Agriculture Reference
In-Depth Information
available to facilitate identifi cation to a species
level and to study phylogenetic relationships
(Subbotin et al., 1996, 1999, 2000, 2001, 2003;
Bekal et al., 1997; Andrés et al., 2001a,b; Mokabli
et al., 2001; Rivoal et al., 2003). Further work is
needed to convert these RFLP-based primers
into the more cost- and time-effi cient PCR-based
probes.
One of the major challenges to controlling
cereal cyst nematodes is occurrence of individuals
within species and also among populations from
different regions that is highly variable in viru-
lence and in reproductive capacity (fi tness) char-
acteristics on the same host (Rivoal et al., 2001;
Mokabli et al., 2002). Moreover, individual
species within the
H. avenae
group are highly
heterogeneous with respect to virulence to spe-
cifi c host genotypes (Cook and Rivoal 1998; Cook
and Noel, 2002; McDonald and Nicol, 2005).
Virulence groups (pathotypes) are differenti-
ated (Cook and Noel 2002; McDonald and Nicol
2005) by testing unknown populations against a
matrix of cereals in “The International Cereal
Test Assortment for Defi ning Cereal Cyst
Nematode Pathotypes,” which was developed by
Andersen and Andersen (1982). The test distin-
guishes three primary groups based on host resis-
tance reactions of three barley cultivars carrying
the resistance genes
Rha1
,
Rha2
, and
Rha3
. Addi-
tional barley, oat, and wheat differentials are used
to defi ne pathotypes within each group. The most
widely distributed
H. avenae
populations in
Europe, North Africa, and Asia are in groups 1
and 2 (Al-Hazmi et al., 2001; Cook and Noel
2002; Mokabli et al., 2002; McDonald and Nicol
2005). Pathotypes in group 3 are prevalent in
Australia, Europe, and North Africa (Rivoal and
Cook 1993; Mokabli et al., 2002). Unfortunately,
the pathotype concept is incomplete because it
was established to differentiate northern Euro-
pean populations of
H. avenae
and is increasingly
incapable of clearly defi ning resistance reactions
achieved with populations in other regions. For
instance, three undescribed pathotypes were
recently reported from China (Nicol and Rivoal
2007; Peng et al., 2007), and the existing pathotype
matrix does not defi ne North American popula-
tions (R.W. Smiley, unpublished data). The Test
Assortment therefore greatly underestimates
polymorphism for
H. avenae
(Cook and Noel
2002; McDonald and Nicol 2005),
H. latipons
,
and
H. fi lipjevi
. The Test Assortment needs to be
revised to capture new sources of resistance and
pathogen variation.
Management
To achieve effective control of cereal cyst
nematodes it is necessary to reduce the popula-
tion below the economic threshold for damage.
This requires defi nitive studies on population
dynamics and yield losses on representative
local cultivars under natural fi eld conditions.
Cultural practices based on rotational combina-
tions of nonhosts (noncereals), resistant cultivars,
and clean fallow can effectively control these
nematodes. Restricting hosts to 50% of the
time in heavier soils and 25% in lighter soils
can cause dramatic reductions in the popula-
tion of
H. avenae
. However, these management
strategies each require a full understanding of
the virulence and diapause characteristics for
the local nematode populations, and of the
effectiveness and durability of the resistance
gene(s)
deployed
against
that
nematode
population.
The use of host-plant resistance is one of the
most effective methods of controlling cereal
nematodes. Resistance is defi ned as the ability of
the host to inhibit nematode multiplication (Cook
and Evans 1987). Ideally resistance should be
combined with tolerance, which is the ability of
the host plant to maintain yield potential in the
presence of the nematode (Cook and Evans 1987).
The use of cultivars that are both resistant and
tolerant offers the best control option, in addition
to being environmentally sustainable and requir-
ing no additional equipment or cost. However,
the use of resistance requires a sound knowledge
of the virulence spectrum for the targeted species
and pathotypes. Wheat cultivars resistant to
H.
avenae
populations in one region may be fully
susceptible to populations in other regions. This
was shown for Australian cultivars evaluated in
Israel (Bonfi l et al., 2004) and for the cultivar Raj
MR1 in India, which is effective in Rajasthan but
