Nematode Resistance in Soybean - Translational Genomics for Crop Breeding: Biotic Stress - page 112

the length of rotations. Resistant cultivars do
not require specialized equipment, and gener-
ally seed costs are similar to that of susceptible
cultivars (Boerma and Hussey 1992). In addi-
tion, they may limit damage from other disease
complexes associated with nematodes (Bond and
Wrather 2004). Cultivars resistant to SCN have
been shown to yield 10-50% more than suscepti-
ble cultivars (Shannon et al. 2004). Soybean cul-
tivars resistant to RKN yielded five times more
than highly susceptible cultivars (Kinloch et al.
1998).

method suggested by Niblack et al. (2002) pro-
vides a system to compare SCN reaction among
genotypes based on the same phenotyping proce-
dure. Recently, Brown et al. (2010) reported that
an automated fluorescent-based imaging system
is just as accurate ( r 2 ≥ 0.95) and more effi-

cient ( 50% faster) than manual counting under

a microscope. This method can greatly improve
the consistency and turnaround of data while
reducing the time and labor commitment associ-
ated with SCN female counting.
Genetic marker technology has facilitated the
identification, location, and characterization of
QTL associated with SCN resistance. As already
mentioned in the chapter, the genes rhg1 located
on Chr. 18 and Rhg4 located on Chr. 8 are
most frequently associated with SCN resistance
from the various sources studied to date (Con-
cibido et al. 2004). Resistance QTL have been
shown to provide resistance to more than one
HG type of SCN (Guo et al. 2006; Vuong et al.
2010, 2011; Wu et al. 2009; Yue et al. 2001).
There are many additional examples of broad-
based resistance provided by specific QTL, as
well as resistance to only specific HG types.
With the availability of genetic markers linked to
SCN resistance genes, marker-assisted selection
(MAS) promises to increase the efficiency and
speed of the development of SCN-resistant cul-
tivars (Mudge et al. 1997). Until recently, most
breeders have selected resistant lines by inocu-
lating plants in a greenhouse with eggs or cysts
of SCN. The SCN greenhouse bioassay is labor-
intensive, costly, and success is dependent on
precise environmental conditions. Conventional
SCN greenhouse screening can cost up to $6or
more per plant and still takes 30 days to com-
plete a screening cycle. With MAS, breeders
are able to select lines on the basis of alleles
at genetic markers linked to SCN resistance. The
lines with a high probability of having resistance
genes will be selected, reducing the number of
lines that need to be evaluated in the greenhouse
(Concibido et al. 2004). MAS using SSRs is
cost-effective, and single nucleotide polymor-
phism (SNP) markers are even lower in cost

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Soybean Cyst Nematode

Developing cultivars resistant to SCN has been
challenging because of the genetic variability of
the pathogen, the exotic and unadapted nature of
resistance sources, and the fact that several genes
are involved in resistance. Resistance is defined
on the basis of nematode reproduction on a geno-
type compared to a susceptible soybean standard.
Reproduction on a range of genotypes or indica-
tor lines is further used to classify susceptibility
or resistance to different HG types (or races). The
SCN female index (FI %), used to classify resis-
tance, is defined elsewhere in this chapter. Four
categories of resistance are designated to aid in
classification of resistance (Niblack et al. 2002;
Schmidt and Shannon 1992). The categories are
FI

< 10% (resistant), FI = 10-30% (moderately

resistant), FI 31-60% (moderately susceptible

or slightly resistant), and FI

=

>

60% (susceptible).
Young (1998) summarized methods for eval-
uating resistance to nematodes that enable
researchers to accurately evaluate numerous
genotypes. Appropriate screening techniques are
essential for the successful development of culti-
vars resistant to nematodes. Screening can be car-
ried out in the field, but so far it is primarily per-
formed in the greenhouse under controlled con-
ditions. Screening is also complicated by the fact
that SCN inheritance is complex, resistance to a
specific HG type must be conducted in separate
tests, and cyst counts must be made for reactions
to each HG type. A standardized greenhouse