Breeding potato varieties for biotic and abiotic stresses
Dalamu, Scientist
Division of Crop Improvement, CPRI, Shimla-171 001
Potato (Solanum tuberosum) is the fourth most important food crop in the world.
Worldwide more than 19 million ha of potatoes are grown with a total economic value
higher than 31 billion US$ (http://www.potato2008.org/en/world/ index.html). Potato is
vulnerable to various biotic and abiotic stresses. Among them late blight, viruses (X, S,
Y and PLRV), bacterial wilt, wart and cyst nematodes cause economic loses worldwide
and in India. Among abiotic stresses, heat, drought and frost are most important.
Resistance breeding by incorporating resistance genes in the host plant is the most
effective, economical and eco-friendly management strategy in any crop. Various biotic
and abiotic stresses of potato, sources of resistance available against them and
breeding strategies followed are as follows:
BIOTIC STRESSES
Late Blight
Late blight caused by Phytophthora infestans (Mont.) caused Irish famine in 1844-45
and within a very short period spread throughout the world. This is the most devastating
disease of potato world over. P. infestans originated in Mexico where lot of wild
Solanum species also grow and co-exist with late blight. Thus, these wild Solanum
species possess a fair degree of resistance to balance the Phytophthora attack. The
pathogen is highly variable and various pathological races exist universally. The fungus
had developed virulences against almost all the known R-genes in potato. This may be
due to the fact that A2 mating type now occurs almost throughout Europe and even in
some Asian countries and therefore, racial variability through sexual reproduction in
these regions exists.
The heritable nature of resistance to late blight was first established in 1909 in wild
species S. edinense. Resistance to blight can occur both in foliage and tubers.
However, breeders have largely neglected the latter. Broadly resistance can be grouped
into two types: (i) race-specific resistance or vertical resistance or major gene
resistance or qualitative or discontinuous resistance: This sort of resistance is based on
gene-for-gene relationship. It was identified in S. demissum (2n=6x=72) and expressed
in the form of hypersensitive response of the tissue to all races of P. infestans that did
not possess the corresponding virulence to the resistance genes (R-genes). Specific
resistance is governed major dominant genes each of which is brought into action by
distinct pathotypes. At present eleven such genes (ex-demissum) are recognized. S.
stoloniferum has also been found to possess similar resistance genes. The process of
transfer of R-genes from S. demissum background was started in mid-fifties. Kufri Jyoti
(possessing R-genes 3.4.7) was released in 1968 and still grown in several parts of the
country. However matching virulences (3.4.7) were detected immediately after 5-6 years
of its cultivation making it completely susceptible by 1988. Till now lot of varieties
carrying R-genes have been bred and deployed across the country.
(ii) race non-specific or horizontal resistance or minor gene resistance or field
resistance or polygenic resistance or quantitative resistance or partial resistance: Such
resistance is quantitative and is governed by many genes. Field resistance to late blight
operates mainly through four factors, viz. infection efficiency, incubation period,
colonization rate and sporulation efficiency. Many host factors, environmental aspects,
edaphic, nutritional and climatic have an effect on these four components of resistance.
Besides, components of field resistance to tuber blight include the depth in the soil at
which the tubers are produced, the ease with which the spores are washed down from
the canopy into the soil, the rapidity of periderm formation and the resistance to
wounding. Although, at the phenotypic level, both types of resistances can be easily
identified, yet at the genotypic level these are almost similar.
Resistance sources:
Several wild Solanum species possesses high degree of resistance to late blight e.g. S.
bulbocastanum, S. demissum and S. stoloniferum. Other wild species possessing
resistance are S. tuberosum ssp. andigena, S. pinnatisectum, S. polyadenium, S.
verrucosum, S. chacoense, S. berthaulti, S. vernei, S. polytrichon, S. simplicifolium and
S. microdontum.
Breeding strategies
S. demissum is the most potential wild species that has been used for imparting late
blight resistance in Solanum tuberosum worldwide due to resistance is controlled by
dominant, major genes and provided complete control of the disease. In the beginning
of 20th century S. demissum was crossed with S. tuberosum and their progenies used in
the resistance breeding programmes in U.K. and Germany. However, in due course of
time the resistance broke down as new compatible physiological races of P. infestans
appeared. Slowly the attention was shifted towards general resistance, which is
controlled by many minor genes, which does not exert any directional selection
pressure on the fungus. However, this type of resistance is difficult to handle in a
backcrossing programme as most of the genes flow only to a low proportion in the
progeny and then quickly disappear upon back crossing. Over the years field resistance
had become the backbone of late blight resistance-breeding programmes. As the diploid
wild species are non crossable with cultivated potato, dihaploids of tetraploid potato can
be crossed with diploid species, creating genotypes with divergent genetic base. These
genotypes are then selfed and subjected to further selection of genotypes having
desirable genes from different species, which in turn are crossed to get genotypes with
more resistant genes. These diploid hybrids are then doubled mitotically to create
tetraploid hybrids that may be used as cultivars or parental lines in the breeding
programmes. Such work has been initiated in species like S. verrucosum and S.
bulbocastanum.
History of late blight resistance breeding:
Initially the genotypes were screened in the fields and subsequent selections
were made. This resulted in finding of immune clones of S. demissum and S.
antipoveizii that were used as parents for late blight resistance breeding. Field
screening led to the selection of two hybrids viz. hybrid 9 and S. 1758 which were later
released under the commercial name Kufri Kundan and Kufri Kumar respectively. From
1956 onwards, both artificial inoculation and field screening were employed and several
field resistant cultures were identified. Six of them Kufri Jyoti, Kufri Khasigaro, Kufri
Naveen, Kufri Jeevan, Kufri Neela and Kufri Muthu were released for commercial
cultivation.
Until early 1970s, all the parental lines used in late blight resistance breeding
programme had S. demissum as source of both major and minor gene resistance.
However, breeding for exclusive field resistance was initiated by using S. verrucosum
as resistance source. Crosses are also made between known late blight resistant
parents including clones of S. andigena and the progenies exposed both in laboratory
and fields to all the available spectrum of races. Using this strategy, potato variety Kufri
Giriraj was released in 1999 deriving durable late blight resistance from S. andigena.
Efforts are also being made to transfer quantitative resistance from diploid species S.
verrucosum and S. microdontum into S. tuberosum background.
Late blight resistant varieties developed for different regions of the country are K. Sutlej,
K. Jawahar, K. Anand, K. Chipsona I, K. Chipsona II and K. Pukhraj for plains and K.
Megha, K. Giriraj, K. Swarna and K. Girdhari for hills:
Role of molecular markers:
Significant correlations exist between the segregation at a certain genetic marker
locus and variation in the trait value, indicating the presence of a quantitative trait locus
(QTL) in the proximity of the marker locus. Identification of a quantitative trait loci and
their large-scale use will go a long way in developing late blight resistant varieties in the
country. Mapping population at diploid level of S. chacoense and S. spegazzinii has
been developed for the identification of QTLs for late blight resistance.
VIRUSES
Viruses have systemic distribution in the host and leads to degeneration of seed stocks.
At least twelve viral diseases are known to infect potato crop. Among them, viruses
PVX, PVY, PVS, PVA and leaf roll (PLRV) are important. Mixed infection of these
viruses causes tremendous reduction in yield compared to losses incurred individually.
Introducing resistant cultivars is one of the most efficient ways of reducing the losses
caused by viruses. Resistance genes to different potato viruses have been identified in
many wild potato species. Some of these genes have been incorporated in many of the
recently released potato cultivars.
The nature of resistance against viruses is either tolerance or resistance or
hypersensitivity or extreme resistance or immunity. Out of the four types of resistance,
immunity gives almost complete elimination of virus and is preferable over other types.
However, in recent years, more emphasis is being given to vector resistance where the
resistance sought is against the vectors (aphids and other vectors), the carrier of
viruses and not against viruses themselves.
Resistance sources:
PVX: S. tuberosum ssp. andigena and tuberosum, S. acuale, S. sucrense, S.
chacoense and S. curtilobum.
PVY: S. stoloniferum, S. demissum, S. chacoense, S. candiophyllum, S. microdontum,
S. polyadenium and S. multidissectum.
PVS: S. tuberosum ssp. tuberosum varieties like Great Scot, Kerries Pink, Red Skin,
Eclipse, British Queen, Up-to-date, Duke of York and Foxlyna etc.
PLRV: S. acuale, S. berthaultii, S. etuberosum, S. brevidense and S. tuberosum ssp.
andigena.
Breeding strategies
The development of cultivars with multiple virus resistance remains a challenge for the
breeders. This may be because the breeder must select for many important characters,
therefore, introducing even a few genes for resistance to viruses becomes a difficult
task. A solution may be provided by parental line breeding. If the breeder receives a
parent whose progeny consists of a large proportion of genotypes with multiple
resistance to viruses, the chances of selection of a variety with multiple resistance
increases considerably. At CPRI, the efforts were directed to develop the parental lines
having virus resistance in duplex/triplex/tetraplex form and developed a parental line
having PVY resistance gene in triplex condition. The progeny of triplex parents when
crossed with nulliplex parents will produce 96 per cent progeny having the resistance
gene.
Transgenics are an alternative to traditional breeding for viral resistance. One of the
most popular transgenic strategies to control viruses in potato is through coat protein
(CP)-mediated resistance. Transgenic potato plants expressing CP gene have been
developed by different workers for individual resistance against PVX, PVY and PLRV.
Bacterial Diseases
Potato is affected by six bacterial diseases viz. bacterial wilt or brown rot (Ralstonia
solanacearum (Smith), soft rot of stem and tuber (Erwinia carotovora, Bacillus spp.,
Pseudomonas spp.), ring rot (Clavibacter michiganensis ssp. sepedonicus (Spieck and
Koth), common scab (Streptomyces spp.), pink eye (Pseudomonas spp) and leaf spot
(Xanthomonas vesicatoria).
Among them Bacterial wilt or brown rot is most devastating. It was first reported in 1892
and is wide spread in all mid hill regions of the country. The disease damages the crop
by premature wilting of standing crop and rotting of tubers in fields, transit and stores.
Premature wilting before tuber setting results in 100% losses. Wilt in later stages results
in an infected produce, which is likely to rot in stores. The bacterial wilt is primarily seed
borne, but survives equally well in soil. Host resistance is hard to find because of lack of
co-evolution of the host and the bacterium, high variability in the bacterium and
instability of the host resistance.
Nature of resistance:
Resistance to bacterial wilt is a partially dominant, polygenic and both additive and nonadditive gene actions are involved, but the latter component is more important.
Significant general and specific combining abilities were observed. The response of
host to the pathogen is altered by nematodes resulting in break down of the resistance.
There are also evidences of interactions between genes for resistance and genes for
adaptation, which shows that genes for adaptation are involved in conditioning the
resistance expression. This highlights the importance of adaptive potential of a resistant
genotype for the expression of resistance. Therefore, epistatic gene effects are large in
inheritance of resistance. These epistatic gene effects are certainly conserved by
artificial clonal selection for resistance and vegetative propagation of the selected
resistant clones in breeding programs. A breakdown of the intact parental genotypes
upon cross breeding will results in a partial loss of favorable epistatic associations for
resistance. Progeny testing, thus, helps in identification of best general combiners for
true potato progenies resistant to bacterial wilt.
Resistance genes
A gene-for-gene relationship is not applicable to bacterial wilt. Certain genes other than
those ‘for resistance alone’ have turned out to have the novel (pleiotropic) effects in
conferring the resistance once the potato plant has come into contact with pathogen
under a certain set of environmental conditions. These genes were eventually called
‘genes for resistance’ once a certain level of resistance was detected. The major or
minor status of these genes depends on the particular genotype of the pathogen, and
the particular environmental conditions that influence their expression. Attempts to
transfer resistance from wild Solanum sp. into common potato resulted in excessive
recombination, resulting in breakdown upon intercrossing. Non-strain specificity and
race-cultivar specificity are the common features required for resistance to bacterial wilt.
Thus, the host genotype x pathogen genotype interaction in potato-R. solanacearum
system seems to be artifactual. Both the host and pathogen are sensitive to
environmental changes. Therefore, host genotype x pathogen genotype interaction may
also be a result of host genotype x environment and / or pathogen x environment
interaction.
Breeding strategies
The resistance in the clones of species like S. phureja mainly and a few other Solanum
species have been exploited extensively in the South American countries, but the Indian
isolates of the bacterium has proved to be strongly virulent making these sources
ineffective. Resistance in S. phureja, the only species where it has been studied in
detail, is strain and temperature-specific and breaks down under the warm climates.
Nematode injury also leads to its break down. A collection of nearly 500 clones of
Solanum species which carry low to moderate degree of resistance i.e. S. phureja, S.
microdontum, S. canasense, S. stenotomum, S. pinnetisectum, S. sparsipilum, S.
kurtzianum, S. jamesii, S. polytrichon, S. vernei, S. acaule and S. stoloniferum and
inter-specific hybrids between a number of above species and also resistant varieties
developed so far in other parts of the world, viz. Prisca, Cruza, Caxamarca, Molenera
and Ampola were screened against different isolates of the pathogen. All these
cultivars/cultures proved susceptible to Indian isolates except for the one clone of
diploid S. microdontum showing a moderate level of resistance. The efforts made to
transfer the useful resistance from this source into the tuberosum background via
dihaploids resulted in the development of two promising meiotic tetraploids. However, in
field tests these were also found to be susceptible.
Wart disease
Wart disease of potato is caused by fungus, Synchytrium endobioticum (Schilb). This
disease was first reported in 1953 in North Bengal Hills. To avoid its further spread to
other parts of the country, domestic quarantine (embargo) was implemented in 1959. At
present this disease is restricted in Darjeeling area only. The wart fungus is an obligate
parasite producing two kinds of sporangia, viz. summer sporangia (active phase) and
winter sporangia (dormant phase). The disease causes cauliflower like growths on
tubers, stolons and stem bases. The heavy infection of disease causes rottage of entire
produce and results in total loss of crop. Cultivation of wart immune varieties on a longterm basis is the only viable alternative.
Resistance genes
The resistance genes are available in a number of varieties of S. tuberosum. Besides, a
number of wild species like S. boliviense, S acaule, S microdontum, S. demissum, S.
sparsipilum, S. polytrichon, S. simplicifolium, S. chacoence f.sp. boergerii, S. vernei and
S. spegazzinii are known to have resistance to the disease.
Monogenic dominant mode of inheritance for at least the control of necrotic response
has been proved. However, modifying genes are also present which condition the
nature and extent of response. Therefore, resistance is determined by a single
dominant gene A, the action of which can be inhibited by three complementary genes B,
C and D. Early workers postulated that immunity is dominant and susceptibility
recessive, and two factors i.e. X and Y can induce immunity independently provided the
‘factor Z’ is present. In absence of factor Z, immunity is only induced by both X and Y
factors acting together. Recent workers have invoked the role of polygenes to explain
that resistance to more than one race is not comprehensive.
Breeding strategies
Systematic breeding programme for wart resistance started in 1964. The crosses
between wart immune/resistant tuberosum parents result in recovery of quite high
percentage of resistant clones than between resistant x susceptible parents. Using
Adina x Ultimus, several late blight resistant and wart immune hybrids were developed.
One of them was released for commercial cultivation under the name Kufri Sherpa in
1983, which did not become popular because of its poor keeping quality, unattractive
dull white skin, round tubers with medium deep eyes. Indian cultivars, viz. Kufri Jyoti,
Kufri Chamatkar, Kufri Muthu, Kufri Sheetman, Kufri Bahar, Kufri Khasigaro and Kufri
Kumar are immune to the race of S. endobioticum prevalent in the Darjeeling hills.
These cultivars except Kufri Jyoti also didn’t establish in the area because of local
preference for varieties with red skin tubers. To develop a red tuber variety, Pimpernel
was used as one of the parents. One hybrid from cross SLB/Z 405a x Pimpernel was
selected and has since been released as cultivar Kufri Kanchan. This variety is immune
to wart and possesses high degree of field resistance to late blight.
Nematodes
About 90 species of nematodes belonging to 38 genera have been reported to be
associated with potatoes. Among these, potato cyst nematode has been recognized as
the major pest.
Potato Cyst Nematodes
Potato cyst nematodes (Globodera spp.) is one of the major potato pest throughout the
world. Quarantine or regulatory actions are imposed against them in most countries. In
India, the potato cyst nematode was first detected in 1961 by Jones at Ootacamund in
Tamil Nadu. Both the species G. rostochiensis and G. pallida are prevalent in these hills
as individual or as mixed populations. Potato cyst nematodes infections are difficult to
recognize or diagnose, as, these are often mistaken for nutrient deficiency. The typical
symptoms of infestation is small patches of poorly growing plants appear in the field
with temporary wilting of plants occurs during hotter parts of the day. Plants have
stunted with unhealthy foliage, premature yellowing, poor development of root symptom,
reduction in size and number of tubers and poor yields. Control of potato cyst nematode
through chemicals is inadequate, expensive and environmentally hazardous; breeding
cultivars resistant to the pest is the most effective way to control it.
Breeding strategies
The efforts to locate the source of resistance to cyst nematodes began in 1968.
The species possessing resistance are S. ehrenberjii, S. vernei, S. chacoense, S.
phureja, S. demissum, S. gourlayi, S. microdontum, S. sucrense, S. tarijense, S. acaule,
S. fendleri, S. multidissectum, S. oplocense, S. sparsipilum and some accessions of S.
tuberosum ssp. andigena. Besides wild potatoes efforts were also made to procure
resistant breeding lines to both the species from different nations. A parental line VTn 2
62.33.3 (S. tuberosum x S. vernei hybrid) received from the Netherlands, having
resistance to both the species, was extensively used in crosses with late blight resistant
cultivar Kufri Jyoti. Four selections from this cross, namely, SON-19, 36, 105, and 110
having combined resistance to cyst nematodes and late blight, were evaluated in field
trials to ascertain their yield potential. The hybrid SON-110 was selected for
multiplication trials in Nilgiri hills and was released as Kufri Swarna in 1985. Nine
resistant cultures with spp. Andigena in their pedigree were imported from Dr. Howard
of Plant breeding Institute, Cambridge in 1976. Of these two cultures, viz., D 40/8 and D
42/9, proved resistant to both the species of cyst nematodes and are presently being
used in the breeding programme.
ABIOTIC STRESSES
Genetic studies on germplasm variability have revealed species or even cultivars
of potato can resist abiotic stress. Variability exists in the related species for heat, frost
and drought tolerance. Therefore, top priority is required to be directed for introducing
tolerance to abiotic stresses through organised breeding programmes so that potato
can successfully be grown in tropical and subtropical areas of the world.
Breeding for heat stress
Potato has temperate origin (in highlands of South America). Minimum night
temperature plays a crucial role during tuberization in potato and largely determines
whether plants will tuberize or not. Tuberization is reduced at night temperatures above
20OC with complete inhibition of tuberization above 25 OC. Thus at lower altitudes in
tropical zones, the yield of potatoes is less than one third compared to that in temperate
zones. Aspects of heat tolerance that are considered important and should be taken into
account in breeding programme are: 1) ability of the plants to tuberize at night
temperature of 22O C and above, 2) low shoot/root ratio at high temperature, and 3)
early maturity of the crop.
Leaf bud cuttings is a model system to study the effect of temperature on tuberization in
potato as well as for screening for heat tolerance. Single leaf cuttings are placed in the
soil/sand medium so that the leaf bud is buried in the soil. These cuttings are exposed
to 22O C night temperatures and leaf buds are examined for tuberization after three
weeks. The presence of tubers in the leaf bud give good indication of the heat
tolerance. Another method to select heat tolerant genotypes is to expose seedlings to
22O C night temperature in the glass house and then check for tuberization after 4 and 8
weeks of treatment. The seedlings that tuberize under these conditions are early and
have tolerance to heat stress. To breed heat tolerant genotypes for Indian conditions,
crosses were made amongst known heat tolerant and local high yielding genotypes.
The known heat tolerant genotypes used in the breeding programme were LT-1, LT-2,
LT-5, LT-7, LT-8, LT-9, DTO-28, DTO-33 (received from CIP) Katahdin, Desiree and
Kufri Lauvkar.
Breeding for drought stress
Drought affects potato growth and production by 1) reducing the amount of productive
foliage, 2) decreasing the rate of photosynthesis per unit of leaf area, and 3) shortening
the vegetative period. All stages of crop development are sensitive to drought stress.
Insufficient water supply in the period between emergence and the beginning of tuber
bulking may therefore lead to a reduced growth rate of foliage and ground cover
resulting in incomplete and yields below optimum. Tuber quality characteristics such as
shape, dry matter content and content of reducing sugars are influenced by water stress
during the vegetative period.
Drought resistance can be the result of drought avoidance (e.g. closure of stomata,
large root system) or drought tolerance (capacity for osmotic adjustment, rapid
resumption of photosynthesis activity etc.). Aspects of drought resistance that are
considered important and should be taken into account in breeding programme are: 1)
the effect of short periods of stress on productivity and tuber quality, 2) survival and
recovery of the plants after water stress, and 3) water use efficiency. Experiments on
reduction of leaf extension rate upon exposure to water stress and its recovery showed
that potato cultivars could be placed in three groups. Group ‘A’ was characterised by
minimum growth reduction under stress and rapid recovery on re-watering with final
increase in the leaf length exceeding that of the unstressed controls. In group ‘B’ plants
stress created moderate reduction in growth and on recovery the increase in leaf length
became comparable to that of controls. Group ‘C’ was characterised by large reduction
in growth and re-watering did not result in final leaf length increases comparable to that
of controls. Both of these characters can be used as selection criteria in the breeding
program for drought tolerance.
Breeding for frost tolerance
Higher crop losses occur in tropical highlands and subtropical plains where frosts can
occur any time during the crop growth period. In India more than 80% of the potatoes
are grown during winter in plains and the crop is prone to frosts during the months of
December and January. Based on the field observations, two types of frosts are often
distinguished. “White frost” occurs when there is a decrease in temperature and relative
humidity is high. “Black frost” occurs under low temperatures and much drier conditions,
hence more damaging and severe because plant tissue is darkened immediately.
Acclimation or hardening may increase the resistance to frosts in many plants.
Exposure of the plants to prolonged low temperature is effective in increasing resistance
to frost injury in S. tuberosum, S. multidisectum, S. chomatophilum, S. acaule and S.
commersonii.
Cultivars Kufri Sheetman and Kufri Dewa possess resistance to frost. High degree of
frost resistance was observed in other 28 hybrids from crosses involving S. acaule.
Conclusion
New pathotypes keeps on emerging in nature. Due to cultivation of resistant
monocultures on a large scale, breeding of resistant varieties against the biotic stresses
is a relay race process. The breeding of new varieties, therefore, always requires
search of new/appropriate genes conferring resistance from wild/ semi-cultivated
species and their introgression in suitable agronomically superior background. In a
number of instances, however, resistant genes are not available and in this context
biotechnological interventions in conjunction with conventional breeding techniques can
be useful in transferring resistance across the species, genera, families etc. Quantitative
Trait Loci (QTLs), a biotechnological tool, in potato holds promise as most of the biotic/
abiotic stresses are polygenic and inherited in quantitative manner. The QTL markers, if
could be exploited in a manner as in case of mendelian factors, can substantially reduce
the time lag in breeding.
Suggested readings
Bradshaw, J.E. and G.R. Mackay. 1994. Potato Genetics, CAB International, UK.
Khurana, S.M.Paul and SK Pandey. Potato in subtropics edited book.
Khurana, S.M.Paul. 1999. Potato Viruses and Viral Diseases, Tech. Bull. No. 35
(Revised), CPRI, Shimla. 94 pp.
Krishna Prasad, K. S. 1993. Nematodes- distribution, biology and management. In:
Advances in Horticulture, Vol 7, Potato, (K.L. Chadha and J.S. Grewal Eds.), pp
635-647. Malhotra Publishing House, New Delhi.
Krishna Prasad, K. S. 1993. Nematode problems of potato. In: Hand Book of Economic
Nematology (R.S. Singh and K. Sitaramaiah Eds.). pp 139-156. Cosmo
Publications, New Delhi.
Shekhawat, G.S., S.M. Paul Khurana and B.P. Singh. 1999. Important diseases of
potato and their management. In: Potato Global Research and Development, Vol. 1
(Khurana, S.M. Paul, G.S. Shekhawat, B.P. Singh and S.K. Pandey, Eds.), pp 281303. Indian Potato Association, Shimla.
Singh, B.P. 1999. Status of late blight in sub-tropics. In: Potato Global Research and
Development, Vol. 1 (Khurana, S.M. Paul, G.S. Shekhawat, B.P. Singh and S.K.
Pandey, Eds) pp 525-533. Indian Potato Association, Shimla.
Singh, B.P. and G.S. Shekhawat (Eds.). 1999. Potato Late Blight in India. Tech. Bull.
No. 27 (Revised), CPRI, Shimla. 85 pp.