Agriculture Reference
In-Depth Information
The importance of stem rust in causing grain
yield losses in wheat is readily apparent since telia
of the fungus can be seen on the stems as the crop
matures. Stem rust infections rupture the host
plant epidermal tissue, causing an increased loss
of water. Nutrients and water diverted by the
fungus in the production of urediniospores also
contribute to added stress of wheat that can con-
tribute to premature death (Roelfs 1985). Stem
rust infected plants are more susceptible to win-
terkill, produce fewer tillers, and have small
heads. Lodging of plants caused by broken straw
can occur due to severe stem infections. Severe
infections in the last few weeks before harvest can
greatly reduce grain yield due to the loss of water
during the critical period of grain fi lling. Grain
from stem rust infected wheat is often shriveled,
which may reduce the market grade.
mating type, resulting in dikaryotic hyphae (
n
+
n
)
that develop into a cluster of tubular or cuplike
aecia on the underside of the leaf surface. Aecio-
spores (
n
+
n
) are produced in chains in aecia and
are windborne to infect the telial hosts (grasses),
but do not infect the aecial host. After an infection
is established on a telial host, blisterlike uredinia
develop at the infection site. Urediniospores (
n
+
n
)
are produced as soon as 7 days after initial infec-
tion. Urediniospores are continuously produced
in a sporulating uredinium for a sustained period
of up to several weeks as long as the host tissue
remains viable. The urediniospores are clonally
produced for a number of generations and initiate
new infections on the telial hosts. Under favor-
able conditions, urediniospores can reproduce
rapidly in wheat crops, generating large quantities
of inoculum. Teliospores are produced as the
wheat plants approach maturity.
In most areas where wheat stem rust is impor-
tant, the pathogen survives through the noncrop
seasons as mycelia in tissues of dormant crop
plants, volunteer plants, or alternative telial hosts
(Roelfs 1985). Urediniospores could also be trans-
ported from one region to another following the
succession of crops within an epidemiological
zone. Thus the disease cycle can be completed
without the presence of the alternate host. In
North America,
P. graminis
f. sp.
tritici
is found
primarily on common and durum wheat, barley,
foxtail barley (
Hordeum jubatum
), and jointed
goatgrass, although artifi cial inoculation can
induce infection on many other grasses.
Taxonomy, life cycle, and host range
Wheat stem rust belongs to one of several
formae
speciales
in
P. graminis
. The fungus is heteroe-
cious, alternating between a telial host in the
Poaceae and an aecial host in the Berberidaceae,
and macrocyclic, with fi ve spore states that are
distinct in morphology and function. Illustrations
of the life cycle of
P. graminis
f. sp.
tritici
can be
readily found in various monographs and text-
books (Roelfs 1985; Agrios 1997).
The dikaryotic (
n
+
n
) teliospore is the dormant
spore stage and germinates after breaking dor-
mancy, usually after overwintering. The fungus
has a brief diploid state when two nuclei fuse in
a germinating teliospore. Meiosis follows, pro-
ducing single-celled, hyaline haploid (
n
) basidio-
spores. Basidiospores are windborne to infect the
alternate host of susceptible species in
Berberis
spp. and
Mahonia
spp. There are a large number
of species in
Berberis
and
Mahonia
listed as sus-
ceptible to
P. graminis
(Roelfs 1985), but the
common barberry,
B. vulgare
, is considered to be
the most important. After infection on the alter-
nate host, fl ask-shaped pycnia develop on the
upper leaf surface, producing single-celled pyc-
niospores (
n
) and receptive hyphae (
n
) that serve
as gametes. Fertilization occurs when a pycnio-
spore fuses with a receptive hypha of the opposite
Genetic variation in
Puccinia graminis
f.
sp.
tritici
Race surveys of
P. graminis
f. sp.
tritici
have been
conducted in the US since 1919 (Stakman et al.,
1919). The initial set of differentials used to iden-
tify wheat stem rust races were the common wheat
cultivars Little Club, Marquis, Kanred, and Kota;
the durum cultivars Arnautka, Speltz Marz,
Mindum, Kubanka, and Acme; the emmers
'White Spring', 'Khapli'; and diploid einkorn
wheat (
T. monococcum
ssp.
monococcum
). These
various wheats had multiple genes for stem rust
resistance and were gradually replaced by differ-
