Agriculture Reference
In-Depth Information
early-generation visual selection. The findings of
this study validate the effectiveness of visual se-
lection in early-generation breeding, surprisingly
even for yield attributes. Brown (1988) had pre-
viously shown that the non-genetic component
of seedling tuber size unduly influenced sin-
gle-hill breeding clone yields, with the recom-
mendation being that little emphasis be placed
on yield at this stage of selection. Yield and its re-
spective components are selected more efficiently
using field-grown tubers in subsequent clonal
generations (Caligari
et al
., 1986; Maris, 1986;
Brown, 1988; Kumar and Gopal, 2006). Selec-
tion against obvious negative attributes such as
tuber malformations are recommended during
single-hill selection, with literature supporting
this approach for improving breeding efficiencies
in the earliest stages of the selection process
(Maris, 1988; Ruiz De Galarreta
et al
., 2006).
Intense selection pressure is applied at this
stage, with typically
1-
4% of evaluated single
hills being selected and advanced based on a sur-
vey of US potato breeding programs (Kathleen
Haynes, personal communication). Bradshaw
(2007a,b) cited literature that stated intense,
early-generation visual selection was inefficient
and discussed an alternative approach using
mid-parent values and seedling progeny tests
to improve efficiencies in the Scottish Crop Re-
search Institute breeding program. Additional
details of this breeding strategy for increasing ef-
ficiencies in combining processing qualities and
disease and pest resistances are described by
Bradshaw
et al
. (2003). The impression of ineffi-
ciency in the breeding process is also supported
by Wenzel
et al
. (1983), with the authors calcu-
lating that 500,000-
2,000,000
seedling tubers
must be evaluated for the selection and release of
one cultivar. Hoopes and Plaisted (1987) stated
greater efficiencies than did Wenzel, with a cal-
culation of approximately
200,000
seedlings
evaluated for every cultivar named and released.
An evaluation of the Northwest Potato Var-
iety Development Program (NPVDP), comprised
of researchers in the USA in Idaho, Oregon, and
Washington, challenges the belief that intense
early-generation selection (
1-
2% of single hills
being selected in the NPVDP) is inefficient in the
identification of new potato cultivars. Since the
inception of the NPVDP in 1985,
41
cultivars
have been released, with six of the fifteen most
widely grown cultivars grown in the USA in
2012 having originated from this program
(USDA, National Agricultural Statistics Service,
Crop Production, December 2012). Based on an
annual evaluation of approximately
210,000
seedling tubers in the NPVDP and the release of
41
cultivars over
27
years, one seedling tuber in
approximately
138,000
will be released as a cul-
tivar. Brown (2011) cited a figure of one cultivar
being identified and released per
60,000
seed-
lings evaluated in a period from 2004 to 2012.
This more recent data on the numbers of seed-
lings evaluated per cultivar released would indicate
much higher efficiencies in traditional breeding
programs than previous citations would indicate.
12-
hill selection (2nd field generation)
On the basis of
1-
2% of single hill clones being
selected in the field and retained, approximately
2000-2250 breeding clones are planted the fol-
lowing year in the Aberdeen potato breeding
program as non-replicated plots consisting of
12
genetically identical plants in what is termed a
12-
hill (12H) plot. This field generation is the
first for which field data on flower color, vine ma-
turity and size, and resistance to diseases such as
foliar early blight (
Alternaria solani
) and verticil-
lium wilt (
Verticillium dahliae
) are collected.
During the field season, 12H plots are also
visually evaluated for the presence of virus—most
notably
Potato virus Y
(PVY), which has become
an increasingly difficult pathogen for breeding
programs due to the introduction of newer strains
of PVY with less obvious foliar symptoms of infec-
tion than the previously prevalent PVY
O
common
strain. Removal or roguing of virus-infected plots
is necessary to prevent the spread of the virus by
aphids throughout the 12H plots; the advent of
enzyme-linked immunosorbent assay (ELISA)
tests that can be conducted in the field have been
adopted by breeding programs to confirm the pres-
ence or absence of virus in “real time” to allow the
removal of infected plants that are asymptomatic
or with weakly expressed symptoms.
At the end of the field season, 12H plots are
dug and laid on the ground for visual evaluation.
Similar to the single-hill generation, selection
against negative tuber attributes is applied. How-
ever, with plots consisting of
12
plants rather
than a single plant, yield now factors into the
decision making to a much greater degree than in
