Agriculture Reference
In-Depth Information
L.), chickpea (
Cicer arietinum
L.), lentil (
Lens
culinaris
Medik.), and soybean [
Glycine max
(L.)
Merr.].
Pythium
species have a worldwide distribution
and are found in most agricultural soils (Hendrix
and Campbell 1970; Martin and Loper 1999). As
many as six species have been isolated from a
single soil sample and more than 30 species have
been isolated from wheat (Farr et al., 2007),
although not all species are equally virulent
(Chamswarng and Cook 1985; Ingram and Cook
1990; Higginbotham et al., 2004b). Because of the
ubiquitous nature of these pathogens and the
chronic nature of the disease, Pythium root rot
was termed “the common cold of wheat” (Cook
and Veseth 1991).
The impact of
Pythium
on wheat was not real-
ized until fi eld trials were conducted with the
fungicide metalaxyl. This oomycete-specifi c fun-
gicide increased yields as much as 0.8 t ha
−1
in the
Pacifi c Northwest US (Cook et al., 1980; Smiley
et al., 1996b). Treatment of
Pythium
-infested
soil with fumigation increased yields 13%-36%
(Cook et al., 1987).
Pythium
root rot has also been
reported on wheat in Australia (Pankhurst et al.,
1995), southeast US (Milus and Rothrock 1997),
and Turkey (Tunalı et al., 2008) but can probably
be isolated from soils in most wheat growing
areas.
but without disease-free plants for comparison,
this generalized mild stunting may not be noticed
in the fi eld. However, because the pathogen
destroys root tips, feeder roots, and root hairs, the
ability of the plant to take up water and nutrients
is reduced and symptoms of nutrient defi ciency
or water stress can become apparent. Maturity
can be delayed, plant height is reduced, plants
have fewer tillers, and heads are poorly fi lled.
Infected roots may appear yellow-brown in color,
but usually the rotted roots quickly disintegrate
and are not recovered.
Pythium
species survive as thick-walled
oospores or sporangia that are produced in
infected roots. When roots decay the inoculum is
released into the soil. Most inoculum is present
in the top 10-15 cm of soil. The pathogen can
colonize clean wheat straw, chaff, or green manure
as a nutrient source to support mycelium growth
and to increase inoculum density (Cook et al.,
1990). In the Pacifi c Northwest,
Pythium
popula-
tions averaged 350-400 propagules per gram of
soil (Cook et al., 1990), exceeding the threshold
of 200 propagules per gram of soil needed to cause
growth reductions (Fukui et al., 1994). When a
seed is placed in the soil, or a root tip grows near
a
Pythium
spore, seed or root exudates stimulate
the germination of the spore, resulting in rapid
chemotrophic attraction to and infection of the
seed or root (Hering et al., 1987; Fukui et al.,
1994; Martin and Loper 1999). Spore germina-
tion can occur within a few hours and infection
within 10-24 hours. Many
Pythium
spp. are also
capable of rapid mycelial growth. In wet soils,
some species can form motile swimming spores
(zoospores) which are chemotactically attracted to
root tips and seeds.
Pythium diseases are favored by cool, wet,
poorly drained soils with high clay content and
low pH (Fukui et al., 1994). These cool wet
conditions are often associated with delayed
sowing of fall-planted crops (Smiley et al., 1996b)
or result from excessive crop residue in no-till
systems, which reduce the warming and drying of
the soil during the spring (Cook et al., 1990).
However, improved water infi ltration often asso-
ciated with long-term no-till may reduce the
occurrence of
Pythium
diseases. Maximum infec-
Symptoms and epidemiology
Pythium
primarily infects juvenile tissues, includ-
ing embryos, emerging seedlings, root tips, lateral
roots, and root hairs. High pathogen populations
can reduce emergence of seedlings and stands of
wheat (Color Plate 14a), due to death of the seed-
ling from infection either before emergence
(preemergence damping-off) or after emergence
(postemergence damping-off). Rotted seeds can
also be found in soil. However, with cereals such
as wheat, these symptoms are rare. Usually, an
embryo-infected seedling will emerge success-
fully and then remain stunted (Fukui et al.,
1994).
Diagnosis of
Pythium
is diffi cult in wheat
because of the lack of distinctive aboveground
symptoms. In general, wheat will appear stunted,
