Nematodes Which Challenge Global Wheat Production - Wheat Science and Trade - page 165

vigor (Color Plate 16). Mature plants are often
severely stunted (Color Plate 17). Plants with
visual damage often occur in patches but may also
occur over entire fi elds, particularly under mono-
cultures of susceptible cereals and when com-
bined with inadequate plant nutrition or other
stress.
Symptoms on roots are specifi c to host species.
Wheat ( Triticum species) and barley ( Hordeum
vulgare L.) roots invaded by Heterodera avenae
branch excessively at locations where juveniles
invade, resulting in a bushy or knotted appear-
ance (Color Plate 18a). Root symptoms often do
not become recognizable until 1-3 months after
planting, depending upon climatic conditions and
spring or winter wheat growth habit. Oat ( Avena
sativa L.) roots invaded by H. avenae are short-
ened and thickened but do not exhibit the knotted
symptom.
Heterodera species complete only one genera-
tion per crop season. Juveniles penetrate epider-
mal and cortical cells of young root segments in
the zone of elongation. They enter the stele,
where they induce the formation of a specialized
feeding cell called a syncytium. Females are fertil-
ized by males and 100-600 eggs are retained in
the female body.
Mature females become sedentary and embed-
ded in the root. The presence of the white swollen
female body (0.5-2 mm; about the size of a pin
head) is diagnostic. It can be seen around the
fl owering time of wheat. One or more females
are generally visible at the point of abnormal
root proliferation. They protrude from the
root surface, glisten when wet, and are white-
gray. They are best viewed by washing a root
sample and observing under low magnifi ca-
tion (Color Plate 18b), because their presence
among knotted roots is often obscured by adher-
ing soil. The females are attached loosely and
are easily dislodged when soil is washed from
roots.
Upon death of host roots the female body wall
dies and hardens into a resistant dark-brown cyst
of a similar size as a soil particle. These cysts
mostly dislodge into the soil as the wheat roots
decompose. The cyst protects eggs and juveniles

during periods between hosts. Eggs inside cysts
may remain viable for several years. Emergence
of juveniles from brown cysts requires a period of
dormancy (diapause) that differs among species
and climatic regions. Diapause characteristics
must be understood before damage by these nem-
atodes can be effectively managed. Emergence of
juveniles is triggered by specifi c interactions
between soil temperature and moisture, and these
conditions may be overcome to some extent by
exudates from host roots (Ismail et al., 2000;
Scholz and Sikora 2004; McDonald and Nicol
2005). Well-established infestations exhibit eco-
typic differences in which peak numbers of infec-
tive juveniles in soil generally coincide with the
traditional wheat sowing and seedling growth
stages in each geographic region (Rivoal and Cook
1993).
Cereal cyst nematode is not strongly restricted
by soil type but damage is often greatest in light-
textured, well-drained soils such as sands. The
damage threshold varies with soil type, climate,
and cultivar, and with nematode species, viru-
lence, and ecotype characteristics. These variable
infl uences on plant damage make it diffi cult to
directly relate initial populations with reduction
in grain yield (Bonfi l et al., 2004).

Causal organisms
The most economically important cyst nematode
species on wheat include H. avenae Wollenweber,
H. latipons Franklin, and H. fi lipjevi (Madzhidov)
Stone. Two important nematodes previously
reported as H. avenae have been reclassifi ed. The
so-called Gotland strain of H. avenae is now
accepted as H. fi lipjevi (Bekal et al., 1997; Ferris
et al., 1999), making this species more reported
than previously thought. Most recently, H. avenae
pathotype Ha13 in Australia was redescribed as
H. australis (Subbotin et al., 2002); that designa-
tion has not yet been widely accepted.
Identifi cation of cereal cyst nematodes is
complex and has traditionally been based on com-
parative morphology and diagnostic keys (Luc
et al., 1988; Handoo 2002). Techniques based on
protein or DNA differences using RFLP are now