Agriculture Reference
In-Depth Information
countries where higher infections of leaf rust on
durum wheat had occurred.
Herrera-Foessel et al. (2005) described up to
fi ve genes in nine CIMMYT durum lines that
conditioned resistance to the
P. triticina
race that
was virulent to Altar C84. Herrera-Foessel et al.
(2007) determined that one of these genes mapped
to the
Lr3
locus on chromosome 6B, and a second
resistance gene was closely linked to the
Lr3
locus.
rounded by chlorosis or necrosis on seedling
leaves. On adult-plant leaves of highly susceptible
plants, uredinia continue to develop in stripes
from the initial infection sites, without necrosis or
chlorosis. On resistant cultivars, necrotic stripes
develop from the initial infection sites. Responses
on resistant wheat genotypes vary from no visible
symptom to various sizes or lengths of necrotic
patches or stripes with varying amounts of sporu-
lation (Color Plate 10b). Uredinia erupt to release
urediniospores. Each uredinium can produce
thousands of urediniospores over a period of days.
Urediniospores are spherical, 15-20 μm in diam-
eter, and echinulate.
Urediniospores are dispersed mainly by wind
but also can be spread by insects, animals, and
humans. A minimum of three hours of dew for-
mation on the plant surface is needed for uredin-
iospores to germinate and infect plants (Rapilly
1979). The optimum temperatures for spore
germination are 10-12 ºC, and the minimum
temperature for germination is just above 0 ºC
(Newton and Johnson 1936). Urediniospores do
not germinate well when the temperature is above
20 ºC. Germ tubes penetrate into plant tissue
through stomata without forming appressoria as
in other wheat rusts (Marryat 1907; Allen 1928).
After a germ tube passes through a stoma, it forms
a substomatal vesicle, from which branched
hyphae grow intercellularly. Haustoria are formed
from the hyphae and grow into host cells. The
uredinia are produced from intercellular hyphae
and emerge on the plant surface. Under optimum
temperature conditions (13-16 ºC), it takes 12-13
days from initial infection to sporulation of new
uredinia (Hungerford 1923). Stripe rust will con-
tinue to produce new uredinia and urediniospores
in stripes further up and down the leaf from the
initial site of infection.
Urediniospores are the sole initial inoculum
source for the stripe rust pathogen. Initial inocu-
lum can be local and/or from outside a region
depending upon climatic conditions that infl u-
ence survival of stripe rust during the summer or
winter. In the Pacifi c Northwest of the US and
the adjacent area of British Columbia and Alberta
in Canada, the stripe rust fungus is able to survive
during the summer and winter most years. In this
WHEAT STRIPE RUST
Distribution and epidemiology
Stripe rust is an important disease of wheat
worldwide. The disease has been reported in
more than 60 countries and has caused yield losses
in Africa, Asia, Australia, New Zealand, Europe,
North America, and South America (Stubbs
1985; Chen 2005). In the US the disease has been
most common in states west of the Rocky Moun-
tains since the late 1950s but has become increas-
ingly frequent in the eastern and midwestern
states since 2000 (Chen 2005). In recent years
signifi cant losses in wheat due to stripe rust
have been reported in California, Oregon,
Washington, Idaho, Montana, Colorado, Texas,
Oklahoma, Kansas, Nebraska, South Dakota,
Louisiana, Arkansas, Missouri, Alabama, and
Georgia.
Stripe rust infection can occur at any growth
stage when green plant tissue is available. The
fi rst visible symptom of infection appears as chlo-
rotic spots that resemble viral symptoms. Ure-
dinia of stripe rust are yellow to orange in color;
thus the disease is commonly called yellow rust.
Stripe rust uredinia, 0.3-0.5 mm by 0.5-1.0 mm,
are much smaller than uredinia of stem rust and
leaf rust. Uredinia can form on both sides of
leaves but are more abundant on the upper surface
(Color Plate 10). Uredinia can also form on leaf
sheaths, glumes, awns, and on immature green
kernels. Uredinia form in patches around infec-
tion sites on seedlings and are arranged in stripes
between leaf veins on adult plants. Depending on
the level of plant resistance, uredinia can be sur-
