Agriculture Reference
In-Depth Information
the head by premature ripening or bleaching of
one or several spikelets any time after fl owering.
Diseased spikelets are often a light yellow color
during the dough stage. As the disease advances,
the light pink or salmon color of the fungus may
appear at the base or along the edges of infected
spikelets. The peduncle below infected heads may
show dark coloration. When the wheat matures,
developing perithecia may be evident on the heads
as tiny purple-black specks. Infected seed is
usually shrunken or shriveled, appearing either
bleached or pink in color.
genic diversity and selection for more complex
chemotypes have been suggested as major factors
in the spread of Fusarium head blight in North
America (Ward et al., 2008).
MANAGEMENT OF RESIDUE-BORNE
DISEASES
Given the many positive aspects of reduced or
no-till farming systems, it is clear that the man-
agement of residue-borne diseases of wheat must
respond to this new paradigm. The management
techniques most likely to impact residue-borne
diseases are crop diversity and host-plant resis-
tance (Cook 2006; Anderson 2008). Used together,
disease resistant crops and diversifi cation of those
crops in time and space should help to minimize
the effects of residue-borne diseases (Holtzer
et al., 1996; Hanson et al., 2007).
Distribution and losses
The disease has worldwide distribution, although
outbreaks are considerably infl uenced by local
weather conditions during fl owering and by crop-
ping history in a given fi eld. Grain yield and grain
weight losses are variable, with average losses of
5%-15%, but losses as high as 70% in epidemic
years (Ireta and Gilchrist 1994). Losses also occur
from mycotoxin contamination of grain, which
can be hazardous to livestock and humans ingest-
ing contaminated grain products. The most pre-
valent toxins produced by
F. graminearum
are the
trichothecenes, in particular deoxynivalenol. The
disease attacks many cereal crops, including
wheat, barley (
Hordeum vulgare
L.), rice, rye
(
Secale cereale
L.), oat (
Avena sativa
L.), and
maize. Other grass species can also serve as alter-
nate hosts (Inch and Gilbert 2003; Goswami and
Kistler 2004).
Crop diversity
The frequency and specifi c crop (or fallow) used
in a crop diversifi cation scheme will infl uence
occurrence, severity, and duration of residue-
borne disease outbreaks (Peairs et al., 2005;
Krupinsky et al., 2007). Rotation to a nonhost
crop can be effective in reducing disease inci-
dence in subsequent wheat crops. However, the
number of rotations depends on how quickly
the wheat residue is disturbed and broken down
(Fernandez et al., 1998). In western Australia,
a 1-year rotation is suffi cient, but in eastern
Australia, a 2- or 3-year rotation is needed
because of the different effects which climatic
conditions have on residue degradation
(Summerell and Burgess 1989; Bhathal and
Loughman 2001).
Pathogen variability
Understanding the population structure of
F.
graminearum
has been somewhat complicated by
differences in many factors including species
groupings, mating types, and chemotypes (Ireta
and Gilchrist 1994; Guo et al., 2008; Miedaner et
al., 2008). Depending on the types and origins of
isolates evaluated, and the methods used for eval-
uation, different conclusions have been reached
as to the nature of genetic diversity within
F.
graminearum
(Xu et al., 2004; Gale et al., 2007;
Leslie et al., 2007; Starkey et al., 2007). Patho-
Host-plant resistance
Powdery mildew
Wheat cultivars vary widely in their reaction to
powdery mildew, and new races of the fungus can
develop to overcome resistant cultivars (Brown
