Agriculture Reference
In-Depth Information
symptoms but fail to produce perithecia in culture,
the isolates should be examined for the possibility
that they are an anamorphic state (
Phialophora
or
Harpophora
spp.; Gams 2000) of recently
described heterothallic species requiring the pres-
ence of both mating types to produce the sexual
stage. These fungi seldom produce a sexual stage
in nature and require mixtures of mating types
to produce the sexual stage in culture. They are
easily misidentifi ed as
Ggt
or
Gga
, may be under-
reported, and are likely to occur on cereals in
nature.
Gaeumannomyces incrustans
Landschoot &
Jackson and
Magnaporthe poae
Landschoot &
Jackson were the fi rst-described heterothallic
species of
Gaeumannomyces
-like fungi (Clarke and
Gould 1993). Anamorphs of
M. poae
, misreported
initially as
P. graminicola
and later shown to be
individual mating types of
M. poae
(Clarke and
Gould 1993), cause summer patch of perennial
grasses (Smiley et al., 2005a). Anamorphic states
of
G. incrustans
,
M. poae
, and related fungi cause
a high-temperature form of take-all on wheat and
other cereals in pot tests (Smiley et al., 1986;
Elliott 1991).
DNA probes have also been developed to iden-
tify isolates of
G. graminis
varieties and related
species (Clarke and Gould 1993; Augustin et al.,
1999; Herdina and Roget 2000; Rachdawong et
al., 2002; Freeman and Ward 2004). Molecular
tools have revealed genetic polymorphism within
Ggt
populations (Ward and Gray 1992; Bateman
et al., 1997). Characterizations of
Ggt
populations
from monoculture wheat crops, using both restric-
tion fragment length polymorphism (RFLP) and
random amplifi ed polymorphic DNA (RAPD)
markers, have revealed two genetic groups called
G
1
and G
2
(Lebreton et al., 2004). Isolates of G
1
were dominant in the fi rst and sixth wheat crops,
and G
2
isolates were dominant in the third and
fourth wheat crops. Aggressiveness of group G
2
was signifi cantly greater than that of group G
1
,
which corresponds with observed peaks of disease
during a wheat monoculture. A linear relationship
between G
1
and G
2
frequencies and disease sever-
ity on wheat roots occurred in fi elds monitored
over three consecutive seasons (Lebreton et al.,
2007).
Disease management
The most important cultural practice used to
control take-all is crop rotation (Asher and
Shipton 1981; Hornby et al., 1998; Ennaïfar et al.,
2007). Other practices that infl uence take-all
severity following the buildup of inoculum during
preceding host crops include tillage (Ennaïfar et
al., 2005), sowing date and density, application of
fertilizer or lime, and grass weed control. Fungi-
cide seed treatment provides consistent but partial
effi cacy by reducing primary infections (Schoeny
and Lucas 1999; Bailey et al., 2005; Ennaïfar
et al., 2005). No resistant cultivar is currently
available and sources of resistance are scarce
(Cook 2003).
Effects of soil cultivation have been variable.
Direct-seeded crops have had a lower disease
incidence in Britain (Brooks and Dawson 1968)
and either a higher level of disease (Moore and
Cook 1984) or no effect (Schroeder and Paulitz
2006) in the Pacifi c Northwest US.
Late sowing will allow a longer period of inocu-
lum reduction and less favorable temperature
conditions at the time of possible infections, thus
reducing the frequency of primary infections.
Reducing the sowing density reduces the amount
of primary infection as well as secondary infec-
tions capable of transmitting the disease from
plant to plant (Colbach et al., 1997).
Take-all generally becomes more severe imme-
diately following application of lime to acid soils,
particularly when applications causing a large
change in acidity occur during intervals in which
environmental conditions and host frequency are
also conducive to disease expression (Asher and
Shipton 1981). However, host genotypes, isolates
of
Ggt
, and other soil microbes each vary in toler-
ance to acidity. In some regions the occurrence
of severe take-all has been associated more with
acid than neutral soils, resulting in less severe
take-all following application of lime (Hornby
et al., 1998).
Application of the ammonium-ion form of
nitrogen fertilizer generally leads to a reduc-
tion of take-all severity compared to the nitrate
form or a mixture of these ions (Huber et al.,
