Agriculture Reference
In-Depth Information
90 cm depending upon variables such as intensity
of surface cultivation or seasonal rainfall (Taylor
and Evans 1998; Thompson et al., 1999; Ophel-
Keller et al., 2008).
The reproductive rate for
P. thornei
increases
as soil temperature increases from 15 to 20 ºC
(Castillo et al., 1996). Irrigated wheat yield in
P.
thornei
-infested fi elds was improved by delaying
planting by one month, presumably because
seedlings overwintered with lower populations
than for early planting (Van Gundy et al., 1974).
In a contrasting use of temperature treatment,
mulching a fi eld with polyethylene fi lm for 6-8
weeks over the hot summer period suppressed
P. thornei
populations by 50% (Di Vito et al.,
1991).
As with cereal cyst nematode,
Pratylenchus
neglectus
and
P. thornei
are often more damag-
ing to crops in drier than wetter regions. The
economic threshold for damage is expected to
be lower for low-rainfall environments than
for crops produced with supplemental irrigation
or in areas of greater precipitation especially
during the growing season (Orion et al., 1984;
Castillo et al., 1995; Nicol and Ortiz-Monasterio
2004).
Chemical nematicides are effective and are
widely used in research (Taylor et al., 1999;
Thompson et al., 1995; Smiley 2005a,b) but are
not economically feasible, registered, or environ-
mentally appropriate for managing these parasites
on wheat. Biological control agents are not com-
mercially available for
Pratylenchus
species on
wheat. Bacterial parasites of
Pratylenchus
have
been reported (Ornat et al., 1999) but are unlikely
to be successfully adapted for managing these
migratory species that are well adapted to highly
diverse soils and climates.
Green-manure crops can be used to sanitize
soil through biofumigation in regions where water
is not a limiting factor for wheat growth. Several
Pratylenchus
species are capable of multiplying in
roots of sudangrass [
Sorghum drummondii
(Nees
ex Steud.) Millsp. & Chase] and many
Brassica
species. Populations may remain high where these
crops are grown to maturity for seed or forage
harvest. However, when green tissue from some
of these crops is incorporated into soil it is, in
some cases, capable of reducing the elevated pop-
ulation to preplant levels or below (Bernard and
Montgomery-Dee 1993; Potter et al., 1998, 1999;
Forge et al., 2000).
FUTURE PERSPECTIVES
In most regions, and especially in the developing
world, the impact of cereal cyst and root-lesion
nematodes on wheat yield has not been docu-
mented, because extraction of nematodes is not a
normal practice for the husbandry of wheat.
Even where nematodes are known to be damag-
ing the identity of the species and pathotype
(for cyst nematodes) complex is often poorly
defi ned.
More intensive surveys are required to more
clearly identify the following: (i) areas where
cereal cyst and root-lesion nematodes are present;
(ii) species and pathotypes; (iii) occurrences of
mixed populations; and (iv) the magnitude and
frequency of yield loss. Further development of
molecular identifi cation and quantifi cation tech-
niques and adoption of these procedures by com-
mercial soil diagnostic laboratories will allow
more rapid and effective surveys of populations
in areas where nematodes are currently not moni-
tored. A commercial testing program in South
Australia is of particular interest (Ophel-Keller et
al., 2008). A DNA extract from soil is used to
quantitatively estimate inoculum levels of multi-
ple fungal pathogens and populations of several
nematode species, including
H. avenae
,
P.
neglectus
, and
P. thornei
. Levels of disease risk,
established from many years of collected fi eld
population and yield loss data, are communicated
back to farmers through a network of agronomic
advisors.
Many wheat breeding programs are not breed-
ing for resistance to cereal cyst and root-lesion
nematodes. Reasons may include a lack of under-
standing of the importance of the issue, limited
fi nancial, technical or institutional support for
this disciplinary research, or lack of fi eld test
