Agriculture Reference
In-Depth Information
of the roles of avenacinase, melanin, and laccases
in take-all, and the link between
G. graminis
var.
tritici
and
Magnaporthe grisea
(Cook 2003;
Freeman and Ward 2004) are likely to provide
new options for achieving genetic resistance.
Likewise, despite the lack of specifi c genetic
resistance to
Pythium
in wheat, a better under-
standing is emerging for innate resistance and the
role of signaling pathways, especially the jas-
monic acid and ethylene pathways (Vijayan et al.,
1998; Okubara and Paulitz 2005). Elicitors pro-
duced by
Pythium
have also been identifi ed,
which may be perceived by the host plant
(Veit et al., 2001). Higher levels of eyespot resis-
tance may result from studies of the determi-
nants of pathogenicity by
Oculimacula yallundae
and
O. acuformis
, including analysis of the
infection process and importance of tissue
susceptibility.
Identifi cation and deployment of genetic
resistance has been especially diffi cult for species
of
Pythium
,
Rhizoctonia
,
Fusarium
, and
Gaeuman-
nomyces
. Strong advances are currently being
made in mapping QTLs for partial seedling
resistance to Fusarium crown rot (Boville et al.,
2006; Collard et al., 2006). This work is also being
extended to include adult-plant resistance.
However, these pursuits need to be balanced
by a deeper understanding of the components
of resistance, including resistance to penetra-
tion, resistance to stem colonization, plant
reaction to infection, and sensitivity to toxins
to enable a more defi nitive identifi cation of
the QTL.
Identifi cation of loci for resistance to
Fusarium
and other pathogens are complemented by
improvements in the precision and speed of
assays to detect disease resistance or to link phe-
notypic reactions to sources of genetic resistance
(Cowger and Mundt 1998; Wildermuth et al.,
2001; Mitter et al., 2006). Continued develop-
ment of markers for detecting the presence of
resistance genes in seedlings will further improve
the effi ciency of wheat breeding programs.
Plant breeders in France (INRA) are also attempt-
ing to clone the
Pch1
gene for resistance to
eyespot.
conspecifi c with others, and other described
species may actually be complexes of cryptic
species. Many sequences of
Pythium
species on
GenBank may also be misidentifi ed. Likewise,
newly recognized polymorphism within
G. grami-
nis
var.
tritici
has been revealed (Lebreton et al.,
2004), and it appears that anastomosis groups of
R. solani
may function as phylogenetic and bio-
logical species which have evolved separately on
different host plants and no longer exchange
genetic material (González et al., 2006). PCR
techniques were used to show that
Cephalospo-
rium gramineum
can become seedborne in wheat
(Vasquez-Siller and Murray 2003), and real-time
PCR procedures were developed to identify and
quantify several of these pathogens in DNA
extracts from soil and plants (Lees et al., 2002;
Schroeder et al., 2006; Okubara et al., 2008).
Real-time PCR is also now used to make routine
farm management decisions. A commercial soil
diagnostic laboratory in South Australia uses a
DNA extract from soil to identify and estimate
population levels of several nematode species and
inoculum levels of fungal pathogens, including
F.
culmorum
,
F. pseudograminearum
,
G. graminis
var.
tritici
,
G. graminis
var.
avenae
, and
R. solani
AG-8
(Ophel-Keller et al., 2008). Predictions of poten-
tial disease risk are communicated back to farmers
through a network of agronomic advisors (Herdina
and Roget 2000). This application of modern
technology will undoubtedly facilitate more rapid
and effective surveys of species distribution and
inoculum density.
The growing body of literature on molecular
aspects of pathogenicity will continue to refi ne
understanding of pathogenic variation within
pathogen populations, as well as to improve
development of genetic resistance. Considerable
new molecular information is available regarding
the pathogenicity and genetic structure of
B
.
sorokiniana
(Kumar et al., 2002),
Fusarium
species
causing crown rot (O'Donnell et al., 2004; Monds
et al., 2005; Chakraborty et al., 2006), and
G.
graminis
var.
tritici
(Lebreton et al., 2004). All
Fusarium
populations studied thus far appear to
be recombining, suggesting an unrecognized role
for ascospores in epidemiology. Current studies
