Nematode Resistance in Soybean - Translational Genomics for Crop Breeding: Biotic Stress - page 114

nematode-free plants (Young 1998). Tolerance
is HG-type independent (Boerma and Hussey
1992), with plants being productive regardless
of field SCN HG type. Ultimately, tolerant geno-
types should be readily recognized by good
performance in SCN-infested, untreated fields.
High levels of tolerance have been reported in
the Japanese cultivar Gendenshirazu (Ichnohe,
1988). PI 97100, MG VII collected from Korea
and the germplasm line G88-20092 derived from
this PI also show tolerance (Boerma and Hussey
1993). Boerma and Hussey (1984) suggest toler-
ant cultivars have an advantage over resistant cul-
tivars by preventing yield losses without impos-
ing selection pressure on SCN field populations.
Growing resistant and tolerant cultivars in alter-
nate years has been suggested to improve the
longevity of resistance genes while providing
stable yield performance without putting selec-
tion pressure on the nematode (Shannon and
Anand 1997).

0-4 scale, with 0 being no galls to 4 with 75%

of the entire root system; and (2) on the basis of
the number of galls on the root system on a 0-5
scale, where 0 means no galls and 5 equals

>

100
galls per root system (Ritzinger et al. 1998). In
addition to root galling, nematode reproducibil-
ity was also assessed to identify resistant geno-
types. Many studies using this screening scheme
identified a number of soybean PIs with high lev-
els of resistance to Mi and Ma nematodes (Harris
et al. 2003; Luzzi et al. 1987; Luzzi et al. 1995a;
Yates et al. 2010). Root-knot screening is most
frequently carried out in greenhouse tests, but it
can also be conducted in root-knot infested fields.
A successful greenhouse system used at the Uni-
versity of Georgia-Atlanta (UGA) in many stud-
ies is detailed in research conducted by Hussey
et al. (1991).
Many soybean varieties resistant to southern
RKN (Mi) are readily available, and some culti-
vars have combined resistance to both RKN and
SCN, primarily in MG V and later MGs (Young
1998). There are few genotypes available in MG
IV and earlier that carry resistance to any species
of RKN. However, a few productive cultivars in
MGs II-IV were identified with resistance to Mi
nematode. Cultivars with resistance to SCN and
Mi RKN are needed for the midwestern United
States (Kuger et al. 2008). Because MG IV and
earlier genotypes are being grown in both North
America and South America where these nema-
tode species are abundant, there is more empha-
sis by soybean breeders to combine resistance to
RKN and SCN in earlier maturities because they
can often occur in the same field.
About 3,000 accessions from the USDA Soy-
bean Germplasm Collection have been screened
for higher resistance to RKN species (Harris
et al. 2003; Luzzi et al. 1987). Among these,
PI 96354, PI 200538, and PI 230977 were deter-
mined to have the highest levels of resistance to
Mi, Ma and Mj nematodes, respectively. These
PIs were found to carry unique resistant alleles
with better resistance than in resistant U.S. cul-
tivars (Luzzi et al. 1994a, 1995a, 1995b). Three
breeding

≥

Root-Knot Nematode

Among common RKN species ( Meloidogyne
spp.), Mi, Ma, and Mj nematodes are the primary
species that cause yield losses in soybean. Mi
nematode is most prevalent, but the other species
can be more prominent, such as Ma nematode in
peanut-growing areas and Mj nematode in more
tropical areas (Kinloch 1998). Resistant cultivars
to date are not immune to RKN species, and 30-
50% yield loss can occur if the nematodes are
in high numbers in the field (Carter et al. 2004).
Resistance to all three species can be found in
the USDA Soybean Germplasm Collection. Pub-
licly released southern U.S. cultivars Forrest,
Bedford, Braxton, Gordon, Jackson, Kirby, and
derivatives from some of these genotypes have
resistance to all three RKN species (Carter et al.
2004).
Assessment of host plants' reactions to vir-
ulence of RKN is generally based on root gall
index scores (Niblack et al. 2004). Gall index
can be rated (1) by the percent of roots with galls
on the entire root system (Stetina et al. 1997) on a lines—G93-9009, G93-9106, and