Synthetic Wheat—An Emerging Genetic Resource - Wheat Science and Trade - page 360

Fusarium and powdery mildew diseases
Fusarium graminearum Schwabe, commonly
known as head scab or Fusarium head blight,
reduces yield and produces toxin-infected grain
in high-rainfall environments. Resistance to scab
is incomplete and a number of quantitative trait
loci (QTL) for resistance has been identifi ed (Liu
and Anderson 2003; Pumphrey et al., 2007). Nev-
ertheless, the search for new additive variation or
immunity has been extended to synthetic wheat.
Although complete resistance has not been found,
signifi cant variation for resistance has been
reported in primary synthetic wheat (Oliver et al.,
2005). In some cases, the resistance is not associ-
ated with Ae. tauschii but with the tetraploid
donor. Hartel et al. (2004) found scab resistance
in T. dicoccoides associated with chromosome 3A
that was expressed in the resultant primary syn-
thetic. Four of these primary synthetics have
since been registered as sources of scab resistance
(Berzonsky et al., 2004). There are a number of
mechanisms of scab resistance (Mesterhazy 1995),
and evidence suggests that the synthetics confer
type II resistance, which limits fungal spread
within the spike (Hartel et al., 2004). Resistance
to powdery mildew [ Blumeria graminis (DC) E.O.
Speer f. sp. tritici ] has also been reported, in
association with chromosomes 5D and 7D (Lutz
et al., 1995).

dicoccum parents were resistant and the Ae. taus-
chii parents were susceptible to RWA. Although
the level of resistance in the resultant synthetics
was marginally less than the tetraploid parent,
effective levels of resistance were observed.
Clearly, this variability in combination with D
genome or rye-segment resistance would improve
the effectiveness of the resistance of cultivated
wheat to RWA.
The same set of 58 primary synthetics was also
evaluated for greenbug ( Schizaphis graminum
Rondani) resistance (Lage et al., 2003). The
greenbug absorbs plant sap and injects a toxin
that causes tissue death; subsequent yield losses
can be signifi cant. Interestingly, while the expres-
sion of resistance was suppressed in some combi-
nations, epistatic interaction gave rise to some
synthetics with higher levels of resistance than
either parent. Similarly, Smith and Starky (2003)
found more than 33% of 149 synthetic materials
based on resistant Ae. tauschii sources were highly
resistant to greenbug.
Hessian fl y ( Mayetiola destructor Say) is wide-
spread across Asia, Europe, and North America.
Larvae feed on stem sap, weakening the plant
and reducing yield. Resistance has been found
and described in Ae. tauschii and primary syn-
thetic wheat (Hatchett et al., 1981; Hatchett
and Gill 1981). A gene for resistance, H32
located on chromosome 3D, was recently identi-
fi ed in the primary synthetic parent of the Inter-
national Triticae Mapping Initiative (ITMI)
population (Sardesai et al., 2005). In addition,
two primary synthetics resistant to Hessian fl y
were recently registered for breeding applica-
tions (Xu et al., 2006), although their respective
resistance genes may be allelic to the previously
described H26 and H13 genes (Wang et al.,
2006).

Insect pests
Insect damage can signifi cantly reduce wheat
yield and quality. Resistance to a range of insect
pests has been reported in primary synthetic
wheat. The Russian wheat aphid (RWA), Diura-
phis noxia (Mordvilko), is widespread across Asia
and North America, causing signifi cant crop
losses when infestations are severe. Resistance in
hexaploid bread wheat has been generally associ-
ated with the D genome or with rye introgressions
(Lage et al., 2004). Resistant primary synthetic
wheat accessions, based on crosses to resistant Ae.
tauschii , have been reported (Nkongolo et al.,
1991; Mujeeb-Kazi et al., 2000b). Lage et al.
(2004) evaluated 58 primary synthetics developed
by crossing T. dicoccum with Ae. tauschii . The T.

Soilborne nematodes
Soilborne nematodes have been shown to limit
wheat production in some areas of the world and
remain undiagnosed in many more. Cereal cyst
nematode ( Heterodera avenae Wollenweber) can
be an important constraint in southern Australia,