potato breeding at the scottish plant breeding
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Potato Breeding at the Scottish Plant Breeding
Station and the Scottish Crop ResearchInstitute: 1920 – 2008
John E. Bradshaw
Received: 10 October 2008 /Accepted: 16 February 2009 /
Published online: 15 April 2009
# EAPR 2009
Abstract Seventy-two potato cultivars have been bred at the Scottish Plant Breeding
Station and the Scottish Crop Research Institute since 1920. The original genetic base
contained resistance to wart disease and to viruses, but not comprehensive resistance
to all strains. Introgression of resistance genes from the wild and cultivated potato
species of Latin America started for late blight in 1932, for viruses in 1941 and for
potato cyst nematodes in 1952. Just seven of the 219 wild tuber-bearing species
recognized by Hawkes in 1990 feature in the pedigrees of our cultivars, with
Solanum demissum for blight resistance in 58, S. vernei for nematode resistance in
19 and S. microdontum for Potato virus Y resistance in 15, the other four species
being S. multidissectum, S. commersonii, S. maglia and S. acaule. Resistance to
other fungal and bacterial diseases has been mainly due to chance rather than
deliberate breeding. From 1970, selection for yield and quality included processing
quality, and despite lack of commercial success, prospects remain good for cultivars
resistant to sweetening during cold storage. Since 1990 prebreeding has combined
desirable traits through efficient recurrent selection based on progeny testing and
provided parents for the commercially funded breeding of finished cultivars. Only
one cultivar is a Neotuberosum – Tuberosum hybrid, whereas 15 cultivars have the H1 gene for resistance to Globodera rostochiensis introgressed from group
Andigena. Long-day Phureja cultivars are finding a market niche for their flavour
attributes. Breeding strategies and methods are critically reviewed from a genetic
viewpoint.
Keywords Breeding methods . Introgression . Neotuberosum . Phureja . Population
improvement . Potato genetics . Progeny testing
Potato Research (2009) 52:141 – 172
DOI 10.1007/s11540-009-9126-5
J. E. Bradshaw (*)
Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, UK
e-mail: john.bradshaw@scri.ac.uk
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Abbreviations
CPC Commonwealth Potato Collection
NL National List
PCN Potato cyst nematode
PLRV Potato leafroll virusPMTV Potato mop-top virus
PVX Potato virus X
PVY Potato virus Y
QTL Quantitative trait locus
SCRI Scottish Crop Research Institute
SPBS Scottish Plant Breeding Station
SSRPB Scottish Society for Research in Plant Breeding
TRV Tobacco rattle virus
Introduction
Foundation of the Scottish Society for Research in Plant Breeding and the Scottish
Plant Breeding Station in 1920
The Scottish Society for Research in Plant Breeding (SSRPB) was formed in 1920 to
establish and run a plant breeding station in Scotland. It was conceived during the First
World War out of concerns about feeding the British people and out of recognition of
the opportunities afforded by the new science of genetics to develop scientific breeding.The cost was shared 50:50 by the Government and the Society, which raised money
from firms, associations and persons involved in agriculture. In 1920, the Society
purchased Craigs House, Corstorphine, Edinburgh, and the Scottish Plant Breeding
Station (SPBS) was born (Gallie 1971). This marked the start of potato breeding
which continues today at the Scottish Crop Research Institute (SCRI). The Station
had moved to Pentlandfield, near Roslin, Edinburgh, in 1954 and was amalgamated
with the Scottish Horticultural Research Institute at Invergowrie, Dundee, to create
SCRI in 1981, when SSRPB relinquished the management of SPBS. Today the
government is the main, but not exclusive, source of funding for research.
Potato Cultivars Bred Since 1920
Seventy-two cultivars have been bred primarily for use in Great Britain over the last
88 years (Table 1; includes three Kenyan cultivars-discussed in the next section),
from The Alness in 1934 to Mayan Twilight in 2008. The 72 cultivars provide a
measure of the success of the potato breeding and of the germplasm available to
other breeders and hence to potato crop improvement. Some cultivars were
commercially more successful than others. Craigs Royal and its red variant Red
Craigs Royal were the most widely grown second early cultivars in the 1960s.
Pentland Crown was the most widely grown cultivar in Britain from 1970 to 1979
and Pentland Dell was still the seventh most widely grown one in 2007 as a result of
its suitability for making French fries. Pentland Beauty and Pentland Javelin were
both popular first early cultivars. Pentland Hawk and Pentland Ivory became
established as early maincrop cultivars, but were not as popular as Pentland Squire,
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Table 1 Cultivars bred at the Scottish Plant Breeding Station/Scottish Crop Research Institute
Cultivar Type Wild species in
pedigree
Female parent Male parent Year
cross
Year
NLa Time
(years)
The Alness SE Abundance Majestic 1927 1934 7
Craigs Defiance EM Epicure Pepo 1933 1939 6
Craigs Bounty M dms cmm mag W967c38 70(13) 1936 1946 10
Craigs Royal SE Craigs Defiance Gladstone 1939 1948 9
Craigs Snow
White
M dms cmm mag Craigs Defiance W800(2) 1939 1948 9
Craigs Alliance FE Craigs Defiance The Alness 1939 1949 10
Pentland Ace SE dms Craigs Defiance 997a44 1942 1951 9
Pentland Beauty FE dms Craigs Royal 1306a2 1946 1955 9
Pentland Crown EM G414a64 11 – 79 1951 1958 7
Roslin Chania Kenya dms 882(5) 1104c2 1945 1960 15
Roslin Eburu Kenya dms 882(5) 1104c2 1945 1960 15
Roslin Sasumua Kenya dms Craigs Defiance 1306a2 1946 1960 14
Pentland Dell EM dms Roslin Chania Roslin Sasumua 1953 1960 7
Pentland Envoy FE Bismark 11 – 79 1953 1961 8
Roslin Riviera EM dms 791a116 1104a3 1946 1961 15
Pentland Falcon EM dms Roslin Riviera Dr McIntosh 1954 1962 8
Pentland Glory FE Craigs Royal 2288a2 1955 1963 8
Pentland Hawk EM dms 2168e3 Roslin Sasumua 1958 1966 8
Pentland Ivory EM dms Pentland Crown Pentland Dell 1959 1966 7
Pentland Javelin FE 2693ac2 11 – 79 1959 1968 9
Pentland Kappa SE dms Roslin Eburu Roslin Sasumua 1960 1968 8
Pentland Lustre FE dms Craigs Royal 2700b8 1960 1969 9
Pentland Marble FE dms 3305(6) 3392(1) 1961 1970 9
Pentland Meteor FE dms 2749c12 2693abc2 1960 1970 10
Pentland Raven EM dms Red Craigs Royal 3323(4) 1961 1970 9
Pentland Squire EM dms Pentland Crown Pentland Dell 1960 1970 10
Croft EM dms 2895fg6 Pentland Dell 1963 1975 12
Sheriff EM dms 3639ab3 3681ad1 1966 1981 15
Baillie SE dms mcd 3071ab1 Maris Piper 1969 1981 12
Provost FE dms 2693ac2 M109 – 3 1965 1981 16
Kirsty M dms Pentland Crown 3683a2 1969 1982 13
Ailsa EM mcd G4324(545) Maris Piper 1971 1984 13
Moira FE dms 3653a1 6669abc8 1970 1984 14
Morag EM dms vrn mlt 8844(11) Pentland Ivory 1973 1985 12
Rhona EM dms 2895fg6 Pentland Ivory 1971 1985 14
Shula M dms mcd Pentland Hawk 8328ab21 1973 1986 13
Teena EM dms mcd G5299(1) Pentland Ivory 1973 1986 13
Shelagh M dms GL71/179 3681ad1 1974 1986 12
Morna FE dms vrn Pentland Javelin 8795a4 1973 1986 13
Glenna SE dms vrn 10223(7) 10300(13) 1975 1987 12
Torridon M dms mcd vrn acl 8372a(17) G5833(5) 1977 1989 12
Brodick EM dms mcd vrn 7683a12 8898abc14 1978 1990 12
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Table 1 (continued)
Cultivar Type Wild species in
pedigree
Female parent Male parent Year
cross
Year
NLa Time
(years)
Stirling M dms mcd 8318(6) 8204a4 1977 1991 14
Eden EM dms mlt 10899ad14 Maris Piper 1979 1991 12
Glamis EM dms Maris Peer 3146a3 1976 1991 15
Provan M dms vrn mlt 11234ab16 Maris Piper 1979 1991 12
Cramond EM dms mcd vrn 7683a12 10173ab19 1978 1992 14
Buchan EM dms Croft Cara 1981 1993 12
Brodie EM dms Croft Cara 1981 1993 12
Othello M dms Croft Cara 1981 1996 15
Derek EM dms Croft Cara 1982 1996 14
Claret EM dms G6755(1) Cara 1982 1996 14Spey M dms vrn 12327a1 Cara 1981 1996 15
Kirrie SE dms Spunta Cara 1986 1996 10
Anya SE Désirée Pink Fir Apple 1986 1996 10
Amour EM dms vrn 9559ab2 Cara 1981 1998 17
Blush FE dms vrn 15005a2 12380ac2 1987 1998 11
Golden
Millennium
M dms mcd vrn
mlt
Brodick 14025a3 1987 1999 12
Harborough
Harvest
M dms mcd vrn mlt Brodick Eden 1990 1999 9
Montrose M dms mcd vrnmlt
Brodick Eden 1987 1999 12
Sebastian M dms vrn 8890ab42 Cara 1981 2000 19
Thyme EM dms vrn Cara 12380ac2 1985 2000 15
Scarborough EM dms mcd vrn
mlt
Brodick Eden 1990 2001 11
Tay EM dms mcd vrn
mlt
83P18a1 Brodick 1990 2001 11
Lady Balfour M dms mcd vrn 8204a4 15119ac5 1988 2001 13
Eve Balfour M dms mcd vrn Stirling 15119ac5 1991 2002 11
Vales Sovereign EM dms 15205ab6 Picasso 1992 2003 11
Vales Emerald FE Maris Peer Charlotte 1997 2005 8
Vales Everest M dms mlt Cara 12674ab1 1988 2005 17
Phureja cultivars
Mayan Gold LM DB270(43) DB220(52) 1986 2001 15
Inca Sun LM 81S66 71T46 1993 2001 8
Inca Dawn LM 80CP23 71T46 1993 2003 10
Mayan Queen LM DB257(28) 84.2P.75 1996 2008 12
Mayan Star LM 84.2P.75 DB257(28) 1995 2008 13Mayan Twilight LM Open-pollinated 1994 2008 14
FE first early, SE second early, EM early maincrop, M maincrop, LM late maincrop, acl Solanum acaule,
cmm S. commersonii, dms S. demissum, mag S. maglia, mcd S. microdontum, mlt S. multidissectum, vrn
S. verneia Year cultivar registered or added to National List
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which was released 4 years later. Remarkably, in 1977, the Pentland series of
cultivars occupied approximately 40% of the British potato acreage (Mackay 2003).
Since 1980, 48 cultivars have been released, but until recently none have achieved
the major impact of the cultivars just considered. Now though, Lady Balfour is the
number one organic cultivar in Britain and Vales Emerald is established as a first early cultivar, Vales Sovereign as an early maincrop for prepack and baking, and
Vales Everest as a maincrop for general use.
A complete record exists of all of the crosses made from 1920 onwards and this
provides a genetic history which allows a number of questions to be asked and
answered about potato breeding at SPBS/SCRI over this period of time. No attempt
is made to review potato breeding in general as this has been done elsewhere by the
author (Bradshaw 2007a, b), but comment is made as appropriate, for example,
where the SPBS/SCRI programmes differ markedly from those being done
elsewhere. Although not all of the parents in Table 1 are shown in Figs. 1, 2, 3, 4,5, 6, 7, 8 and 9, the latter do provide an accurate genetic history of the breeding
programmes without going into even greater detail. Two types of code are used: in
429a8, 429 is the cross and a8 the clone from that cross; likewise in 571(18), 571 is
the cross and 18 the clone from that cross.
Fig. 1 Use of Solanum demissum for late blight resistance and S. rybinii (= S. phureja) in breeding
Pentland Ace, Roslin Chania, Roslin Sasumua, Pentland Dell and Pentland Ivory
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Cultivars for Overseas
Potato breeding at SBPS/SCRI has always been done primarily in the context of
providing new cultivars better adapted to seed production in Scotland, ware growing
in Britain, and the requirements of end users. However, in the 1950s and 1960s
germplasm was sent to East Africa and India, to help these countries establish their
own breeding programmes, and from this germplasm cultivars were selected. Three
‘Roslin’ cultivars (Chania, Eburu and Sasumua for Kenya) are shown in Table 1 as
they appear in the pedigrees of other cultivars; but there were others: cvs Kufri Jyoti
(India), Kufri Moti (India), Roslin Bvumbwe (Malawi), Roslin Elementieta (East
Africa), Roslin Mount Kenya (East Africa), Roslin Tsangano (Malawi) and Kenya
Akiba (Kenya), and six more cultivars in Kenya in the early 1970s (Black 1971;
Holden 1977).
Since 1980 cultivars have also been identified as suitable for seed export to the
Mediterranean and North Africa: cvs Baillie, Kirsty, Ailsa, Rhona, Shelagh, Morna,
Torridon, Stirling, Glamis, Othello, Amour, Sebastian, Lady Balfour, Eve Balfour
and Vales Everest. Brown et al. (1996) analysed some of the trial results and found a
greater correlation for total marketable yield between the Scottish ware site where
clones were selected and sites in England (r = 0.43 to 0.70) than with sites in the
Mediterranean (r = 0.00 to 0.67). Although the very best clones from the Scottish
ware site performed reasonably well in the Mediterranean, the results supported the
idea that selection would be optimized by selecting in environments more similar to
those in which the cultivars were to be grown. Stewart et al. (1994) did, however,
find that some useful selection for resistance to early blight ( Alternaria solania), a
warm climate disease of potato, could be done on seedlings in a glasshouse at SCRI.
Fig. 2 Selection for field resistance to late blight: pedigree of clone 8204a4, the parent of Stirling and
Lady Balfour [2323c1 is derived from 1531(3) and 11 – 79]
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The selected individuals were more resistant in the field in Israel than unselected
controls (35.4% leaf necrosis compared with 66.9%); hence, it is important to
determine by experiment the optimum strategy for selecting for target environments.
An account of breeding for overseas can be found in Mackay (2003).
Government Testing and Registration of New Cultivars
Two years before the foundation of SPBS, in 1918, the Board of Agriculture for
Scotland set up a Government Plant Registration and Seed Testing Station to decide
what are and what are not new cultivars of plants, and also to grant certificates for
those that after careful trial were recognized as new and improved cultivars. SPBS
became the neighbour of this station at East Craigs, Edinburgh. Hence, the breeding
of new cultivars has always finished with official government testing and
registration, and this process has always involved an official description of the
new cultivars. This is now done through national listing in which new cultivars must
be shown to be distinct, uniform and stable and to have value for cultivation and use.
Likewise, the multiplication of new cultivars always involved official government
testing of seed stocks. This is now done through the Seed Potato Classification
Scheme which is administered and managed by a division (Science and Advice for
Fig. 3 Use of S. simplicifolium (= S. microdontum) and S. rybinii (CPC 979) as sources of resistance to
Potato virus Y (PVY) and clones 41956 and 44/1016/10 (from S. acaule) as sources of resistance to Potato
virus X : pedigree of G5833(5), the parent of Torridon
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Scottish Agriculture) of the Scottish Government, which is the direct descendant of
the 1918 Government Plant Registration and Seed Testing Station.
Multiplication and Marketing of New Cultivars
From 1920 until 1967, SSRPB utilized the expertise and facilities of its members to
multiply and market the cultivars produced by SPBS, an arrangement which was
formalized in 1952 in terms of official agents appointed from the membership. After
the 1964 Plant Varieties and Seeds Act established plant breeders’ rights, the
National Seed Development Organisation was set up and marketed state-bred
cultivars from 1967 to 1987, with royalty income returned to central government.
Pentland Falcon was the first cultivar to receive plant breeders’ rights. Since 1987
the breeding of finished cultivars has been funded by commercial partners who are
also responsible for multiplication and marketing (Mackay 2003). Royalties from
Fig. 5 Use of H1 gene from Commonwealth Potato Collection (CPC ) 1673 in breeding cultivars resistant
to Globodera rostochiensis, the golden potato cyst nematode
Fig. 4 Use of clones from G1921 as Ry sto source of resistance to PVY: pedigrees of progenies G8866 and
G8867 which provided clones that were used as parents in the multitrait programme
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plant breeders’ rights are negotiated and shared with Mylnefield Research Services,
the commercial wholly owned subsidiary of SCRI.
Today we have commercial breeding contracts with international processing
companies who produce crisps and French fries in Britain, with national companies
who pack table cultivars for the main supermarkets, and with seed producers. In one
sense we have moved from government-funded breeding aimed at farmer and public
good needs, as well as market requirements, to commercial breeding aimed at end-
user needs. However, in practice the difference in breeding objectives is not that great, rather it is a question of who pays for the breeding in relation to who benefits
from new cultivars. Commercial success for new cultivars has always required
attention to market requirements and commercial breeding cannot ignore environ-
mental and health issues. It is important, however, to maintain germplasm collections
such as the Commonwealth Potato Collection (CPC) of wild and cultivated potatoes
from Latin America to provide the ultimate raw materials for breeding, to do parental
breeding (prebreeding) to provide improved germplasm for the breeding of finished
Fig. 7 Andigena accession CPC 2802 as a source of resistance to G. pallida, the white potato cyst
nematode (not shown are clones 12601ab1 and 12636a2, which have pedigrees similar to that of
12674ab1)
Fig. 6 Use of S. vernei as a source of resistance to potato cyst nematodes: pedigree of clone 15119ac5,the parent of Lady Balfour and Eve Balfour
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cultivars, and to do research to develop the genetic knowledge and tools for more
efficient and novel breeding. These activities are still considered appropriate for
government funding and are done at SCRI.
Genetic Base in 1920: Wart and Virus Resistance
Genetic Base
Potato breeding in Britain up to the early twentieth century was reviewed by
Glendinning (1983). Here we start with the first cultivars from SPBS. The Alness
(Fig. 1, Table 1) came from the cross Abundance by Majestic, Craigs Defiance from
Epicure by Pepo and Craigs Royal from Craigs Defiance by Gladstone (Table 1). We
shall consider cvs Craigs Bounty and Craigs Snow White shortly. The other
cultivars which feature in the pedigree of Pentland Dell in Fig. 1 are Kerr ’
s Pink,Shamrock and Dr McIntosh, and through clone 121(2), Witchhill, Flourball and
Fig. 9 The multitrait programme: fourth cycle clone 03C4a3 was used as a parent in commercial breeding
programmes in 2008 as it has good resistance to late blight and G. pallida and is resistant to cold
sweetening (virus resistance status unknown). The two sources of potato cyst nematode resistance are S.
vernei (V ) and Andigena CPC 2802 ( H3). PCN potato cyst nematode
Fig. 8 Pedigree of cv. Brodick, which is resistant to cold sweetening
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Immune Ashleaf. Holden (1977) lists the 26 initial parents used in breeding cultivars
down to Croft (Table 1).
Cvs Majestic, Gladstone, Flourball, Immune Ashleaf, Witchhill, Kerr ’s Pink and
Shamrock were, so far as is known, derived from earlier British cultivars. Pepo was
of European origin and Abundance and Epicure each have Magnum Bonum as thefemale parent. Its female line traces back through Early Rose and Garnet Chili to
Rough Purple Chili and the year 1851 and is the source of Chilean Tuberosum
cytoplasm. Clone 70(13) (Table 1), a parent of Craigs Bounty, came from The
Bishop × Laures, the former being bred by J.H. Wilson of St Andrews (see
“Resistance to late blight ”) from the cross Up to Date × Rector (a New Zealand
variety × Maincrop, the latter having Magnum Bonum as the female parent). In
summary, the parentages of cultivars used in the early breeding work at SPBS trace
back either to old European/North American cultivars or to cv. Rough Purple Chili,
with the exception of cv. Dr McIntosh, which came from back-crossing a hybrid between Herald and Solanum rybinii (= S. phureja, see “Phureja”) to Herald.
Ultimately, like all breeding programmes outside South America, the European and
North American cultivars were derived from some of the original introductions of
potatoes from South America from the sixteenth century onwards, and hence have
sampled the same gene pool (Rios et al. 2007; Ames and Spooner 2008). Except
where there have been introgressions from the wild and cultivated species of Latin
America, or base broadening, the same is true for the wider range of cultivars now
being used as parents in our commercially funded breeding work. Our processing
partners have provided modern parents (both cultivars and breeding clones) from North America and mainland Europe and our other partners have also provided
European cultivars.
Wart Resistance
Among the cultivars available for use as parents in 1920 was resistance to wart,
which was inherited in a simple dominant manner. Back in 1920, wart (Synchytrium
endobioticum) was considered a serious soil-borne disease of potato. It also provides
the first example of success in breeding for disease resistance at SPBS. Wart was
largely eliminated from Britain during the first half of the twentieth century by
coupling the breeding of cultivars with immunity/resistance to race 1 (the only race
found in Britain) with scheduling of land, which prohibited the cultivation of
susceptible cultivars on land known to be infested (Simmonds 1969). Until as
recently as 1990, all our cultivars were resistant, with Stirling our first susceptible
cultivar to be released. However, we may need to reconsider breeding for resistance
if we want our new cultivars to be grown in European countries where wart is still a
problem and other races occur.
Virus Resistance
In 1921 Salaman (1921) recognized that the ‘degeneration’ of potato seed stocks
which worsened with successive generations was the result of virus infection, and
this led to targeted breeding for resistance. Present in the cultivars available for use
as parents at SPBS in 1920 were strain-specific resistances to the mosaic viruses,
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Potexvirus Potato virus X (PVX) and its variant Potato virus B, and Potyvirus
members Potato virus Y (PVY) strain C and Potato virus A. Resistance to all these
viruses was achieved in the second cultivar to be released, Craigs Defiance, and was
also present in cvs Pentland Dell, Pentland Kappa and Croft. Resistance is conferred
by four Tuberosum genes, Nx, Nb, Nc and Na (Davidson 1980). The source of the Nx gene was Paterson’s Victoria, assuming cv. Magnum Bonum came from the cross
Early Rose by Paterson’s Victoria and was not an open self of Early Rose.
Subsequently, genes conferring comprehensive resistance to all strains of these
viruses became preferred, although the Nytbr (Tuberosum) gene for resistance to the
common PVYO strain of PVY proved extremely useful. It was introduced into the
SPBS breeding programme in 1948 through a clone from Australia, 11 – 79 [USDA
41956 × (Katahdin × Snowflake)], whose full pedigree is given by Holden (1977).
The first cultivar with virus resistance from this source was Pentland Crown,
followed by cvs Pentland Envoy, Pentland Ivory and Pentland Javelin. Interestingly,14 out of 15 parental clones from those chosen to start a new breeding programme in
1991 (see “Multitrait Programme”) were resistant to PVYO and 11 probably
contained Nytbr , thus confirming its relatively high frequency in SBPS/SCRI
germplasm (Solomon-Blackburn and Bradshaw 2007). Furthermore, 23 out of our
50 cultivars starting with Pentland Javelin are considered resistant to PVY.
Breeding Methods
Pair Crosses
All of the cultivars bred at SPBS/SCRI have come from pair crosses. Clones from
bud selfs [e.g. 571(18) in Fig. 1] do feature in pedigrees and berries from open selfs
were collected as late as 1965 and also feature in pedigrees (e.g. 1306a2 in Fig. 1).
The crosses are either between clones and cultivars that complement each other for
desirable characteristics, or backcrosses where desirable genes are being introgressed
from the wild and cultivated species of Latin America, maintained in the CPC since
1939 (Bradshaw and Ramsay 2005). Introgression of blight [ Phytophthora infestans
(Mont.) de Bary] resistance began in 1932, followed by virus resistance from 1941,
and finally potato cyst nematode (PCN) resistance from 1952. PCN resistances
provide good examples of the use of the CPC (Bradshaw and Ramsay 2005), which
has been maintained at SPBS/SCRI since 1965.
Size of Programme and Time to Breed a New Cultivar
In the early days the programme was small in size and from 1920 to 1939 a total of 908
crosses were made, an average of 45 per year with a range from 1 to 88. Craigs Royal
and two other cultivars came from a set of 83 crosses. Typically, the year after crossing,
seedlings would be raised in a glasshouse and about 2,500 transplanted to the field.
Around 500 would be selected for planting in small plots the next year, followed by one
or more years of trial and multiplication plots before 3 years of official registration
trials, with registration the next year. Although plot size nominally increased from five
to 12 to 25 tubers, more rapid multiplication took place from clones producing many
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tubers. Hence, the time from crossing to registration varied from 6 to 10 years, with an
average of 8.5 years for The Alness and Craigs series. The Roslin series took longer
because they were crosses made available for selection overseas, although cv. Roslin
Riviera proved suitable as an early maincrop in Britain.
The times shown in Table 1 are from crossing to registration or since 1973 tonational listing. Official registration trials usually took 3 years, whereas National List
(NL) trials are for 2 years. However, as the change resulted in breeders doing an
extra year of pre-NL trials, the times in Table 1 are all comparable. Cultivars would
be added to the Register or NL the year after completing official trials and I have
endeavoured to be consistent throughout Table 1 in giving this date, but there may
be a few examples where I am a year early by mistake.
The Pentland series of cultivars took 7 – 10 years, again with an average of 8.5. Cv.
Pentland Crown came from a set of 59 crosses. The cultivars starting with Croft took
longer, from 8 to 19 years, with an average of nearly 13 years. The higher averagelength of time was due to the logistics of increases in the size of the programme, more
traits being assessed, and potential cultivars being trialled in more environments. The
greater than average times were the result of more seed of promising crosses being
sown and more trialling being done before deciding to submit a potential cultivar to
official trials. By 1981, advances in computing hardware and the in-house development
of suitable software allowed randomization and replication of all trials of all clones
undergoing selection, as well as facilitation of overall planning and data management
(Mackay 2003). The target length of time with the breeding scheme, achieved with a
number of cultivars, was 12 years: year 1 crossing, year 2 seedlings in a glasshouse,year 3 single spaced plants at the seed site, year 4 unreplicated small plots at the seed
site, years 5 – 7 ware (yield) trials at the breeding station, seed production at the seed
site and disease and quality testing, years 8 – 10 multisite trials in Britain and overseas
and larger-scale seed production at the seed site, years 11 and 12 NL trials and year
13 new cultivar(s) added to the NL, 12 years after crossing (Mackay 2005). This
timescale can be reduced to 11 years by having only 2 years of multisite ware trials
(Bradshaw and Mackay 1994; Mackay 2003). Simmonds (1969) was certainly of the
view that greatly increased numbers of seedlings were required in potato breeding
because success is proportional to number, and raising 100,000 seedlings (from about
200 crosses) became normal. However, research in the 1980s found that intense early-
generation visual selection for most quantitative traits was very ineffective,
particularly between seedlings in a glasshouse and spaced plants at a seed site
(Bradshaw and Mackay 1994; Bradshaw et al. 1998). Selection for tuber skin and
flesh colour and shape can be done if these are important for particular consumers.
Use of Progeny Testing
The solution to ineffective early-generation visual selection developed and
implemented at SCRI from 1985 was the use of progeny tests to discard whole
progenies (= full-sib families) before starting conventional within-progeny selection
at the unreplicated small-plot stage. More true seed of the best progenies would be
sown to increase the number of clones on which to practise selection in seeking new
cultivars (Bradshaw and Mackay 1994; Mackay 2003). Currently, seedling progeny
tests are used at SCRI for resistance to late blight (both foliage and tuber), resistance
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to the white PCN and tuber yield and appearance, as visually assessed by breeders.
Tuber progeny tests are used for fry colour and a second visual assessment of tuber
yield and appearance (Bradshaw et al. 2003). Mackay (2005) describes how the
three cvs Golden Millennium, Harborough Harvest and Montrose were bred from 43
crosses and just 3,100 seedlings by using progeny tests for fry colour after storage at 4 and 10 °C for 3 months. Cvs Lady Balfour and Vales Everest came from a set of
30 crosses that was made in 1988 and included in all of the seedling progeny tests in
1990 (not 1989 as that year the programme moved from Edinburgh to Dundee).
Today we consider 40 crosses adequate for a particular target, with approximately
2,000 seedlings from all crosses raised in the non-destructive seedling and tuber
progeny tests and a further 2,000 seedlings subsequently raised from the best ten
progenies. Thus, in total, 20,000 rather than 100,000 seedlings would be raised from
200 crosses, and in this respect our breeding programmes differ from traditional
potato breeding (Bradshaw and Mackay 1994; Mackay 2005).
Rapid Multiplication
Where commercial partners are very focused on a specific market and are prepared to
take the risk of rapidly multiplying a promising clone as a potential cultivar, a few
crosses can quickly result in a new cultivar, as happened with Anya (3,351 seedlings
from seven crosses each with Pink Fir Apple as one parent, aimed at an improvement
on Pink Fir Apple) and Vales Emerald (1,440 seedlings from seven crosses, three with
Maris Peer as one parent, aimed at an improvement on Maris Peer).
Rate of Progress
Today, once promising clones have been identified after both the first and the second
year of ware (yield) trials (i.e. 5 and 6 years, respectively, after crossing), they are used as
parents in the next round of crossing and selecting to keep the momentum of the
programme going. In this sense recurrent selection is operating on a 5- or 6-year cycle.
However, successful potato cultivars can have a long life (e.g. Maris Piper has been the
number one cultivar in Britain for 30 years), as do clones maintained in our museums
because they possess interesting traits, and both continue to be used as parents, a practice
that must be hindering progress, along with longer cycle times than should be necessary.
For example, in the pedigree of cv. Vales Sovereign (Fig. 5), one sees gaps of 11 and
10 years in the two generations preceding the cross that resulted in Vales Sovereign.
Introgression of Genes for Disease and Pest Resistance from the Wild
and Cultivated Species of Latin America
Resistance to Late Blight
Use of R-genes
Late blight, caused by the oomycete Phytophthora infestans (Mont.) de Bary, first
made its impact outside Mexico in 1845 and 1846 when severe epidemics swept
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through North America and Europe and resulted in the Irish potato famine. The
apparent breakthrough in finding genetic resistance came when J.H. Wilson, of
St Andrews, like R.N. Salaman in England, discovered late blight resistance in
S. demissum during the early years of the twentieth century. When Wilson died in
January 1920, SPBS inherited his breeding material, including clone W800(2) [andclone W967c38 which came from The Bishop × W800(2)] which traces back to
Myatt ’s Ashleaf × New Zealand Red, S. commersonii × S. demissum and S. maglia ×
S. edinense, the latter being a hybrid of S. tuberosum and S. demissum, so named
because it was first described from material in the Edinburgh botanic garden.
However, an early setback in breeding for blight resistance at SPBS proved to be a
bad omen. In 1932, hitherto resistant material succumbed to a new race of blight,
whereas the original S. demissum source remained resistant. Eventually cvs Craigs
Bounty and Craigs Snow White were released in 1946 and 1948, respectively. Cv.
Craigs Snow White came from a cross made in 1939 between Craigs Defiance andWilson’s W800(2). It contains what became known as the R1 gene for blight
resistance from S. demissum and was designated the R1 differential once appropriate
genetic studies had been done.
In 1932 W. Black crossed S. demissum (6 x) as the female parent with the first
cultivar from SPBS, The Alness (4 x), and secured a pentaploid (5 x) clone 429a8
from which he was able to introgress blight resistance. Later, in 1937, he secured a
few artificial tetraploid seedlings, including clone 735, by hybridizing S. rybinii
(Ru.159 from Russia) (= S. phureja) (2 x) with S. demissum (6 x), despite their
difference in endosperm balance number. Cv. Pentland Ace (the R3 differential, withgene R3a) was released in 1951 after just three backcrosses to S. tuberosum, starting
with clone 735 (Fig. 1), and genetic analysis tells us that this is as fast as one can
expect to do an introgression with modest but realistic population sizes (Bradshaw
and Ramsay 2005). Marker-assisted selection is faster only when genotypic selection
(for desirable genes and against undesirable ones) is more effective than phenotypic
selection. This is because the number of generations and population sizes required
for a successful introgression depend upon the frequencies of the desirable products
of meiosis during sexual hybridization, which in turn depends upon the number of
chromosomes and the number and distribution of chiasmata.
The commercially more successful cv. Pentland Dell (with genes R1, R2 and R3)
traces its pedigree back to both clone 429a8 and clone 735 and required five
generations (Fig. 1). It entered ware production in Britain in 1963 when race 4 was
the prevalent race of P. infestans. It succumbed to blight in 1967, but nevertheless
went on to become one of SPBS’s most successful cultivars and is still widely grown
in Britain for making French fries, albeit under the protection of fungicides.
By 1968 the number of R-genes identified had risen to the 11 which are currently
recognized, but races of P. infestans overcoming the more recently discovered
R-genes ( R5 to R11; clone 2182ef7 in Fig. 2 is R7 differential) were already widely
distributed in Britain, despite the R-genes not being present in common commercial
cultivars (Malcolmson 1969). It was clear that they would not provide durable
resistance, either singly or in combination, owing to the evolution of new races of
P. infestans. As a consequence, many breeders started to select for quantitative field
resistance, either by using races of P. infestans compatible with the R-genes present
in their material or by creating R-gene-free germplasm so that screening could be
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done with any race. The former strategy, which was adopted at SPBS, is probably
better as there is evidence that defeated R-genes may still contribute some resistance,
so combining R-genes with high levels of field resistance is a desirable goal (Stewart
et al. 2003). Cultivars up until Croft were nevertheless tested for their resistance to
common race 4 (can overcome R4) and the following proved resistant (i.e. had at least one S. demissum derived R-gene other than R4): Craigs Bounty, Craigs Snow
White, Pentland Ace, Pentland Beauty, Pentland Dell, Pentland Falcon, Pentland
Hawk, Pentland Ivory, Pentland Kappa, Pentland Meteor, Pentland Raven, Pentland
Squire and Croft.
Use of Field Resistance
Breeding for field resistance to late blight was started at SPBS in 1954 by Black
(1970). This was possible because he had races of P. infestans compatible with theR-genes for hypersensitive resistance that were present in his germplasm. He defined
field resistance as the degree of resistance exhibited by a plant to all races of
P. infestans to which it is not hypersensitive. The main objective of the breeding
programme was the production of new cultivars of commercial standard possessing
enough inherent resistance to blight to make protective spraying of crops against the
disease unnecessary in any environment. Black thought that field resistance would
be more enduring than hypersensitivity and provide a more reliable protection against
crop failure. He advocated that protection afforded by R-genes for hypersensitivity
must be supplemented by inherent field resistance. Arguments over the durability andeffective use of these two types of resistance intensified with the publication in 1968 of
Van der Plank ’s book on disease resistance in plants (Van der Plank 1968), and have
continued to the present day (Solomon-Blackburn et al. 2007).
By the time when Black retired in 1968, he had shown that field resistance could
be built up rapidly to a high level by appropriate hybridizations (Black 1970) and
had achieved commercial success with Roslin Eburu in East Africa and Roslin
Bvumbwe in Malawi. The breeding material used by Black consisted mainly of
seedling selections and commercial cultivars bred from S. demissum at SPBS. The
seedling selections were six or more generations removed from S. demissum and had
complex pedigrees involving other species such as S. rybinii (= S. phureja) and
S. simplicifolium (= S. microdontum) (Fig. 2). The breeding material also included
blight-resistant derivatives of S. demissum (e.g., M109 – 3, M124 – 2, M136 – 6 and
MRu18) obtained from Mexico through J.S. Niederhauser of the Rockefeller
Foundation. Further progress from 1968 to 1990 was reviewed by Wastie (1991),
including selection for field resistance in tubers as well as foliage. Tubers can be
infected by spores from the slowly spreading sporulating lesions of a partially leaf-
resistant cultivar and resistance in the foliage does not guarantee resistance in the
tubers, although the two traits can be correlated. Promising blight-resistant cultivars
from SCRI were Teena (released 1986), Shelagh (1986), Torridon (1989), Brodick
(1990), Stirling (1991) and Cramond (1992) (Table 1). However, none of them had
sufficient table or processing quality for commercial success, and none of them had
resistance to the white PCN [Globodera pallida (Stone)], now the major pest of
potato in Britain. Commercial success was finally achieved with cv. Lady Balfour
(released 2001), which is now the number one organic cultivar in Britain. It also
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performs well in Egypt. It has partial resistance to both species of cyst nematodes
derived from S. vernei (as does cv. Eve Balfour, which was also blight resistant) .
However, its resistance in the field to the new blue_13 genotype of P. infestans was
disappointing in 2007 and it appears that field resistance is not necessarily more
durable than R-gene resistance. Interestingly, clone 8204a4 (Fig. 2), a parent of cvsLady Balfour and Stirling, retained its field resistance, which was first recognized by
Malcolmson (1976) in 1976.
Four types of field resistance can now be recognized. Firstly, some field resistance
is associated with late maturity (Bradshaw et al. 2004) and cannot be utilized in
earlier-maturing cultivars. Secondly, there is field resistance associated with defeated
R-genes (Stewart et al. 2003), but it is not clear whether this increased resistance is
conferred by the defeated R-gene or linked genes. Although small in effect, it is
nevertheless useful resistance. Thirdly, there are quantitative trait loci (QTLs) which
have a large effect on field resistance, such as the one on linkage group IV of cv.Stirling (Bradshaw et al. 2004) and clone PDH247 (Bradshaw et al. 2006) that has
been overcome by genotype blue_13 of P. infestans. Thus, QTL × isolate
interactions can occur, and this type of field resistance is no more durable than
major R-gene resistance. Finally, there is field resistance that does not yet display
resistance × isolate interactions (e.g. clone 8204a4) and is still effective. Is this type
of field resistance, which is not associated with QTLs of large effect, horizontal
resistance as proposed by Van der Plank back in 1968?
New Sources of Resistance to Late Blight
In 2008 we screened the CPC with P. infestans blue_13 because this aggressive
metalaxyl-resistant A2 genotype accounted for 70% of the blight population in
Britain in 2007, a rapid increase from 12% in 2005 (Cooke et al. 2008). New
populations of P. infestans comprising both mating types have been spreading from
Mexico to the rest of the world since 1984 and have given concerns about
the consequences of sexual reproduction in this devastating pathogen (Goodwin
and Drenth 1997). We found resistance in S. bulbocastanum, S. chacoense,
S. commersonii, S. demissum, S. okadae, S. polyadenium, S. stoloniferum and
S. verrucosum. But can these resistances be used to achieve that elusive durability?
In the meantime susceptible cultivars are grown in Britain and protection with
ten fungicide sprays is normal practice, and something the government wishes
to reduce.
Resistance to Viruses
PVY, PVX and Potato Leafroll Virus
Introgression of Ny genes for comprehensive resistance to all strains of PVY started
in 1941 with S. microdontum (CPC 51A, S. simplicifolium = S. microdontum) and
S. chacoense (CPC 51B), followed by S . salamanii (CPC 23, a natural hybrid of
S. demissum and S. tuberosum subsp. andigena) in 1942 and S. demissum (CPC 4,
source of Nydms) in 1943. Cockerham (1970) showed that the genes in CPC 51A and
CPC 51B are allelic and functionally identical. The locus is known as Nychc. Both
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the gene from CPC 4 and the one from CPC 51A were utilized in a background of
field resistance to PVY from S. rybinii (= S. phureja) CPC 979. Interestingly,
Pentland Glory has field resistance to PVY which is probably derived from CPC 979
via clone 2288a2 (Fig. 3). Later, in 1954, a cross [(S. antipoviczii × Erika)selfed ×
Tuberosum hybrid, where S. antipoviczii = S. stoloniferum] coded R49/272(= G1921) segregating for the Ry sto gene for comprehensive resistance was obtained
from R.W. Ross in Cologne. This became more commonly used than accessions of
S. stoloniferum in the CPC as the source of the Ry sto gene, for example, in clones
G8866(1), G8866(11) and G8867(15) (Fig. 4), which were used as parents in the
multitrait breeding programme (see later) and more recently in commercially funded
breeding programmes. Although clone G8884(2), another multitrait parent, has S.
verrucosum var. spectabilis (= S. hougasii) in its pedigree, it does not have extreme
resistance to PVY and hence does not possess Ryhou.
Introgression of Rx genes for immunity to all strains of PVX also started in the1950s. USDA 41956, a parent of 11 – 79, was one source of Rxadg (Fig. 3); another
was Andigena potato CPC 1673 (see the next section). The most widely used source
of Rxacl , the gene from S. acaule, came from R.W. Ross in 1955 (44/1016/10, Fig. 3,
Fig. 4), although it was also present in S. acaule CPC 379 (Davidson 1980).
In contrast, breeding for resistance to Potato leafroll virus (PLRV) involved
selection for field resistance (Davidson 1980). Useful levels of resistance were
achieved in some cultivars such as Pentland Crown, Pentland Envoy, Sheriff, Kirsty,
Morag, Torridon and Kirrie, but this is not many in total. However, clones have been
bred with resistance to PLRV infection, accumulation or movement, and these areavailable for future use. Of particular interest are clones such as G8107(1), which
has strong resistance to infection following aphid inoculation as well as resistance to
virus accumulation (Solomon-Blackburn et al. 2008).
Although many parents with resistances to PVX, PVY and PLRV are maintained
at SCRI (G clones), just four G clones appear in Table 1, together with G414a64,
one of the parents of cv. Pentland Crown, which is a cross between cvs Mauxman
and Pepo. Clones G4324(545), G5299(1) and G5833(5) have pedigrees which trace
back to CPC 51A. The pedigree of G5833(5) also traces back to clones 41956 and
44/1016/10 (Fig. 3), whereas G6755(1) traces back to 11 – 79. Thus, cv. Torridon
(Table 1), which is resistant to PVX, is the only cultivar with S. acaule in its
pedigree, along with three other wild species. Interestingly, it has been shown to
possess two extra chromosomes compared with normal cultivars, 50 rather than 48
(Wilkinson 1992).
Breeding for virus resistance was considered important until our breeding
programmes became commercially funded after 1987. Our commercial partners
produce their seed in Scotland and feel that they can control virus infection through
seed certification with roguing. They also fear more farm-saved seed would result
from resistant cultivars. However, this attitude is changing as they target new
markets in parts of Europe where virus diseases are endemic, particularly PVY, and
resistant cultivars are an obvious solution. Hence, we are maintaining and using our
collection of G clones and six of them have been used in recent years in commercial
breeding. The exceptions to these commercial attitudes are Tobacco rattle virus
(TRV) and Potato mop-top virus (PMTV) because in tubers they cause spraing
symptoms, which are a particular problem for processors.
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TRV and PMTV
Today in our breeding programmes for processing companies we assess the TRV
resistance of clones in advanced trials in an infested field near SCRI. We would do
likewise for PMTV if we had sources of resistance. Field tests for both TRV andPMTV were developed and used from 1971 (Simmonds 1972). Since 1981, SCRI
annual reports have sometimes given the resistance/susceptibility of new cultivars,
but claims of resistance have been few and far between: TRV resistance in cvs Moira
and Vales Emerald and putative PMTV resistance in cv. Ailsa. Susceptibility to TRV
has been a criticism of SCRI processing cultivars such as Pentland Dell, Ailsa,
Brodick and Spey, and a feature of Pentlandfield cultivars such as Pentland Crown,
Dell, Hawk, Ivory and Squire. Likewise, popular cultivars used for French fries have
also tended to be susceptible to spraing, with the exceptions of Shepody and
Markies. In contrast, popular crisping (chipping) cultivars such as Hermes, LadyRosetta, Record and Saturna have good resistance and are now being used as sources
of TRV resistance, as well as processing quality.
Resistance to PCNs
H1
Cyst nematodes (Globodera rostochiensis and G. pallida) started to become a
serious problem in Britain in the early 1950s. The first source of resistance to beused successfully came from a CPC accession (CPC 1673) of Andigena potatoes. It
proved to be a simply inherited major dominant gene which was named H1, from the
old name for the nematode Heterodera, and which was effective against what are
now known as pathotypes Ro1 and Ro4 of G. rostochiensis. Following a cross
between CPC 1673 and cv. Kerr ’s Pink in 1952, it took three backcrosses to the
European cultivated potato, with selection for commercially desirable traits as well
as for resistance in nematode-infested soil, before cv. Pentland Javelin was released
from SPBS in 1968 (Fig. 5). The Plant Breeding Institute, Cambridge, had achieved
the same feat a year earlier with cv. Maris Piper. After the H1 gene has been
incorporated into a number of cultivars and breeding lines, these can be intercrossed
in a breeding programme and offspring sought with two copies of the gene, through
test crosses to a susceptible line. These duplex lines can, in turn, be intercrossed and
offspring sought with three or four copies of the H1 gene. Whilst only one copy is
required for resistance, such clones are extremely useful as parents in a breeding
programme because all, or nearly all, of their progeny are resistant even when the
other parent is susceptible, thus avoiding the need to screen the progeny for
resistance or waste resources on raising susceptible seedlings. Thus, cv. Pentland
Javelin was crossed with cv. Maris Piper and clone 10341ab18 was selected as
having two copies of H1, then 10341ab18 was crossed with cv. Cara (which has one
copy of H1 as well as Rxadg from CPC 1673) and clone 15205ab6 was selected as
having three copies of H1. Finally 15205ab6 was crossed with cv. Picasso (two
copies of H1) and cv. Vales Sovereign was selected and National Listed in 2003 as a
new early maincrop cultivar (Fig. 5). The copy number in Vales Sovereign is still to
be determined, but it is known that cv. Spey (Table 1) has three copies (Mackay
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2005). The resistance has proved durable in the UK, where Ro1 appears to be the
only pathotype, but pathotypes such as Ro3, which can overcome H1, have been
found on the European mainland. However, tolerance to nematode invasion is also
required to avoid yield loss. Deliberate selection for this requires expensive yield
trials on infested soils, with and without nematicide control, and is not routinelydone, although new cultivars are assessed for tolerance as part of commercialization.
Fifteen out of our 50 cultivars starting with Pentland Javelin have the H1 gene:
Pentland Javelin, Pentland Lustre, Pentland Meteor, Glenna, Eden, Provan, Buchan,
Brodie, Spey, Kirrie, Amour, Harborough Harvest, Sebastian, Tay and Vales
Sovereign.
S. vernei
PCN populations which could overcome the H1 gene were soon found, and provedto be what is now known as pathotype Pa2/3 of G. pallida, the white PCN. Over
60% of the potato fields in England and Wales are now infested with G. pallida,
which growers try to control with nematicides, but again the government would like
to see a reduction in pesticide use, as is clear from the 2008 EU Pesticide Directive.
Quantitative resistance to both G. pallida and G. rostochiensis was found in a
diploid wild species from South America, S. vernei (CPC 2487 and CPC 2488).
Colchicine treatment of seed from these and another S. vernei produced tetraploid
plants of S. vernei which were crossed with Tuberosum potatoes in 1957 and 1958.
The resulting hybrids were intercrossed and also outcrossed to other cultivars andclones for four generations with selection for resistance and other desirable traits,
before cvs Morag and Glenna were released in 1985 and 1987, respectively. Whilst
these were commercially acceptable cultivars, it took longer to produce a cultivar
with the potential to be commercially successful. Clone 10300(13), a parent of cv.
Glenna and its sister clone 12288af23, was crossed with cv. Cara to produce clone
15119ac5 (Fig. 6). Then clone 8204a4 (Fig. 2), with S. demissum derived field
resistance to late blight, was crossed with 15119ac5 to produce cv. Lady Balfour,
which was National Listed in 2001. SCRI cultivars with partial resistance to Pa2/3
from S. vernei (other cultivars have this source in their pedigrees but lack resistance)
are as follows: Morag, Glenna, Spey, Blush, Sebastian, Thyme, Lady Balfour and
Eve Balfour.
H3
The third main source of resistance to be successfully incorporated into the European
potato was quantitative resistance to G. pallida (Pa2/3), but not to G. rostochiensis,
from Andigena (CPC 2802), and is known as H3 as it was initially thought to be a
major gene. Starting in 1969, a self from CPC 2802 was crossed with cv. Maris
Piper, followed by a cross to K5/2 to produce a progeny segregating for the H1 gene
as well as the H3 genes. One parent of K5/2, P55/7, also has the H2 gene from S.
multidissectum for resistance to pathotype Pa1 of G. pallida. The H2 gene has not
been incorporated into any SPBS/SCRI cultivars as pathotype Pa2/3 proved to be the
main problem in Britain, but S. multidissectum does appear in the pedigrees of nine
cultivars (Table 1) as a consequence of using K5/2. Two backcrosses of H1H3
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clones to S. tuberosum cultivars gave clones 12601ab1, 12636a2 and 12674ab1 and
cv. Eden, which was National Listed in 1991 (Fig. 7). However, again a further
generation was required to produce a commercially successful cultivar. Cv. Cara was
crossed with clone 12674ab1 in 1988 to produce cv. Vales Everest, which was
National Listed in 2005. It is a maincrop potato which is suitable for processing.Fortuitously, clones 12601ab1, 12636a2 and 12674ab1 and cv. Eden also produce
light-coloured fry products after storage at 4 °C, an important processing trait.
Summary of Introgression
Considering that 219 wild tuber-bearing species were recognized by Hawkes (1990), it
is striking that just seven species feature in the pedigrees of SPBS/SCRI cultivars
(Table 1), despite ready access to around 80 species in the CPC, and evidence from
our crossing books that many have been tried. Of the seven species, S. demissum(including S. salamanii) for late blight resistance features extensively, in 58 pedigrees,
S. vernei for cyst nematode resistance in 19, and S. microdontum (= S. simplicifolium)
for PVY resistance in 15. S. multidissectum occurs in nine pedigrees, but as mentioned
earlier, there has been no selection for the H2 gene for G. pallida (Pa1) resistance.
S. commersonii and S. maglia occur in the pedigrees of cvs Craigs Bounty and Craigs
Snow White, but not in subsequent cultivars. S. acaule for PVX resistance occurs just
once, in the pedigree of cv. Torridon. S. chacoense, S. hougasii (= S. verrucosum var.
spectabilis) and S. stoloniferum have been used as sources of virus resistance and do
feature in the pedigrees of G clones currently being used as parents. This paucity of wild species in the breeding of finished cultivars is still typical of potato breeding
worldwide (Bradshaw 2007b). The conclusion must be that, with a few exceptions, it
has proved difficult to successfully utilize wild species in potato breeding. If this
difficulty is due to the retention of undesirable genes from the wild species through
linkage drag, then gene cloning may be the best way forward.
Resistance to Fungal and Bacterial Diseases
The priorities in breeding for disease and pest resistance have been late blight,
viruses and cyst nematodes. For all of the other diseases of potato it has not been
possible to practise selection for high levels of resistance. Rather, until very recently,
clones in intermediate and/or advanced trials were assessed for their level of
resistance to problem-causing diseases to discard any with extreme susceptibility
(Mackay 1987, 2003). Bradshaw et al. (2000a) described screening for resistance at
SCRI with powdery scab (Spongospora subterranean) taken as typical of a
persistent soil-borne fungus and gangrene ( Phoma foveata) as typical of a tuber-
borne disease that develops in store, together with foliage blight as typical of an air-
borne fungal disease. Now, with powdery scab as one exception, our commercial
partners prefer to simply rely on observations in their own trials, and potential new
cultivars are only assessed as part of official National Listing.
High levels of resistance in new cultivars occurred largely by chance and
susceptibility was accepted if the cultivar had other desirable traits, e.g., cv. Pentland
Dell’s susceptibility to TRV. This is largely what happened for gangrene ( Phoma
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foveata), now considered a less important tuber-borne storage disease in northern
Europe, but one that cannot be ignored; dry rot ( Fusarium sulphureum and F.
sambucinum), a tuber-borne storage disease often associated with warmer climates;
common and powdery scab (Streptomyces scabies and Spongospora subterranea),
both persistent soil-borne blemish-forming diseases of tubers associated with dryand wet growing conditions, respectively, and stem blackleg ( Pectobacterium
carotovorum), a potentially serious bacterial disease in temperate climates. Skin spot
( Polyscytalum pustulans), silver scurf ( Helminthosporium solani) black scurf
( Rhizoctonia solani) and black dot (Colletotrichum coccodes) are all tuber-
blemishing diseases which require more attention now that supermarkets want
potatoes with a good skin finish.
The Pentland and subsequent cultivars (Table 1) were assessed for dry rot, gangrene
and skinspot; those from cv. Pentland Meteor for common scab and those from cv.
Ailsa for Pectobacterium soft rot and blackleg, with the other blemish diseases addedmore recently. More extensive data became available on those cultivars that were
commercially successful. Cv. Red Craigs Royal had good resistance to powdery scab,
as does Lady Balfour, and Pentland Dell and Vales Emerald have useful resistance.
Pentland Crown had good resistance to common scab, as does Anya (and three
others), and Pentland Javelin, Claret and Vales Sovereign (and six others) have useful
resistance. Gangrene resistance is not uncommon (15 cultivars, including Pentland
Ivory) and neither are blackleg and soft rot resistance (15 cultivars). Dry rot resistance
is less common (eight cultivars, including Brodick) and skin spot resistance
apparently rare (four cultivars). Cultivars have not been extensively assessed for resistance to the other blemish diseases, but it is encouraging to see that Vales
Sovereign is resistant to black dot and Vales Everest is resistant to Black Scurf.
In conclusion, there would appear to be resistance to fungal and bacterial diseases
available among Tuberosum cultivars, but with more choice of resistant cultivars for
growing and use as parents for some diseases than others. However, the inbuilt
resistance of new cultivars to the fungal and bacterial diseases just considered is
unlikely to improve dramatically unless end users demand it, because they can see
economic advantages, or governments legislate on it because they want environmental
benefits. This is because breeders do not want to devote limited resources to traits that
will not lead to commercially successful cultivars and hence financial rewards. One
possible exception in the immediate future is resistance to common scab as a result of
pressure to reduce the use of water for irrigation during drier summers as a result of
climate change. In the meantime, control measures other than resistant cultivars are
required for all of the fungal and bacterial diseases considered in this section.
Neotuberosum
We have a Neotuberosum population (long-day-adapted Andigena) dating back to the
one started by N.W. Simmonds from the CPC in 1959. One aim was broadening the
genetic base of European and North American Tuberosum breeding programmes. A
gene pool of Andigena potatoes with origins approximately 45% Bolivian accessions
from the CPC, 35% south Peruvian, 10% north Peruvian and 10% Colombian was
subjected to recurrent mass selection in outdoor plots. Within four generations,
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Simmonds (1969) reported good progress, with the better Andigena clones com-
parable in yield and maturity to Tuberosum cultivars and better on average in terms of
late blight resistance. These clones were more variable in tuber shape than modern
cultivars and inferior in regularity of tuber shape, but of similar cooking quality. As a
consequence of their rather ‘rough’ appearance, it subsequently proved difficult to breed successful cultivars from crosses of this Neotuberosum material with
intensively selected Tuberosum clones, despite yield heterosis (hybrid vigour) in
crosses to modern cultivars (Glendinning 1981). Just one cultivar in Table 1, Shelagh,
has a Neotuberosum parent, GL71/179, despite such parents being used over the
period from 1969 to 1998, with intensive evaluation of hybrids from 1979 to 1987.
Neotuberosum needed improving for traits in addition to tubering in long days to
have real impact because it was not as good as intensively selected Tuberosum.
However, recurrent mass selection was discontinued in 1978, when the emphasis
changed to evaluating the potential of this improved material in breeding cultivars, atask that was essentially completed by 1987. Eventually, in 1996, the collection of
318 elite Neotuberosum clones from the 200 ‘ primary’ selections, taken in the 1960s
and 1970s from the main mass-selected population, was converted to true botanic
seed for long-term storage as a potential genetic resource if required in the future
(Glendinning 1987; Mackay 2003). A bulk seed harvest was also taken from the
original mass-selected Neotuberosum population for long-term storage. This
biodiverse population has now been recovered, tested for current quarantine diseases,
and is entering genetic research programmes at SCRI. An important question that
now arises is whether it should be selected for further improvement, particularly intuber shape and appearance, or maintained without conscious selection as a
biodiverse population that can be evaluated in long days for new traits. The latter
is our current intention, given the ease with which desirable genes such as H1 have
already been introgressed by backcrossing from Andigena potatoes into successful
cultivars, and the same is likely to be true for new traits. It is unlikely in current
breeding programmes that Neotuberosum × Tuberosum crosses would survive
progeny tests in which they were compared with Tuberosum × Tuberosum crosses.
Likewise, Tarn and Tai (1983) concluded that first-generation hybrids between
Neotuberosum and Tuberosum were not the best use of their Canadian Neotuberosum
available in 1983. They also advocated further improvement of their Neotuberosum
and/or backcrosses of the hybrids to Tuberosum. A brief review of other
Neotuberosum programmes can be found in Bradshaw and Mackay (1994).
Phureja
S. tuberosum group Phureja is the second most widely cultivated type of potato in the
Andes of South America. We have already met group Phureja as S. rybinii in Fig. 1
(Ru.159 from Russia), Fig. 2 (crossing book does not state which source) and Fig. 3
(CPC 979 from the CPC). These were short-day-adapted potatoes which feature
extensively in the pedigrees of SPBS/SCRI cultivars. We also have a long-day-
adapted population of Phureja potatoes. During the period from 1962 to 1979, Carroll
(1982, 1987) employed a mass-selection method to produce a long-day-adapted
population of diploid group Phureja potatoes (with a small contribution from other
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diploid-cultivated material) from CPC accessions. The population rapidly adapted to
long-day conditions and yield improved over several generations, mainly as a result
of increase in tuber size without a reduction in tuber numbers. It was also possible to
demonstrate variation for late blight resistance and to select for this in the field. The
proportion of oval/long oval, regular-shaped tubers increased, but further selection isnow being made for improved dormancy to provide better germplasm for direct use
in breeding finished tetraploid cultivars. Nevertheless, hybridization of members of
the original long-day diploid population with tetraploid Tuberosum cultivars via
unreduced pollen grains did produce tetraploid hybrids which were superior to
standard tetraploid cultivars in both total and marketable yield, generally producing
more tubers per plant with slightly lower mean tuber weights (Carroll and De Maine
1989), and we now have three such hybrids in NL trials. Furthermore, since 2001, six
diploid Phureja clones have been added to the UK NL as cvs Mayan Gold, Inca Sun,
Inca Dawn, Mayan Queen, Mayan Star and Mayan Twilight (Table 1). Their yieldsare lower than those of tetraploid Tuberosum cultivars, so they are being targeted at
niche markets for their flavour attributes.
The SCRI collection of long-day Phureja clones is proving extremely useful for
breeding research and use in breeding programmes as it contains a range of useful
traits, including high levels of tuber carotenoids, improved flavour, reduced cooking
times and resistance to blackleg (and tuber soft rot), common scab and powdery scab.
In contrast, dihaploids of Tuberosum (i.e. diploids), despite their production at
SPBS/SCRI and potential as described by Simmonds (1969, 1971), do not feature in
the pedigrees of any SBPS/SCRI cultivars. Hence, they are not considered further inthis review, except to say that progress in the use of dihaploids was hampered by
most primary dihaploids being weaklings and male-sterile, so improvement by
intercrossing could not be done on an extensive scale. For a review of diploid
breeding elsewhere, the reader is referred to Haynes and Lu (2005).
Consumer Quality
Consumer quality has always been a key objective for achieving commercial success
with new cultivars, and only cultivars with acceptable quality traits have been released.
The period since 1960 in Britain has seen a steady increase in the proportion of the crop
which is processed, to about 50% in 2008. The main change in the production and sale
of fresh potatoes is that today 63% are supplied by just four supermarket chains
(Anonymous 2008), having been packed by a similar small number of companies
with their dedicated seed producers and ware growers. In addition to prepacked
potatoes with improved flavour and texture, these supermarkets want to provide
convenience foods and novel products with nutritional and health benefits.
Up until 1970 new cultivars were assessed primarily for cooking and keeping
quality, although cv. Pentland Dell went on to be primarily used for processing into
French fries, and was still the seventh most widely grown cultivar in Britain in 2007.
From 1970 onwards, new cultivars were assessed for consumer quality, which included
processing quality as well as cooking quality for table use. Cvs Sheriff, Baillie, Provost
and Teena were identified as have crisping potential and cvs Ailsa, Torridon, Derek and
Spey were identified as having potential for making French fries. However, none of
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these cultivars became established as major processing cultivars. Cv. Pentland Marble
was identified as a specialist cultivar for canning and cv. Anya was bred as a salad
cultivar in a commercially funded programme targeted at this market.
Cold Crispers
Potato stores have also improved since the 1960s with the development of
electronically controlled environments, including refrigerated long-term storage,
and there are fewer losses of yield and quality. However, potatoes for processing are
stored at temperatures from 6 to 10 °C to prevent cold sweetening and dark products
on frying, and this necessitates the use of sprout suppressants, but there is
government pressure for a reduction in their use.
Screening for resistance to low-temperature sweetening started in 1982 and cvs
Brodick and Eden were our first crisping varieties that could be stored at 4 °Cwithout sweetening, a major breakthrough which unfortunately did not translate into
commercial success for various reasons (Mackay 2003). The source of the genes for
resistance to sweetening is not certain. Although both cultivars have wild species in
their pedigrees, the trait is present in cultivated potatoes such as Neotuberosum clone
GL76B/102 (Dale and Mackay 1994). Cvs Brodick (Fig. 8) and Eden (Fig. 7) both
have cv. Pentland Ivory and cv. Maris Piper as parents or grandparents, but their
genetic contributions to cold sweetening resistance, if any, is unknown. It may be
that selection for good fry colour eventually reached the point where reducing sugar
concentration was low enough to be manifest as resistance to sweetening. In NorthAmerica, Love et al. (1998) reported steady progress since 1960 in improving potato
chip quality through lower reducing sugars and better chip colour.
The breakthrough led to commercially funded breeding programmes targeted at
resistance to cold sweetening with McCain for French fries and Golden Wonder for
crisps (Mackay 2005). The target was met in cvs Montrose and Scarborough for
French fries and in Golden Millennium, Harborough Harvest and Tay for crisps.
Their parentage (Table 1) is such that they all have the same four wild species in
their pedigrees, but it is not known if they contribute any useful genes for processing
quality. Although they were not adopted by these companies for various reasons,
they have proved useful parents in further breeding work for major processing
companies, and commercially successful cultivars that do not sweeten remain an
achievable goal. Furthermore, recent research has shown that selection in SCRI
germplasm for lower levels of asparagine as well as lower levels of reducing sugars
results in lower acrylamide in fried products, a desirable outcome because of current
concerns that its presence may harm people’s health (Friedman 2003).
Yield and Changes in Growing Potatoes
Changes in Potato Growing
High yield has also always been a key objective for the commercial success of new
cultivars and only cultivars with acceptable yields have been released, albeit within
the context of maturity group, which is a guide to days to maturity and hence harvest
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quality. Phosphate enrichment of surface water is a problem and there is competition
for water resources between agricultural, industrial and domestic users. Climate
change is also likely to result in different growing conditions, with northern Britain,
for example, likely to have warmer and drier summers, wetter winters with less
snow, earlier springs and more extreme temperature and rainfall events. There willalso be higher CO2 levels, higher UV-B and reduced ozone.
We have therefore started a research programme to look at variation in our
germplasm for rooting traits that could affect water and fertilizer use. It is too early
to say if we can find the necessary variation in cultivated potatoes or if we need to
go to wild species, but now is certainly the time to start if we are to meet the
challenge of continuing to breed new cultivars with inbuilt disease and pest
resistance and capable of higher yields and better quality. It is also too early to say
what rate of change we can achieve, but in another context, the results from our
multitrait programme (see the next section) are encouraging in terms of combiningefficient recurrent selection with cultivar production. We have completed five cycles
of recurrent selection in 18 years, which is a short length of time compared with the
13 years it has taken to produce new cultivars in recent years. Furthermore, we know
from our Neotuberosum work that a major adaptation, namely to long days, was
achieved in this number of generations.
Multitrait Programme
By 1990 cultivars and clones were available with disease and pest resistances which
had been introgressed from the wild and cultivated species of Latin America over
many years, starting in the 1930s (Fig. 9). However, no systematic attempt had been
made to combine them in a single cultivar, although parents had been chosen for
crossing that complemented one another for desirable characteristics, as seen in
Figs. 1, 2, 3, 4, 5, 6, 7 and 8. Therefore, in 1991, a multitrait breeding programme
was started to combine quantitative resistances to late blight and the white PCN
(G. pallida) with commercial worth as judged by breeders through a visual assess-
ment of tubers (breeders’ preference) (Bradshaw et al. 2003). The parents with
resistance to G. pallida also had resistance to G. rostochiensis, the golden PCN.
Parents were also included with resistance to PLRV, PVY and PVX, but time and
resources did not permit direct selection for virus resistance in each generation
(Solomon-Blackburn and Bradshaw 2007). Such an overall combination of traits was,
and still is, lacking in European potato cultivars, despite 50 years of breeding effort.
The breeding programme has made use of the progeny tests described earlier and has
involved cycles of crossing, selection between progenies (= full-sib families) and clonal
selection within the selected progenies. Crosses were made in 1991 (36 parents), 1994
(108 parents), 1997 (108 parents), 2003 (15 multitrait parents + 13 other parents) and
2006 (108 parents + cv. Vales Everest). Glasshouse seedling progeny tests for breeders’
preference, G. pallida and late blight resistance were done in 1992 (120 progenies),
1995 (137 progenies), 1998 (145 progenies), 2004 (122 progenies) and 2007 (132
progenies). Tuber progeny tests at a seed site were done in 1993, 1996 and 1999 for
breeders’ preference and in 2005 and 2008 for fry colour as well as breeders’
preference. In all years clonal selection for breeders’ preference was practised within
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the best progenies at the seed site to provide parents for the next round of crossing. In
1996 there was the addition of clonal tests for G. pallida and G. rostochiensis
resistance, and these were done early in 1997 and completed in time to select parents
for crossing. Thirteen out of the 15 clones used as parents in 2003 came from raising
a further 2,178 seedlings from the best 12 progenies and practising sequential clonalselection in 2000 (breeders’ preference at the seed site), 2001 (G. pallida and foliage
blight) and 2002 (tuber blight and G. rostochiensis).
We have thus shown that efficient multitrait genotypic recurrent selection based
on progeny testing with limited within-progeny selection can operate on a 3-year
cycle and full combined selection between and within progenies can operate on a 6-
year cycle. Furthermore, the breeding scheme was opened up to new germplasm in
2003, when 54 successful crosses were made in addition to the 68 from the 15
multitrait parents. The subsequent progeny tests ensured the 54 progenies were
compared with the 68, and as a result, just six out of the 54 were good enough toenter the breeding programme along with 19 from the 68. Likewise, new cultivars
selected from earlier cycles could be used as parents, but their progenies would
survive only if they were superior to those from the most recent cycle. Six clones
selected from the 1997 crosses and eight selected from the 2003 crosses have now
been used as parents in our commercially funded breeding programmes, but it is
clearly too early to predict which will be the parents of new cultivars. The pedigree
of one of these parents is shown in Fig. 9.
Genetics
Since the founding of SPBS in 1920, genetic knowledge has been sought for use in
breeding. From the 1930s, Mendelian analysis of major genes was done, but not
until the end of the 1930s did geneticists recognize that the potato was a tetraploid
(2n = 4 x = 48) which displays tetrasomic inheritance (Lunden 1937; Cadman 1942).
Such knowledge has proved useful in designing introgression programmes and
progeny testing for determining copy number of major genes in potential parents
(Bradshaw and Mackay 1994; Mackay 2005).
Some of the problems and complexities of working with a tetraploid were overcome
after 1958 with the production of haploids (also called ‘dihaploids’) of tetraploid
S. tuberosum (Hougas et al. 1958) and genetic studies at the diploid level involving
crosses with other diploid Solanum species. The dihaploids were, however, usually
male-sterile, and most dihaploids and diploid species were self-incompatible.
Therefore, true breeding lines which could readily be selfed and crossed and which
displayed disomic inheritance could not be produced, and inheritance studies
remained difficult. Furthermore, most economically important traits displayed
continuous variation, which required biometrical rather than Mendelian analysis.
At SPBS/SCRI from 1973 (Killick and Malcolmson 1973) until 2000 (Bradshaw
et al. 2000b), we relied on combining ability analysis for most of our genetic
knowledge of potato. Such analyses can be done on tetraploids as readily as on
diploids. The concepts of heritability, additive and non-additive genetic variation,
genotype × environment interaction, and population improvement proved useful in
determining our breeding strategies, choosing parents, and in predicting and
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improving responses to selection and rates of progress, as discussed by Bradshaw
and Mackay (1994). They proved particularly useful at the start of new programmes
such as the multitrait programme (Bradshaw et al. 1995, 2000b).
Genetic knowledge of the potato has increased dramatically since the first
molecular-marker map appeared in 1988 (Bonierbale et al. 1988), as brieflyreviewed by Bradshaw (2007b). Today the genes underlying qualitative traits can
be mapped directly onto the dense molecular-marker maps as individuals can be
classified into distinct categories for trait and marker. QTLs can be mapped
indirectly through associations between trait scores and molecular markers. At SCRI
we have developed the theory and user-friendly software for linkage and QTL
analysis in tetraploid potato (Hackett et al. 2007), and used them to analyse a cross
between processing clone 12601ab1 and table cv. Stirling, typical in potato breeding
(Bradshaw et al. 2008). One QTL allele of large effect was found for early maturity,
one for quantitative resistance to late blight and two for quantitative resistance to thewhite PCN. For these alleles we would like to use molecular breeding methods (gene
cloning and marker-assisted selection) to ensure that they are introduced into new
cultivars as quickly as possible. However, many more QTL alleles of small effect
(38 in total) were found for yield, agronomic and quality traits, and to combine them
into a new cultivar requires increasing their frequencies in our breeding programmes
by efficient multitrait genotypic recurrent selection. It is simply not possible to do
this through one round of crossing and selection in typical potato breeding
programmes with
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