Agriculture Reference
In-Depth Information
Plant-parasitic nematode species that live in
the soil represent one of the most diffi cult pest
problems to identify, demonstrate, and control. A
particular challenge is to clearly identify damage
by cereal nematodes, as the symptoms are non-
specifi c and easily confused with other ailments
such as nitrogen defi ciency, water availability,
and other disease. Farmers, pest management
advisors, and scientists routinely underestimate
or fail to recognize their impact on wheat.
Procedures to sample, extract, identify, and
quantify plant-parasitic nematodes are both tech-
nically challenging and time-consuming. Exten-
sive training is necessary to distinguish genera
and species (Varma 1995; Mai and Mullin 1996;
Siddiqi 2000), which is essential for implementa-
tion of appropriate management strategies.
The damage threshold, defi ned as the number
of nematodes to give a specifi c yield loss, is deter-
mined by both environmental and genotypic
factors. The threshold generally is decreased
when plant growth is stressed by drought, poor
soil nutrition, impediments to root penetration,
or adverse temperature. The threshold is increased
by partial or full resistance reactions by a given
cultivar. Damage caused by cereal nematode is
likely to be greater where limited rotation or
cultivar options exist, especially in rainfed
cereal monoculture, including “rotations” of
winter wheat with summer fallow. Unlike the
visually obvious and more vastly studied cereal
rusts, the global knowledge of economic impor-
tance is less well known and understood due
to diffi culties working with soil-inhabiting
nematodes.
The most important plant-parasitic species
affecting wheat are in the genera
Heterodera
(cyst),
Pratylenchus
(root-lesion),
Meloidogyne
(root
knot),
Ditylenchus
(stem),
Tylenchorhynchus
and
Merlinius
(stunt),
Paratrichodorus
(stubby-root),
and
Anguina
(seed-gall) (Rivoal and Cook 1993;
McDonald and Nicol 2005; Nicol and Rivoal
2007; Bockus et al., 2009).
This chapter will focus on two nematodes of
primary global importance to wheat. The global
distribution of cereal cyst nematode species and
pathotypes is clearly a major economic constraint
to rainfed wheat production systems, especially
where monocultures are dominant. Root-lesion
nematode species are also important but appear
to have a more restricted distribution.
CEREAL CYST NEMATODE
Cyst nematodes are the most studied plant-
parasitic nematodes on wheat (Cook and Noel
2002; Nicol 2002; Nicol et al., 2003). Although
the “
Heterodera avenae
group” (Handoo 2002) is
a complex of 12 species and intraspecifi c
pathotypes that invade roots of cereals and grasses,
three main species are the most economically
important:
Heterodera avenae
,
H. fi lipjevi
, and
H.
latipons
(Rivoal and Cook 1993, McDonald and
Nicol 2005).
Heterodera avenae
is economically important in
temperate wheat-producing regions throughout
the world, including North and South Africa,
East and West Asia, Australia, Europe, the Indian
Subcontinent, the Middle East, and North
America.
Heterodera latipons
occurs mostly
throughout the Mediterranean region but also in
Asia and Europe. This species was recently
described as widespread and economically impor-
tant in key wheat growing provinces of China
(Peng et al., 2007).
Heterodera fi lipjevi
was recently
detected in North America (Smiley et al., 2008)
and also has an increasingly recognized wide dis-
tribution across northern Europe and continental
climates of Central and West Asia, as well as the
Middle East and Indian Subcontinent. Only one
species is generally identifi ed in most regions but
mixtures of species may also occur in individual
fi elds (Abidou et al., 2005).
Less prevalent species of cyst nematode associ-
ated with wheat include
H. arenaria, H. bifenestra
,
H. hordecalis
,
H. mani
,
H. pakistanensis
,
H.
pratensis
,
H. zeae
, and
Punctodera punctata
.
Symptoms and epidemiology
Plants with roots heavily damaged by
H. avenae
appear initially as pale green seedlings that lack
