Agriculture Reference
In-Depth Information
Globose, greyish brown apothecia bearing
ascospores of
O. yallundae
develop on decaying
stems and leaf sheaths late in the season and are
mainly found on stubble during the intercrop
period (Wallwork and Spooner 1988; Hunter
1989). Although
O. yallundae
has been considered
the more important causal agent of eyespot,
O.
acuformis
increased signifi cantly to become the
dominant species during the 1990s in the Pacifi c
Northwest US (Douhan et al., 2003) and Western
Europe (Lucas et al., 2000).
The forms which are more commonly observed
in a growing wheat crop are the anamorphs
H. herpotrichoides
and
H. acuformis
. They can be
differentiated through examination of conidial
and cultural characteristics:
H. herpotrichoides
produces either curved or curved and straight
conidia (4 septate, 35-80 μm × 1.5-2.5 μm) on
fast growing, even-edged colonies and squirrel-
grey or olive-grey mycelia, and
H. acuformis
with
only straight conidia (4-6 septa, 43-120 μm ×
1.2-2.3 μm) on slow-growing, feathery or uneven-
edged colonies and grey to brown-grey mycelia
(Nirenberg 1981). The two species can be identi-
fi ed more rapidly and accurately by molecular
markers used in a PCR assay combined with
restriction enzyme digestion of an amplifi ed ribo-
somal DNA fragment (Gac et al., 1996a,b). A
real-time PCR assay now allows workers to simul-
taneously identify and quantify
O. yallundae
and
O. acuformis
(Walsh et al., 2005).
group). In the early 1990s the effi ciency of these
DMIs was also compromised by fungicide resis-
tance in some regions (Leroux and Gredt 1997).
The most-used current fungicide is cyprodinil,
but decreased sensitivity from repeated applica-
tions has also been observed with this fungicide
(Babij et al., 2000).
Moderate resistance to eyespot, provided by
the gene
Pch2
, was fi rst incorporated in the French
winter wheat cultivar Cappelle-Desprez (Muranty
et al., 2002). This gene remained durable despite
widespread exploitation and has been transferred
into many cultivars. The gene
Pch1
was trans-
ferred (Maia 1967) from
Aegilops ventricosa
Tausch. into
Triticum persicum
Vavilum ex Zhuk.,
and the F
1
hybrid was backcrossed with the bread
wheat cultivar Marne, producing the resistant
line VPM
1
; initials refer to ventricosa, persicum,
and Marne. The resistance to eyespot conferred
by
Pch1
is higher than that conferred by
Pch2
(Hollins et al., 1988; Jahier et al., 1989), which
only acts at the seedling stage (Muranty et al.,
2002), and now is being emphasized in wheat
breeding programs in Europe and the US (Allan
et al., 1993). These eyespot resistance genes
remain durable and there is no evidence of differ-
ences with respect to
Oculimacula
species.
FUTURE PERSPECTIVES
Knowledge of the etiology and control of root,
crown, and culm rots continues to improve
with advances in technology. The following
examples illustrate promising areas of emerging
research.
Development of molecular procedures is
greatly expanding the precision of pathogen iden-
tifi cation and, consequently, also supporting a
constant evolution in pathogen taxonomy and
phylogeny. Examples include identifi cation of
DNA sequences of the ITS region of
Pythium
(Martin 2000; Lévesque and de Cock 2004;
Schroeder et al., 2006),
Rhizoctonia
(González
et al., 2001),
Gaeumannomyces
(Freeman and
Ward 2004), and
Fusarium
(O'Donnell et al.,
2004). Results of these tests suggest that many
Pythium
species reported in the literature may be
Disease management
Crop rotation remains the best preventive method
for managing this disease. Intensifi cation of agri-
culture in Western Europe led to an increase in
grain yield losses due to eyespot and this disease
became, from the 1970s, the main target of fun-
gicides applied between the tillering and stem-
extension stages of wheat growth. The most active
fungicides have been the antimicrotubular benz-
imidazole group, especially carbendazim. In the
early 1980s, because of the selection of resistant
strains, the benzimidazoles were replaced by C-14
demethylation inhibitors (DMIs) such as pro-
chloraz (imidazole group) or fl usilazole (triazole
