Agriculture Reference
In-Depth Information
Distribution and losses
Although distributed worldwide, powdery mildew
is economically important in most temperate
regions of the world, particularly on winter wheat
(Bennett 1981). Because of its common occur-
rence, powdery mildew can cause extensive plant
damage to susceptible cultivars under climatic
conditions conducive to disease. Early infection
can cause losses of entire leaves. Under conducive
conditions, the disease can weaken stems and
thereby intensify lodging. In addition to reducing
leaf area available for photosynthesis, the fungus
also serves as a metabolic sink, drawing photosyn-
thates to itself. Reductions in grain yield, grain
weight, and grain quality can occur. Reported
yield losses have varied from 5 to 45% (Fried
et al., 1981; Leath and Bowen 1989; Griffey et al.,
1993; Conner et al., 2003). The disease also
reduces kernel softness, fl our yield, and protein
content in soft wheat (Johnson et al., 1979; Everts
et al., 2001).
blotch) has the telomorphic stage of
Phaeosphae-
ria nodorum
(E. Muller) Hedjar. (Shoemaker and
Babcock 1989; Cunfer and Ueng 1999). This
heterothallic ascomycete produces ascospores in
pseudothecia that mature in wheat residue. Asco-
spores were shown to be produced and deposited
on wheat from August to October in the northern
hemisphere and from February to April in the
southern hemisphere (Arseniuk et al., 1998).
Following primary infection of the leaves, the
fungus forms asexual pycnidia and pycnidio-
spores of the anamorphic stage,
Stagonospora
nodorum
(Berk.) Castellani and Germano.
Changes in fungal nomenclature, coupled with
the use of the full Latin name of the anamorphic
stage in the common name of the disease, has
caused some confusion in what to call the disease
(Cunfer and Ueng 1999). Pycnidiospores serve
as the repeating stage of the fungus over the
growing season and are spread by splashing rain-
fall (Shah et al., 2001). Epidemics are favored by
wet, windy conditions and by air temperatures
between 20 and 27 ºC. Infected seed can serve as
a source of primary inoculum (Bennett et al.,
2007).
Pathogen variability
Blumeria graminis tritici
has acquired a high degree
of specialization on wheat, with races attacking
specifi c cultivars. In Hungary, between 1971 and
1999, small shifts in virulence in the powdery
mildew population were found every 3 to 5 years,
while major shifts were found about every 5 to 7
years (Szunics et al., 2001). Genetic drift, muta-
tion, and directional selection all contribute to
shifts in powdery mildew populations (Limpert
et al., 1999; Paillard et al., 2000). The frequency
of virulence to specifi c
Pm
genes changes over
time and space (Niewoehner and Leath 1998;
Imani et al., 2002; Brown and Hovmoller 2002;
Parks et al., 2008).
Identifi cation and symptomology
The disease is also known as glume blotch. On
wheat heads, it is fi rst apparent on glumes as
small, irregular, gray to brown spots, which can
enlarge and take on a chocolate-brown color.
These small spots or blotches then coalesce, often
producing small black-brown fungal fruiting
bodies in the center of the lesions. Severe head
infections occur when there is excessive rainfall
between fl owering and maturity. The fungus
can infect the seed, which can serve as a source
of primary inoculum for subsequent wheat
crops.
The lesions also can occur on leaves, leaf
sheaths, and stem nodes. Early leaf symptoms
appear as tan-brown spots having an oval or lens
shape. As these spots enlarge and merge, the
necrotic spots turn a light-gray color and form
irregular patterns on the leaf. Pycnidia may be
apparent and scattered within the lesions. As the
STAGONOSPORA NODORUM BLOTCH
Taxonomy and life history
The causal fungus of stagonospora nodorum
blotch (previously known as septoria nodorum
