Agriculture Reference
In-Depth Information
the most visible and is the disease stage on green
plants. Urediniospores are elongated, 10-15 μm
long, and echinulate. The urediniospores are dis-
seminated to newly emerged tissues of the same
plant or adjacent plants, where these spores are
the source of new infections, or the spores can be
windblown over long distances. In the case of
long-distance dispersal, spore depositions on
crops in a new area are often associated with rain
showers. Stem rust pustules develop mostly on
the underside of leaves, but may penetrate and
sporulate on the upper side. In general, leaves of
adult plants are not as receptive as stem tissue for
stem rust infection. As infected plants mature,
uredinia are replaced by telia, changing color
from red to dark brown to black; thus the disease
is also called black stem rust. Teliospores are
fi rmly attached to plant tissue. The telial stage is
not important in the epidemiology of the disease
unless the alternate host common barberry (
Ber-
beris vulgaris
) is present.
The infection process and specialized infection
structures for
P. graminis
f. sp.
tritici
are the same
as previously described for
P. triticina
, except that
a 3-hour period of light is required following 6-8
hours of dew to complete the development of
appresoria and penetration peg. Spore germina-
tion is optimal at 15-24 ºC, and can occur up to
30 ºC. The optimal temperature for sporulation is
30 ºC and can occur up to 40 ºC. A dew period
of 6 hours is optimal for spore germination and
the subsequent infection process (Roelfs et al.,
1992).
In the US, stem rust is rarely observed on
winter wheat in the fall. Infections on susceptible
cultivars are generally not obvious until the spring
after the wheat crop has reached heading. The
disease is most obvious when a crop is approach-
ing maturity. Infections of stem rust can severely
damage crops that are within 2 weeks of harvest.
Infections in winter wheat along the Gulf Coast
and Texas in early spring are likely due to infec-
tions that survived the winter on fall-planted
winter wheat or volunteer plants (Roelfs 1989),
though fi eld observations of overwintering events
are rare. Stem rust infections on wheat are usually
fi rst observed in Texas and Louisiana in the last
two weeks of April or the fi rst two weeks of May
and reach maximum severity by the middle of
May.
Stem rust epidemics during and after the 1930s
were due to the presence of a large pathogen
population on winter wheat in the southern states
that was windblown to the spring wheat region in
the northern states and Canada. In years follow-
ing the 1950s epidemics, however, there has been
an increase of resistant cultivars in the southern
US. This has reduced the opportunity for the
pathogen to infect and overwinter in the south,
resulting in very small population size (Kolmer et
al., 2007a). There is only a small likelihood that
stem rust overwinters on fall-planted winter
wheat in the central Great Plains and the Midwest.
In the northern spring wheat regions, stem rust
occurs most frequently on susceptible wheat lines,
and the initial inoculum for the region is almost
exclusively from infected wheat in the southern
and central Great Plains. In Minnesota and North
Dakota, stem rust on wheat is usually fi rst
observed in mid-to-late June, with maximum
severity in the last week of July or the fi rst week
of August. In the Pacifi c Northwest where plants
of the alternate host common barberry are present,
both overwintering urediniospores and aecio-
spores can contribute to the initial infections, but
stem rust does not normally develop to epidemic
status in this region. Comparisons of area of dis-
tribution, optimal temperatures for infection and
growth, and alternate hosts for
P. triticina
,
P.
graminis
f. sp.
tritici
and
P. striiformis
f. sp.
tritici
are summarized in Table 5.2.
Origin and historical importance
Although the origin of
P. graminis
f. sp.
tritici
, one
of the specialized forms in the
P. graminis
species
complex, is not clear, one or more of the suscep-
tible alternate hosts of
Berberis
spp. was the likely
source of the fungus (Leppik 1967). The rust
most likely evolved in a region where the aecial
and telial hosts overlapped (Wahl et al., 1984).
However, it is diffi cult to restrict the center of
origin to a specifi c region because of the large
number and broad distribution of susceptible
species of both the pycnial and telial hosts. Likely,
P. graminis
f. sp.
tritici
originated on a close wild
