Agriculture Reference
In-Depth Information
the fall-planted winter wheat crop. The optimal
conditions for infection are temperatures near
20 ºC with free moisture on the leaf surface for at
least 8 hours, and 25 ºC is the optimal tempera-
ture for growth. Overnight periods of dew forma-
tion are optimal for germination of urediniospores.
The infection process can occur at temperatures
from 2 to 30 ºC; however, longer periods of dew
are required at the lower temperatures. Tempera-
ture and moisture conditions in the fall months
throughout much of the winter wheat region of
the US allow
P. triticina
to become established
over a large geographical area.
Throughout the US winter wheat region leaf
rust inoculum sources are a combination of (i)
infections that survive the winter either as ure-
diniospores or mycelia and (ii) windblown ure-
diniospores carried in the southerly winds from
infected winter wheat to the south. The relative
importance of overwintering infections and exog-
enous sources of inoculum will vary based on
conditions in the fall that affect leaf rust infection,
temperatures during the winter that allow leaf
rust to survive, and the maturity of the crop.
Wheat cultivars with later maturity will be more
affected by leaf rust infections from exogenous
sources. Severe epidemics result from the combi-
nation of overwintering leaf rust infections, wind-
blown urediniospores deposited in rain events,
regular dew periods, and temperatures greater
than 25 ºC during the time when winter wheat is
breaking dormancy and resuming growth and
development.
In south Texas and along the Gulf Coast,
winter temperatures often exceed 20 ºC, with
only infrequent freezes. In this region where
P.
triticina
regularly overwinters, leaf rust can reach
high severity levels in March and April (Roelfs
1989). From Oklahoma to eastern Virginia, winter
temperatures usually do not exceed 20 ºC and
freezing temperatures are common. In this area
leaf rust overwinters in isolated pockets, resulting
in foci of leaf rust infections that are apparent in
the spring when temperatures are regularly above
20 ºC and leaf rust is rapidly increasing. Leaf rust
severities in this region usually reach maximum
levels in April and mid-May. In the area from
Kansas north to South Dakota and east to the
Ohio Valley, winter low temperatures range from
−10 to −20 ºC, and high temperatures rarely
exceed 15 ºC. Leaf rust can survive in winter
wheat in this region as mycelium resulting from
infections during the fall months. The overwin-
tering survival of leaf rust in this region is
also dependent on adequate snow cover to protect
the wheat leaves from freeze damage during
periods of extreme low temperatures. Leaf rust
severities are at maximum levels in May in Kansas
to June in South Dakota and the Ohio Valley
states.
In Minnesota, South Dakota, and North
Dakota, leaf rust infection on the spring wheat is
usually fi rst observed in mid-to-late June, with
maximum severity levels in mid-to-late July.
Daytime temperatures in the summer in this
area are often greater than 25 ºC, with frequent
rain events and dew periods. All leaf rust infec-
tions on spring wheat originate from windblown
urediniospores from winter wheat growing in the
southern Great Plains or from local fi elds of
winter wheat. Leaf rust epidemics in the spring
wheat region are most severe when initial infec-
tions occur at the tillering stage, which allows
additional generations of urediniospores to infect
the crop.
Origin and historical importance
Puccinia triticina
was introduced to North America
with the fi rst agricultural settlements and wheat
cultivation in the early 17th century (Chester
1946). The origin of
P. triticina
is likely the Fertile
Crescent region of southwest Asia and the Middle
East (Wahl et al., 1984), which is also the origin
of diploid, tetraploid, and hexaploid wheat. The
most susceptible alternate host of
P. triticina
is
Thalictrum speciosissimum
(=
Thalictrum fl avum
),
which is native to southern Europe, western Asia,
and Turkey. It is likely that the center of origin
of
P. triticina
is a region where the alternate and
telial hosts overlap, which would be southwest
Asia.
In early rust research in the US, the impor-
tance of leaf rust was not recognized since all yield
