Agriculture Reference
In-Depth Information
changing the soil environment in ways that either
infl uence the survival of the pathogen between
susceptible host crops or the pathogen's virulence
during the infective stage (Cook and Veseth 1991).
The soilborne plant-pathogenic fungi that cause
the seven root, crown, and culm rots summarized
in this chapter (Fig. 6.1) are heavily infl uenced by
soil physical and chemical properties, by interac-
tions with associated microbes and microfauna in
soil and on plant surfaces, and by the capacity of
plants to serve as hosts for growth and multiplica-
tion. The complexity of factors affecting these
pathogens before and during pathogenesis is
immense.
Updated summaries of these diseases cannot be
complete without acknowledging the great contri-
butions that have been made by large numbers of
practicing farmers, soil microbiologists, soil ecol-
ogists, soil physicists, soil chemists, agricultural
engineers, agronomists, plant pathologists, bota-
nists, geneticists, and wheat breeders. An intro-
duction to these founding contributions can be
found in Butler (1961), Baker and Snyder (1965),
Garrett (1970), Griffi n (1972), Bruehl (1975,
1987), Schippers and Gams (1979), Krupa
and Dommergues (1979), Cook and Baker (1983),
and Parker et al. (1985). Additional reference
topics are cited in appropriate sections of this
chapter.
Many similarities occur among pathogens and
diseases of wheat and other grasses. Complemen-
tary insights into the biology of pathogens causing
four of these wheat diseases are summarized from
a different perspective in treatises by Smith et al.
(1989), Clarke and Gould (1993), Couch (1995),
and Smiley et al. (2005a).
The content of this chapter also would
be incomplete without referencing guidelines
for studying these pathogens and gaining
a greater visual appreciation of disease symp-
toms and pathogen characteristics. Methods to
isolate and study the pathogens are provided
by Singleton et al. (1992), among others.
Excellent color images are available in Zillinsky
(1983), Murray et al. (1998), Wallwork (1992,
2000), Bailey et al. (2003), and Bockus et al.
(2009).
COMMON ROOT ROT
Common root rot is a name originally applied to
a complex of diseases caused by several species of
Bipolaris
('
Helminthosporium
') and
Fusarium
(Butler 1961). Current usage restricts this name
to root and stem base diseases caused by
Bipolaris
sorokiniana
, although this pathogen frequently
occurs in association with Fusarium crown rot
(Windels and Wiersma 1992; Smiley and
Patterson 1996; Fernandez and Chen 2005).
Common root rot was considered a very serious
disease, especially in Canada and Australia, in the
early- to mid-20th century (Butler 1961; Tinline
et al., 1991) and can cause grain yield losses as
great as 25% (Wildermuth et al., 1992). Common
root rot has received less attention in recent years,
possibly because of the increasing importance of
Fusarium crown rot, and because most research
on this pathogen now focuses on the foliar disease,
spot blotch (Kumar et al., 2002). The signifi cance
of common root rot for modern cultivars in con-
temporary cropping systems is not well known
and is probably underestimated.
Symptoms and epidemiology
All parts of the wheat plant can be infected. Initial
symptoms are dark necrotic lesions. The classic
symptom in soilborne infections is a dark lesion
on the subcrown internode (Color Plate 12a),
which can extend up to the crown and, in severe
infections, up the lower internodes on the stems.
Plants with severe subcrown internode symptoms
have reduced root growth, especially of crown
roots (Kokko et al., 1995). The roots are not
usually the major site of infection, although they
may show some browning (Fedel-Moen and
Harris 1987; Kokko et al., 1995). Lesions of
common root rot (Color Plate 12a) are much
darker than those of Fusarium crown rot (Color
Plate 12b). On the stems, they appear streaky
rather than uniform around the circumference of
the stem. Severe common root rot is often associ-
ated with water stress (Piccinni et al., 2000).
Plants may be stunted, and whiteheads (Color
