Agriculture Reference
In-Depth Information
tion by
P. ultimum
and
P. irregulare
occurs at
10 ºC and 5 ºC (Ingram and Cook 1990), but some
species can act as snow molds, infecting under
snow cover at 0-3 ºC.
management option at the present time. Crop
rotation is not used for management because of
the wide host range of
Pythium
species. However,
different hosts select for different
Pythium
species
(Ingram and Cook 1990) and recent surveys
have demonstrated shifts in species composition
resulting from different crop rotations or crop-
ping systems (Schroeder et al., 2007). For
example,
P. irregulare
Group I is strongly associ-
ated with legume rotations (lentil and pea).
Because of long survival of
Pythium
oospores in
dry soils during the summer, traditional summer
fallow may not be effective, although many species
decline to low numbers in a fallow system (Schro-
eder and Paulitz 2006).
Seed treatments reduce early damage to seed
and seedlings (Smiley et al., 1996b; Cook et al.,
2002b) and can increase grain yield through
improved stand establishment (Color Plate 14a),
but they will not reduce root rot in mature plants
because no commercially registered fungicide is
systemically translocated downward into the
roots. Effective seed treatments include the
oomycete-specifi c metalaxyl and mefanoxam, and
the broad-spectrum thiram. Biological seed
treatments with bacteria (
Pseudomonas, Bacillus,
Enterobacteria
) have controlled Pythium root rot
in the greenhouse and have resulted in some yield
increases in the fi eld (Weller and Cook 1986; Kim
et al., 1997; Cook et al., 2002b; Kageyama and
Nelson 2003).
Some management practices can mitigate the
effects of Pythium root rot, such as banding a
starter fertilizer directly below the seed to main-
tain seedling vigor when a portion of the root
system becomes rotted (Cook et al., 2000). Because
old seed germinates more slowly, giving a greater
window of opportunity for
Pythium
to infect, only
new seed should be planted. Volunteer crop plants
and weeds should be killed with preplant herbi-
cides at least 3 weeks before planting to minimize
the “green-bridge” effect (Smiley et al., 1992;
Pittaway 1995). Plants that are dying from treat-
ment with the herbicide glyphosate can serve as
reservoirs of
Pythium
inoculum, because the
necrotrophic pathogen can extensively colonize
the root system when the plant defense system is
reduced by inhibition of a key enzyme in the
Causal organisms
Although 30 species of
Pythium
have been associ-
ated with wheat, most reports focus upon
P.
arrhenomanes
Drechs.,
P. graminicola
Subr.,
P. ultimum
Trow,
P. aristosporum
Vanterpool
, P.
irregulare
Buisman,
P. torulosum
Coker & Patter-
son,
P. sylvaticum
Campbell & Hendrix, and
P.
heterothallicum
Campbell & Hendrix.
Pythium
debaryanum
Hesse is frequently cited in older lit-
erature but this is not currently recognized as a
valid species and many records may be incorrect.
Recent surveys using classical and molecular
techniques have identifi ed 13 species of
Pythium
on wheat in the Pacifi c Northwest US, including
a new species,
P. abappressorium
(Paulitz and
Adams 2003, Paulitz et al., 2003a). Most species
are capable of causing signifi cant reductions
in root biomass (Higginbotham et al., 2004b),
but the most virulent were
P. ultimum
,
P. irregu-
lare
group 1, and
P. irregulare
group IV sensu
Matsumoto (identifi ed as
P. debaryanum
in that
paper).
Pythium
species produce oospores, which result
from the fertilization of oogonia by antheridia.
Oospores are generally spherical, from 15-40 μm
in diameter, and result from sexual recombina-
tion. Asexual sporangia can germinate directly to
form hyphae or indirectly to form zoospores, and
can be spherical, fi lamentous, or lobed in shape.
Identifi cation is based on morphology of sporan-
gia, oospores, oogonia, and antheridia (van der
Plaats-Niterink 1981), or on molecular methods
based on sequencing of the internal transcribed
spacer (ITS) region of the rDNA (Lévesque and
de Cock 2004; Schroeder et al., 2006).
Disease management
Although no high-level resistance or tolerance is
found in adapted wheat cultivars, minor differ-
ences do occur in susceptibility among cultivars
(Higginbotham et al., 2004a). Resistance is not a
