Diseases Which Challenge Global Wheat Production—Root, Crown, and Culm Rots - Wheat Science and Trade - page 125

mainly on cultural practices and resistance.
Because incidence of disease caused by F. pseudo-
graminearum , F. graminearum , and F. avenaceum
is strongly correlated with the quantity of infested
residue (Windels and Wiersma 1992; Smiley
et al., 1996a; Wildermuth et al., 1997; Backhouse
2006), practices which either remove the residue
or allow its decomposition are effective manage-
ment tools. Stubble burning, either after harvest
or immediately before sowing, can maintain the
disease at low levels even in continuous wheat
production (Burgess et al., 1993, 1996). However,
this practice has become unacceptable in many
areas. Amounts of surface residue and stubble
burning may have less effect where F . culmorum
is present (Windels and Wiersma 1992; Smiley
et al., 1996a), presumably because of the presence
of chlamydospores in the soil (Bateman et al.,
1998).
Rotation with nonhosts allows time for natural
mortality of the pathogen. The rotations that can
be used, and their effectiveness, differ between
pathogens. Fusarium pseudograminearum has the
narrowest host range, and almost any noncereal
host, including grain legumes and oilseeds as well
as sorghum [ Sorghum bicolor (L.) Moench] can be
used (Burgess et al., 1996; Kirkegaard et al.,
2004). Maize ( Zea mays L.) has not been recorded
as a host for F . pseudograminearum , but would be
a poor choice if F . culmorum or F . graminearum ,
which do infect maize, are present. Crop rotation
may have less effect on F . culmorum than on F .
pseudograminearum , presumably because of the
longevity of chlamydospores relative to mycelium
in stubble (Sitton and Cook 1981; Summerell and
Burgess 1988).
Nitrogen management has a strong impact on
disease incidence and severity, particularly in
low-rainfall environments where plants mature
without the benefi t of late-season rainfall (Cook
and Veseth 1991; Smiley et al., 1996a). Applica-
tion of nitrogen at rates greater than required for
the expected or attained grain yield typically
increases the expression of whiteheads, reduces
grain yield and test weight, and increases grain
protein content. In low-rainfall regions where low
nitrogen inputs are required to produce low-

protein soft-white wheat, severity of crown rot is
greatly increased when growers apply higher rates
of nitrogen to produce high-protein market classes
of wheat.
Wheat cultivars differ in susceptibility to crown
rot, although the range available among released
cultivars is usually small (Wallwork et al., 2004).
Moreover, cultivars or lines expressing resistance
in some instances are often highly variable in
responses over seasons and geographic areas
(R.W. Smiley, unpublished data). Durum wheat
( T. turgidum ssp. durum ) tends to be more sus-
ceptible than bread wheat ( T. aestivum L.)
(Kirkegaard et al., 2004) and should be avoided
in high disease-risk situations. Wheat lines with
higher levels of resistance have been identifi ed in
a wide range of backgrounds, including bread
wheat, T. zhukovskyi Menabde et Ericzjan, T.
dicoccum Schrank, and synthetic wheat (Wallwork
et al., 2004; Nicol et al., 2007). Resistance appears
to be expressed against all pathogen species
(Miedaner 1997; Wallwork et al., 2004). Both
seedling and adult-plant forms of partial resis-
tance have been identifi ed for F. pseudogra-
minearum . For screening purposes, seedling but
not adult-plant resistance has been inversely cor-
related with the genetically determined depth at
which crown tissue is formed for each wheat gen-
otype (Wildermuth et al., 2001). Genotypes with
seedling resistance form crowns at more shallow
depth than susceptible genotypes, possibly
enabling them to partially escape or delay infec-
tion. Most work on resistance to Fusarium in
wheat has been done with head blight, but the
lack of correlation between resistance for head
blight and resistance for crown rot in rye ( Secale
cereale L.) (Miedaner et al., 1997) suggests that
head blight resistance will not necessarily be
effective against crown rot.

PYTHIUM ROOT ROT

Pythium root rot of wheat is caused by numerous
species of Pythium . These species have a broad
host range including maize, barley, oat, rye, and
many broadleaf crops such as pea ( Pisum sativum