Agriculture Reference
In-Depth Information
infect other grass genera, including
Agropyron,
Agrostis, Brachypodium, Bromus, Dactylis, Festuca,
Hordeum, Glyceria, Poa
, and
Secale
, these alter-
nate hosts are not believed to play an important
role in septoria tritici blotch distribution and epi-
demiology (Eyal 1999). The importance of seed-
borne inoculum of
S. tritici
is somewhat unclear
(Eyal 1999). Signifi cant losses to both grain yield
and grain weight can be realized.
by this asexual stage are typically found in lesions
on leaves, sheaths, and stems and make up the
repeating cycle of the disease throughout the
growing season. Conidia are primarily wind-
disseminated within and between wheat crops or
from other grasses to wheat (Schilder and Berg-
strom 1992). Environmental conditions, host
genotype, and pathogen virulence all infl uence
disease onset, spread, and severity (Ciuffetti and
Tuori 1999).
Pathogen variability
Genetic diversity is very high in
M. graminicola
(Schnieder et al., 2001; Zhan et al., 2003);
however, pathogenic variability is more limited.
Early research found distinct physiological spe-
cialization in
S. tritici
isolates at the genus level in
wheat (
T. durum
- and
T. aestivum
-specifi c iso-
lates) (Eyal et al., 1985; Kema et al., 1996a).
However, later studies showed distinct, qualita-
tive differences in
S. tritici
× wheat cultivar inter-
actions (Eyal 1999).
Identifi cation and symptomology
The disease is also known as yellow leaf spot.
In the fi eld, the disease appears on leaves as
oval- to irregular-shaped, yellow, tan, or brown
lesions, each containing a small dark spot and
surrounded by a yellow border. Individual lesions
can coalesce over part of, or an entire, leaf surface.
On more resistant cultivars, lesions are typically
smaller and darker in color. The disease pro-
gresses from the lower leaves to the upper leaves
as the plant grows and matures. The disease pro-
gresses rapidly under frequent rains with cool,
cloudy, and humid weather conditions. The tan
spot pathogen can also infect seed and be dis-
persed by seed.
Pyrenophora
tritici-repentis
infec-
tion can produce a red smudge or black point
symptom on seed that can impact grain
grading as well as subsequent seedling infection
(Fernandez et al., 2001).
TAN SPOT
Taxonomy and life history
The causal fungus of tan spot is typically referred
to by its telomorphic stage,
Pyrenophora tritici-
repentis
(Died.) Drechs. [syn.
P. trichostoma
(Fr.)
Fuckel], a homothallic ascomycete. Female and
male gametes from the same mycelium are formed
in an ascocarp called a pseudothecium. The
fungus saprophytically colonizes dead and dying
leaves, leaf sheaths, and stems to form mature
perithecia-containing ascospores (Odvody et al.,
1981). Pseudothecia require a cold period to
mature and are visible as dark, raised pinpoints
on wheat straw. Ascospores produced in pseudo-
thecia can serve as primary inoculum and infect
a wheat crop in the fall, winter, or spring
depending on crop rotation, environmental con-
ditions, and geographic location (Wright and
Sutton 1990). The anamorphic stage of the
fungus is
Drechslera tritici-repentis
(Died.) Shoe-
maker (syn.
Helminthosporium tritici-repentis
Died.). Conidiophores and conidia produced
Distribution and losses
Tan spot is an important disease on wheat
throughout the world (Ciuffetti and Tuori 1999).
The disease tends to be more damaging in con-
tinuous wheat areas and where the previous
wheat crop residue is not incorporated into the
soil or burned off (Bockus and Shroyer 1998;
Carignano et al., 2008). Grain losses are most
severe when the disease spreads to and damages
the fl ag leaf. The fungus attacks a wide range of
grass species that can serve as alternate hosts
(Krupinsky 1992; Ali and Francl 2003). Grain
yield reductions varying from 3% to 50% and
kernel weight reductions up to 13% have been
