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containing the mutant allele for the
ahFAD2B
gene. During the process of developing Tif-
guard High O/L, this CAPS marker was con-
verted to a gel-free single nucleotide polymor-
phism (SNP) assay using HybProbe design (Chu
et al. 2011).
An accelerated backcross breeding program
with MAS was used to develop Tifguard High
O/L. Tifguard was used as the recurrent female
parent and two high O/L cultivars were used as
donor parents for the high O/L trait. F
1
,BC
1
F
1
,
and BC
2
F
1
individuals carrying the marker alle-
les for both nematode resistance and high O/L
were selected for use as male parents in the next
round of crossing. BC
3
F
1
seedlings heterozy-
gous for high O/L were selected and allowed to
self. Homozygous BC
3
F
2
seedlings were iden-
tified as Tifguard High O/L. Three cycles of
backcrossing were deemed adequate based on
the high coefficient of coancestry between recur-
rent and the donor parents (Chu et al. 2011).
The effectiveness of selection of nematode
resistance has been the most successful use of
MAS in peanut to date. However, the use of
a single gene trait that confers near-immunity
may be subject to breakdown of resistance under
high selection pressure, and has been cause
for concern even before the release of COAN.
Therefore, new sources of resistance for nema-
todes, such as amphidiploids derived from
A.
stenosperma
, which is highly resistant to fungi
and nematodes (Proite et al. 2008, Leal-Bertioli
et al. 2010; Santos et al. 2011), would be a useful
resource for peanut breeding.
The previously mentioned markers for nema-
tode resistance (Burow et al. 1996) were identi-
fied using bulked segregant analysis. This is effi-
cient for identifying markers with major effects
but is less successful at identifying markers with
smaller effects. Evidence for presence of a sec-
ond, recessive resistance gene was provided by
Church et al. (2005). QTL analysis of a seg-
regating BC
3
F
1
population developed from the
TxAG-6 x Florunner cross has revealed the pres-
ence of three additional QTLs, with QTLs now
from both A and B genomes (Burow et al. 2012).
The previously known marker contributes more
to the explanation of phenotypic variance than
the newer markers; however, newer markers may
be of use to develop a variety with a more durable
resistance. It is possible that the presence of these
additional genes for resistance could explain
in part the linkage drag for yield observed in
COAN.
Leaf Spot Resistance: Two Complex
Traits Controlled by Many Genes
Etiology
The foliar diseases of early leaf spot (caused
by
Cercospora arachidicola
S. Hori) and late
leaf spot (caused by
Cercosporidium person-
atum
[Berk. and Curtis] Deighton), also known
as
Phaeoisariopsis personata
([Berk. and Curt.]
Deighton), are two of the most limiting biotic
stresses in peanut production known worldwide
(Shokes and Culbreath 1997), causing yield
losses of up to 50% (Smith 1984; McDon-
ald et al. 1985). In West Africa, yield losses
can be as high as 70% (Waliyar et al. 2000).
Both diseases often occur together in the
same field, even though one may predominate
(Hassan and Beute 1977). The result of the dis-
ease is defoliation, reducing yield through reduc-
tion of photosynthesis, death of the plant, and
pod loss.
Although these diseases can be controlled
using fungicides, their application is costly in
the United States (Coffelt and Porter 1986). A
study in Ghana (Naab et al. 2005) has confirmed
that foliar application of fungicides can increase
biomass and kernel yields in rainfed peanuts by
39% and 75%, respectively. However, the use of
fungicides, though allowing to increase yields,
is not feasible for many farmers in West Africa,
where poverty is prevalent. Credit facilities for
the purchase of inputs, as well as the input avail-
ability and delivery system, are not adequately
developed. The most practical control method
for these farmers would be the use of host plant
resistance (Holbrook and Stalker 2003).
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