Agriculture Reference
In-Depth Information
Cultivars tolerant to
P. neglectus
are not necessar-
ily tolerant to
P. thornei
, and vice versa. Further-
more, resistance and tolerance to each of these
root-lesion nematode species are genetically
independent; a cultivar can be resistant and
intolerant, susceptible and tolerant, or another
combination.
Successive or frequent crops of susceptible
wheat cultivars elevate populations of
P. neglectus
and
P. thornei
and increase the level of risk to
subsequent intolerant crops. Many cultivars of
mustard [
Brassica juncea
(L.) Czern.,
Sinapsis alba
L.], canola (
Brassica napus
L.), lentil (
Lens culina-
ris
Medik.), and chickpea (
Cicer arietinum
L.) also
increase the population of
P. neglectus
or
P. thornei
,
or both, with multiplication capacities differing
greatly for each combination of
Pratylenchus
species and host cultivar (Bernard and Montgom-
ery-Dee 1993; Castillo et al., 1998; Potter et al.,
1999; Fatemy et al., 2006).
Crops that restrict multiplication of
P. neglectus
and/or
P. thornei
, such as some cultivars of barley,
saffl ower (
Carthamus tinctorius
L.), triticale, fl ax
(
Linum usitatissimum
L.) and fi eld pea (
Pisum
sativum
L.), reduce the nematode population and
improve the yield potential for subsequent intol-
erant wheat crops (Van Gundy et al., 1974; Heide
1975; Esmenjaud et al., 1990; Lasserre et al.,
1994; Farsi et al., 1995; Thompson et al., 1995;
Hollaway et al., 2000; Taylor et al., 2000;
Riley and Kelley 2002; Smiley et al., 2004b).
However, results from these studies indicate
hosting ability is species- and cultivar-specifi c,
within both legumes and cereals. Therefore,
hosting-ability studies must be conducted with
local cultivars.
Wheat cultivars exhibiting tolerance to
P.
neglectus
and
P. thornei
have been deployed effec-
tively in Australia (Vanstone et al., 1998, 2008;
Thompson et al., 1999, 2008; Nicol et al., 2001),
and studies are underway in the US to identify
tolerant germplasm (Fig. 8.4). However, toler-
ance alone is not considered an effective long-
term management strategy.
Resistance is the most important and economi-
cal strategy for reducing populations of root-
lesion nematodes. Unlike the single-gene
inheritance of cereal cyst nematode resistance,
root-lesion nematode resistance is quantitative
and controlled by several genes, making the pros-
pect of developing effective resistance more chal-
lenging. However, many sources of resistance to
P. thornei
have been identifi ed in commercial
wheat cultivars, in Middle East landrace lines,
and in wheat relatives such as
Aegilops
species
(Table 8.2) (Thompson and Haak 1997; Nicol
et al., 1999, 2001, 2003; Nombela and Romero
1999; Thompson et al., 1999; Hollaway et al.,
2000; Zwart et al., 2004, 2005; Tokay et al., 2006;
Sheedy et al., 2008). Several lines are especially
interesting in that they exhibit resistance to both
P. neglectus
and
P. thornei
(Zwart et al., 2005;
Nicol et al., 2007; Sheedy et al., 2007, 2008).
Introgression of dual-resistance sources (Fig. 8.3)
into commercial cultivars would eliminate the
need for farmers to identify
Pratylenchus
to the
species level before selecting a resistant cultivar.
Also of particular interest are lines that convey
high levels of both tolerance and resistance to
P. thornei
or
P. neglectus
.
Phenotypic identifi cation of resistance, coupled
with molecular biology, has been used to investi-
gate the genetic control and location of resistance
genes and the identifi cation of resistance markers.
Quantitative trait loci (QTLs) for resistance to
P.
thornei
have been identifi ed on chromosomes 1B,
2B, 3B, 4D, 6D, and 7A (Schmidt et al., 2005;
Zwart et al., 2005, 2006; Tokay et al., 2006). The
P. neglectus
resistance gene
Rlnn1
occurs on
chromosome 7A, and a molecular marker can
identify its presence in seedlings (Williams et al.,
2002). A resistance gene for
P. neglectus
was also
identifi ed on chromosome 4D (Zwart et al.,
2005).
Research on resistance to
P. neglectus
and
P. thornei
has included development of simple
sequence repeat (SSR) markers for tracking
QTLs in breeding programs. Associations
between markers and resistance reactions have
been suffi ciently consistent to demonstrate the
potential for applying marker-assisted selection to
the improvement of
Pratylenchus
resistance in
wheat. This process is actively practiced by Aus-
tralian and CIMMYT international wheat breed-
ing programs, using the
Rlnn1
marker (Williams
et al., 2002).
