Potato Breeding at the Scottish Plant Breeding

8/18/2019 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: [email protected]


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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

Documents

Potato Breeding at the Scottish Plant Breeding