of the effective use of seedling resistance.

Full text

Special
Report
Successes in Breeding for and Managing Durable Resistance to Wheat Rusts
Roland
F.
Line
and
Xianming Chen,
USDA-ARS,
Washington
State University,
Pullman 99164-6430
Stripe rust, leaf rust, and stem rust-
caused by Puccinia striiforrnis Westend.,
Puccinia recondita Roberge ex Desmaz.,
and Puccinia graminis Pers., respec-
tively-have been considered to be the
most widely destructive diseases of wheat
(Triticum aestivum
L.)
in the world. Rust
epidemics have been common throughout
history and have frequently caused severe
yield losses. During the first half of the
twentieth century, rust resistance was
short-lived. Within a few years after the
release of a new resistant cultivar, new,
virulent races (pathotypes) of the rust
pathogens would appear and severely
damage the previously resistant cultivar.
Within the last 30 to 40 years, major
progress has been made in developing
cultivars with superior, more sustainable
resistance to the rusts, and in the applica-
tion of improved methods of managing
that resistance. Consequently, rust epidem-
ics have been infrequent, and when they
have occurred, damage caused by the rusts
has been less severe and less extensive.
The following is a brief discussion of
some of the successes and a few of the
failures in controlling the rusts. The in-
formation is based on firsthand knowledge
of the rusts and corroborative information
from many other rust specialists. Emphasis
will be on control of the diseases in North
America.
Wheat production and rust develop-
ment
in
North America.
The environ-
mental conditions that affect wheat pro-
duction and rust epidemics have a major
impact on the strategies and methods of
breeding for rust resistance and the man-
agement of that resistance. Wheat is grown
in many regions of North America under a
wide range of environmental conditions. It
is the environment within those regions
that determines the wheat types and mar-
ket classes that are grown, the importance
of each of the three rusts, and the effec-
Corresponding author:
R.
F.
Line
E-mail: rline@wsu.edu
Accepted for publication
10
September
1995
This article is in the public domain and not
copyrightable. It may be freely reprinted with
customary crediting of the source. The American
Phytopathological Society,
1995.
tiveness of different types of rust resis-
tance (10). It also is the diversity of envi-
ronment, market classes, and management
conditions that make breeding for rust
resistance difficult. Newly developed cul-
tivars of each wheat class and type must be
adapted to the environment and manage-
ment systems of each specific region and
must have resistance to other diseases and
pests in that region as well as to the rusts.
Since rust spores can be wind dissemi-
nated long distances, cultivars developed
for some regions must also be resistant to
races from other regions.
All three rusts are obligate parasites, re-
quire free water on the foliage for devel-
dpment of epidemics, and have the ability
to increase from a few primary uredia
(pustules) early in the growing season to
high disease intensities later in the grow-
ing season. Each rust species has a unique,
optimum temperature range for disease
development; stripe rust is a cool tempera-
ture rust, leaf rust is a moderate tempera-
ture rust, and stem rust is a warm tempera-
ture rust. Because of these environmental
requirements, stripe rust is most destruc-
tive in the West, especially the cool Pacific
Northwest, and is sometimes destructive in
the south central United States. Stem rust
is most destructive in the north central
United States and adjacent Canada, can be
destructive in the Pacific Northwest, and is
infrequently destructive in the southern
United States. Leaf rust epidemics can
occur in all of the wheat growing regions
of North America.
In the Pacific Northwest, stripe rust and
leaf rust start in the fall, increase slowly
during the winter, and increase rapidly in
the spring (10). Stripe rust develops most
rapidly during the cooler temperatures of
early spring. Leaf rust develops most rap-
idly during the warmer temperatures of
late spring. Stem rust epidemics, which
start from inoculum produced on barberry
leaves, develop most rapidly when the
weather is hottest during the late spring
and summer. In the southern United States,
all three rusts usually survive during the
winter, but the spring temperatures are
often too high for development of severe
stripe rust epidemics, and the crop usually
matures before severe stem rust epidemics
develop. In the north central and north-
eastern United States and adjacent Canada,
winter survival of the rusts is rare; the
primary inoculum usually originates from
wheat in the southern United States and
arrives in the region in late spring when
temperatures are too high for stripe rust
epidemics.
Resistance to stripe rust.
In the late
1950s and early 1960s, stripe rust caused
losses in excess of 70% in the Pacific
Northwest (12). Those epidemics were the
impetus for a major effort in breeding for
stripe rust resistance. Of the various types
of resistance that have been identified,
seedling resistance and high-temperature,
adult-plant (HTAP) resistance are the most
important (3-6,11,14,15,17). Seedling re-
sistance is characterized by race specificity
and low infection types at all stages of
plant growth and a wide range of tempera-
tures. When used extensively over time
and space, new races usually circumvent
the seedling resistance within 3 to
4
years
after the release of cultivars with that resis-
tance (9,ll). Managing the seedling resis-
tance has extended the life of the cultivars.
Use of the race-specific seedling resistance
in a multiline cultivar has provided pro-
tection for more than 10 years. Use of
cultivar mixtures also has extended the
duration of the effective use of seedling
resistance.
HTAP
resistance is characterized by a
range of infection types and a shift in the
range of infection types depending upon
temperature and stage of plant growth
(17). As plants with HTAP resistance be-
come older, they become more resistant at
higher temperatures, but they remain sus-
ceptible when grown at low temperatures.
Seedlings of cultivars with HTAP resis-
tance are susceptible at all temperatures.
At higher temperatures, flag leaves are
most resistant. HTAP resistance can be
considered to be a type of "slow rusting"
resistance, since it decreases the rate of
rust development. Currently, more than
90% of the cultivars grown in the Pacific
Northwest have HTAP resistance. The
cultivars with HTAP resistance have re-
mained resistant for more than 30 years,
even when grown extensively in the region
and exposed to numerous races of the
pathogen. The durable, HTAP resistance
incorporated into adapted cultivars has
prevented major stripe rust epidemics and
widespread losses and has prevented mul-
1254
Plant Disease
/
Vol. 79 No.
12