Agriculture Reference
In-Depth Information
the reactions of all sources of resistance in wheat
to greenbug biotypes reported by Harvey et al.
(1991), Porter et al. (1994), and Harvey et al.
(1997).
Molecular markers for greenbug resistance in
wheat have seen limited use to date. The
Gb5
resistance gene on chromosome 7S of
T. speltoides
(Tausch) Gren. was transferred to an interstitial
chromosome segment of 7AL in 'Pavon' wheat
via the
ph1b
mutation (Dubcovsky et al., 1998).
Gene
Gb5
, located on the resultant 7AS·7AL-
7S#1L·7AL chromosome, provides resistance to
greenbug biotypes C, E, I, and K (Table 9.2). In
the presence of the wild-type
Ph1
locus, chromo-
some segment 7S#1 does not recombine with
wheat chromosome 7A. Therefore, only one of
several RFLP markers associated with 7S#1 is
needed to track
Gb5
(Dubcovsky et al., 1998). A
wheat germplasm line (designated UCRBW98-2)
carrying 7AS·7AL-7S#1L·7AL with
Gb5
was
released for breeding purposes (Lukaszewski et
al., 2000).
Markers have also been identifi ed for resis-
tance genes
Gb2
and
Gb6
located on the 1RS
arm of the wheat-rye translocation chromo-
some T1AL·1RS (Graybosch et al., 1999). As
in the previous example of a translocated
chromosome segment, in the presence of the
wild-type
Ph1
locus, 1RS is inherited as a non-
recombined block of genes. Graybosch et al.
(1999) identifi ed secalin proteins and rye-specifi c
PCR markers on 1RS that could effectively
track
Gb2
and
Gb6
.
(Walters 1984). The RWA gained international
pest status after being discovered in Mexico
infesting wheat in 1980. From there it moved into
the US via Texas in 1986 and rapidly spread
throughout the primary wheat producing states in
the western half of the US by 1987 to 1988 (Mor-
rison 1988). The RWA continues to be one of the
most important pests of dryland wheat and barley
in the US and South Africa, although it is of
minor importance elsewhere.
Nearly 1 million hectares of the total 27 million
hectares of wheat planted in the western US was
treated for RWA at a cost of $17 million in 1987.
Insecticide costs in combination with wheat yield
losses caused by RWA damage exceeded $53
million, with about one-half of this total incurred
by the state of Colorado alone (Webster et al.,
1994). Losses to cereal crops totaled $893 million
from 1987 to 1993. The impact of the RWA on
wheat production became negligible in the central
US Great Plains after 1994, following the release
of 'Halt', a RWA-resistant wheat cultivar which
carried the
Dn4
resistance gene (Quick et al.,
1996).
Wheat and barley are the main cultivated hosts
for RWA. Triticale and oat also can serve as hosts.
However, the RWA must utilize volunteer cereal
growth and other Graminaceous hosts to survive
between summer grain harvest and the next plant-
ing event in the fall. Just as important to RWA
ecology are the wild grass hosts in the US that
include
Agropyron
spp.,
Elymus
spp.,
Pascopyrum
spp.,
Bromus
spp., and
Aegilops
spp. (Armstrong
et al., 1991; Hammon and Bishop 1997). With its
broad host range, RWA can exist in the US
without the presence of wheat or barley. However,
cultivated hosts such as wheat and barley provide
RWA with the opportunity to exploit a host crop
monoculture and thus become economically
signifi cant.
RUSSIAN WHEAT APHID
Economic impact and distribution
The Russian wheat aphid (RWA),
Diuraphis noxia
(Mordvilko) (Color Plate 23), is a signifi cant pest
of wheat and barley. This aphid occurs through-
out the major wheat producing areas of the world
except Australia. Its origin is thought to be in the
wheat producing region of central Asia. The
RWA was not considered a serious pest of cereal
crops until 1978, when it was discovered causing
extensive damage to wheat in South Africa
Biology, plant damage, and
control methods
The life cycle of the RWA is typical of most
aphids, whereby it reproduces parthenogenically
as female viviparae from spring to fall under warm
temperatures. Generation times range from 8 to
