The problem of seed degeneration in potato: An alternative paradigm for management in developing countries
S. Thomas
1
, A. Abdurahman
2
, S. Ali
3
, J. Andrade-Piedra
4
, P. Kromann
4
, D. Crook
5
, L.
Torrance
6
, M. Kadian
3
, K. A. Garrett
1
and G. A. Forbes
5
1
Kansas State University, Manhattan, KS;
2
Wageningen University, Wageningen, Netherlands;
3
International Potato Center, New Delhi, India;
4
International Potato Center, Quito, Ecuador;
5
International Potato Center, Beijing, China;
6
James Hutton Institute, Scotland UK
Proposed outline:
1.
Degeneration and its importance a.
Definitions in the literature i.
Etiological diversity ii.
Geographical and environmental influencers iii.
Propose definitions iv.
Physiological vs. pathological degeneration b.
Comparison with true seed from major grain crops i.
Seed quality vs. seed health (include International Seed Testing
Association framework) c.
Consequences to ware potato production i.
Estimating losses due to degeneration ii.
Efficacy of clean seed systems
2.
Evolving paradigms of degeneration management a.
Historical models of management i.
Seed management in ‘ancient’ times (before seed certification) ii.
Clean seed systems in developed countries iii.
Seed systems in developing countries- within and outside the Andes b.
Managing potato seed systems in developed countries i.
Evolution of clean seed systems (e.g., recent “relaxation” of seed legislation in England) ii.
Exclusive use of clean seed systems to manage degeneration
1.
Agro ecological risk factors for seed degeneration
2.
A priori need for seed purchase
3.
Minimal use of resistance, inherent conflict of interest in resistance, role of breeders’ rights
4.
High input systems with minimal dependence on alternative onfarm management approaches c.
Managing potato seed degeneration in developing countries i.
Characteristics of potato production systems in developing countries
1.
Subsistence farming
2.
Climatic diversity (include discussion on agro ecological risk factors for seed degeneration) ii.
The technology transfer approach – clean seed systems for developing countries (potential commercial and institutional interests behind the use of certified seed that systematically boycott alternative approaches)

iii.
Failure of clean seed systems – examples; no evidence it will work in the future; difficulties of lack of rotation to control disease and high infection pressure iv.
Factors influencing uptake of management methods (such as financial risk/market chain) v.
Generally not managed – probably biggest source of yield gap
3.
Reducing potato seed degeneration: need for a paradigm shift? a.
Clean seed systems paradigm b.
Alternative paradigm: balance between host resistance, on-farm management and clean seed systems i.
Evidence of usefulness of on farm management ii.
Evidence of usefulness of resistance iii.
Evidence of usefulness of combining host resistance, on-farm management and clean seed systems c.
In addition to failure of clean seed systems there has been huge opportunity cost i.
No research on other approaches – little information on seed degeneration epidemiology ii.
Minimal research has demonstrated interesting phenomena – disease recovery, cross protection, pathogen evolution. iii.
No time or money for implementing techniques known to work: resistance, positive selection, disease avoidance
4.
Conclusions and considerations a.
Devising a research and development agenda i.
Critical analysis of the knowledge gap (include challenges of estimating losses due to seed degeneration) ii.
Identify current and future research foci iii.
New partnerships (e.g., with agro dealers, AGRA, etc.) b.
Need for farmer knowledge and capacity building c.
Perfect is not the enemy of the good – finding the balance d.
Need for modeling to understand how factors work
1. Seed degeneration in potato and its importance
As the fourth most important global food crop, potatoes ( Solanum tuberosum L., Family:
Solanaceae) are among the top 20 agricultural commodities produced in the world (FAO, 2010).
Its ability to grow in harsh climates, high yield potential per hectare of arable ground, good nutritive value and cooking versatility have led to a doubling of potato consumption in the developing world (Lutaladio & Castaldi, 2009). The yield of the crop however can be markedly different around the world and especially low in developing countries (Table 1) (FAO, 2009).
Thus, given the crucial role this tuber plays in feeding the 870 million undernourished people in the world, improving potato productivity is of utmost importance.
Among the many factors contributing to low productivity such as poor management of bacterial wilt, late blight and soil fertility, degeneration of potato seed tubers is a critical factor
(Gildemacher et al. 2009b). Pathogen-related seed degradation can cause up to 85% reduction in potato yields (Rahman, Akanda, Mian, Bhuian, & Karim, 2010a) and is identified to be an important cause of poor productivity in many developing countries. We discuss here the complex

nature of this problem steeped in many epidemiological, socio-economic and management predicaments. Specifically, this paper (1) describes the key factors causing seed degeneration, discussing its importance in potato production systems (2) evaluates how seed degeneration is conceptualized and how the resulting paradigm shapes research and development agenda, ultimately influencing management (3) critically compares management paradigms in welldeveloped systems compared with low input systems in developing countries and (4) identifies knowledge gaps and research challenges for seed degeneration management, now and in coming decades. a. Comparisons with true seed
Potatoes can be propagated either using clonally propagated tubers or sexually propagated botanical seed (also called ‘true potato seed’ (TPS))(Almekinders, Chujoy, & Thiele, 2009).
Clonal propagation has many disadvantages such as loss of diversity, accumulation of deleterious mutations, and of particular interest here, the accumulation of pathogens (McKey,
Elias, Pujol, & Duputié, 2010). While TPS produces relatively disease free seed, seed tubers harbor a wide range of pathogens. Mother plants, which get infected during their lifetime, produce diseased tubers that grow into diseased daughter plants. Thus production of disease-free tubers is particularly challenging. Despite this, the ease of clonal propagation, the ability to fix favorable traits and the familiarity of agronomic practices and production technology, makes clonal propagation the predominant method of planting commercial potato fields. Thus understanding and dealing with seed degeneration is an integral part of increasing potato yields. b. Concepts from literature
Historically, some authors have included physiological disorders, particularly physiological aging of tubers under seed degeneration (Iritani, 1968; Kawakami, 1962). We consider physiological disorders, which generally disappear after one season of adequate management to be a factor in seed quality, but not an aspect of degeneration, which under normal conditions tends to increase over time. Thus, here, seed degeneration refers to ‘the loss of yielding potential of planting material (seed) of vegetatively-propagated crops due to pathogen build-up caused by re-use of infected planting material’.
Seed degeneration in potato has a complex etiology. Since potato propagules, are tubers that mature in the soil, they are exposed to many soil-borne pathogens. Up to 40 different pathogens cause soil borne diseases of potato (Fiers et al., 2012), however not all are significant, nor do all survive well in seed and therefore are not seed borne pathogens (Table 3). Furthermore, although all degeneration causing pathogens are seed borne by definition, not all seed borne pathogens contribute significantly to degeneration because they do not readily increase in frequency in the seed lot over subsequent generations. It is for this reason, that although many potato pathogens may be considered seed borne, potato viruses, which readily spread systemically from mother tubers to daughter tubers, have long been recognized as the primary causes of seed degeneration
(Solomon-Blackburn & Barker, 2001). Of the ~30 viruses infecting potatoes, Potato virus Y
(PVY), Potato leaf role virus (PLRV) and Potato virus X (PVX) are economically important, with PVX considered more important in combination with PVY rather than by itself (Fuglie,
2007). Nonetheless, other organisms may also contribute to degeneration, if they effectively spread to daughter tubers. For example, in the high-altitude production areas of the Andes,

degeneration by viruses is not as bad as in other parts of the world, and in Ecuador, significant seed degeneration due to Rhizoctonia solani has been demonstrated (Fankhauser, 1999).
The rate of seed potato degeneration is directly influenced by numerous environmental parameters and geographical characteristics that affect host, pathogen and vector dynamics. In the Andes, seed degeneration is much slower at heights >2800 m above sea level (masl) and even much so at heights above 3500 masl (Thiele, 1999). Consistent with this observation are studies that common viruses are found at very low incidence in potato landraces or varieties that have been exposed to natural conditions for untold generations (ref). Research done in Peru indicated that virus incidence may often decrease in subsequent generations and that this is strongly favored by higher altitudes (Bertschinger Lukas, Keller Ernst R., & Gessler, 1995). This phenomenon has been known to traditional farmers and a common practice in the Andes is to sow degenerated seed at higher altitudes clean it and then bring it down to lower altitudes
(Thiele, 1999). Seed stock renewals, also affect seed degeneration and can be quite variable with regard to frequency and source of renewed seed, the latter rarely being certified seed. In subsaharan Africa for example, small numbers of farmers renew their seed stock every 6-7 seasons
(Gildemacher, Demo, et al., 2009) while in higher altitudes of Peru this can be as few as every 7 years (Thiele, 1999). Consequently, predicting the rate of seed degeneration in potato is very challenging. c. Consequences to ware potato production
Degeneration as a consequence of composite pathogens has not been widely researched in recent years. It is in part for this reason that there is little quantitative information on the importance of seed degeneration in potato, although workers routinely indicate that poor quality seed is one of the major constraints of potato production (Fuglie, 2007). Nonetheless, numerous controlled degeneration studies have been conducted in which clean seed is compared with seed of a known number of generations of exposure (Hossain, Ali, & Rashid, 1994; Rahman et al., 2010a;
Whitehead, 1930; Whitworth, Nolte, McIntosh, & Davidson, 2006). These studies, although difficult to interpret in the context of what actually happens on a farmer’s field where factors such as selection may mitigate degeneration, do however, give an idea of the potential of degeneration in potato. Accordingly, 3-4 successive seasons of reusing potato seed can result in high incidence of viruses and leading to high yield losses (Biniam & Tadesse, 2010). In a different study, PVY caused up to 85% yield reduction and PLRV up to 79% in reused fifth generation seeds exposed to natural conditions in Bangladesh (Rahman, Akanda, Mian, Bhuian,
& Karim, 2010b). In Colorado, a comparison between cultivars with different levels of symptom expression revealed that PVY related yield reduction was similar in the high and low severity cultivars (Nolte, Whitworth, Thornton, & McIntosh, 2004). (Table?)
2. Evolving paradigms of degeneration management a. Historical models of management b. Managing potato seed systems in developed countries c. Managing potato seed degeneration in developing countries
Global potato seed systems include both formal and informal systems (Thiele, 1999). Formal systems are strictly regulated systems for the breeding, production and distribution of certified, high quality seed while informal systems involve the use of farmer saved seeds for planting. In

North America and Western Europe, formal seed potato systems were established in the early
1900’s (Monares, 1988) but in many countries of Africa, Asia, Middle-East and Eastern Europe repeated attempts to promote formal seed systems have had little success (Jaffee & Srivastava,
1994). The bulky nature of seed potato, high costs of formally regulated seeds and limited infrastructure and resources for formal seed programs are some reasons for the low demand and supply of certified seeds to smallholder farmers (Thiele, 1999). Consequently, informal seed systems account for 70-99% of potato seed supply in many developing countries (Table 2).
In reality, there are many shades of semi-formality in seed production in developing countries, as many specialized seed producers do not participate in any type of formal regulation. Since these small-scale producers are, nonetheless, specialized in seed production, we distinguish this class of seed from that produced on-farm as part of a ware production system, or purchased locally from others who are primarily ware potato producers. Thus, for developing countries, it may be more realistic to classify seed as either coming from specialized seed producers (not formally certified seed) and as “on-farm seed”, i.e., produced within the ware production system.
Although formally certified seed is a common input for potato production in the industrialized nations, formal seed systems generally produce less than 5 % of potatoes planted in developing countries, and in many developing countries the percentage is much lower.
3. Reducing potato seed degeneration: need for a paradigm shift? a. Disease-free seed systems paradigm b. Alternative paradigm: balance between host resistance, on-farm management and clean seed systems i. Evidence of usefulness of on-farm management
Various on-farm management tools can be used to lower seed degeneration, although their epidemiological consequences are not always clearly understood.
Roguing: Removal of infected plants, reduces inoculum sources in the field but could negatively affect overall crop yield if the infected crop produces useable yield (Sisterson & Stenger, 2013).
The method works best when growers synchronize rouging over large areas and requires farmer training in recognizing disease symptoms. In potato, studies on the usefulness of roguing against viral diseases indicate variable success. The method reduced tuber infection by PLRV about 30-
45%, but this depended on the level of vector infestation in the field (Ioannou, 1989). Further, since aphid alighting can be affected by gaps in the crop canopy, rouging resulting in gaps
≥0.6m
2
can result in greater incidence of PVY (Davis, Radcliffe, & Ragsdale, 2009). Thus allowing weeds to fill the gap or replacing plants to fill these gaps may reducing vector alighting and also counter yield losses.
Positive selection: In this method, only healthy looking potato plants, pegged before senescence, are used as the source of seed potato for the next season. In field studies, this method led to an average increase in yield of 23-35% (Schulte-Geldermann, Gildemacher, & Struik, 2012) and
28-53% (Gildemacher et al., 2011a) at minimal additional costs, while simultaneously lowering virus incidence.

Crop hygiene: ‘Groundkeeper’ tubers that produce volunteer potato plants in the field can serve as sources of primary inoculum (Thresh & Cooter, 2005). This is especially of concern in cool temperature regions where vectors overwinter on hosts that do not harbor virus populations
(Robert et al., 2000). In warmer climates, removal of alternate hosts is important in reducing initial inoculum (Robert et al., 2000), but workers in developing countries have noted that farmers rarely remove any plant that may serve as a source of food ( ).
Vector management: Vectors play an important role in the secondary spread of viruses within a field. Potato viruses are vectored by insects, fungi and nematodes although the most economically important vectors are insects, more specifically aphid vectors (Salazar, 1996). The most predominant of these is the green peach aphid Myzus persicae, a polyphagous aphid with nearly 875 hosts (Radcliffe & Ragsdale, 2002). Virus transmission by aphids can be nonpersistent or persistent, and M. persicae for example transmits PLRV persistently and PVY nonpersistently (Radcliffe & Ragsdale, 2002). This in turn can affect disease control by vector management because persistent transmission, associated with longer latent periods are easier to control with contact insecticides (Boukhris-B, Rouze-Joua, Souissi, Glais, & Hulle, 2011).
Paraffinic mineral oils on the other hand, that affect aphid behavior, preventing aphids from probing, have been used against non-persistently transmitted viruses (Dessureault, Prasad,
Meberg, & Teasdale, 2011). In addition to this, growing border crops such as soybean and wheat around the edge of a potato field can limit non-persistently transmitted viruses that are attracted to the contrast (soil brown vs. crop green) at the edges (Dessureault et al., 2011). Other management strategies such as using pheromone traps and insecticidal soap to alter aphid feeding and straw mulching to affect aphid flight activity can also be used (Dessureault et al.,
2011; Saucke & Döring, 2004).
Farmer knowledge and capacity building.
(need something on this to be sure that readers know that none of the above really happens on farmers’ fields in developing countries). ii. Evidence of usefulness of resistance
Potato plants are most susceptible to virus infections during vegetative growth and get resistant with maturity (Radcliffe & Ragsdale, 2002). This, also called mature plant resistance, prevents late infections from translocating into tubers and is known to be cultivar- and virus strainspecific (Radcliffe & Ragsdale, 2002; Robert et al., 2000). Classical resistance breeding programs against potato viruses have been effective since the 1930’s using resistance genes present in wild relatives (such as Solanum demissum ). In potatoes, resistance to viruses is categorized as extreme resistance (ER), hypersensitivity response (HR), resistance to infection, virus accumulation, virus movement and tolerance and has been discussed in detail by Soloman-
Blackburn and Barker (Solomon-Blackburn & Barker, 2001). Accordingly, ER prevents early virus multiplication, without causing cell death while classical HR prevents spread of infection by the death of a few cells around the site of infection. Both HR and ER are durable, simply inherited and relatively easier to breed. Resistance to infection on the other hand, is more difficult to breed and in case of PLRV has polygenic inheritance. Resistance to virus accumulation has been identified against PLRV, PVX and PVY in S. brevidens . In PLRV, some plants accumulated only 1-5% of virus concentration found in susceptible plants. Further, this

resistance is often manifested as a combined effect of virus multiplication and resistance in cellcell movement. Tolerance in plants is less preferred than resistance but has been used where resistance is absent. Tolerant plants continue to serve as sources of inoculum and are predicted to allow for the evolution of high titer virus strains (van den Bosch, Jeger, & Gilligan, 2007). In addition to these, pathogen derived resistances such as post transcriptional gene silencing, coat protein-, movement protein-, polymerase- and ribozyme- mediated resistances have been used to generate transgenic resistance lines (Solomon-Blackburn & Barker, 2001).
Need to say something no scales used –probably not really quantitative iii. Evidence of usefulness of combining host resistance, on-farm management and diseasefree seed systems c. Opportunity costs of limiting attention to disease-free seed systems
4. Conclusions and considerations a. Devising a research and development agenda i. Critical analysis of the knowledge gap (include challenges of estimating losses due to seed degeneration) ii. Identify current and future research foci iii. New partnerships (e.g., with agricultural seed dealers, AGRA, etc.) iv. Need for models to understand how factors work b. Need for farmer knowledge and capacity building c. Perfect is not the enemy of the good – finding the balance
Text from previous write-up that doesn’t directly fit under outline:
Improved detection and prevention of seed degeneration
Detection techniques
Early detection of viruses in potato based on symptomatology had the disadvantage of missing plants with masked symptoms and latent infections (Robert, Woodford, & Ducray-Bourdin,
2000). This problem was overcome with the introduction of serological techniques such as
ELISA (enzyme-linked immuno-sorbent assay), where specific, sensitive and quick detection of tuber infection was possible (Robert et al., 2000). Introduction of polymerase chain reaction
(PCR) based techniques further improved the sensitivity and specificity allowing for strain-level detection of potato viruses (Robert et al., 2000). Current advances in detection, to name a few, include microsphere immunoassay (MIA), an alternative to ELISA that allows multiplex detection of viruses; cDNA macroarray, a nucleic acid based method that similarly allows the detection of multiple viruses simultaneously (Maoka, Sugiyama, Maruta, & Hataya, 2010); and

reverse transcription with loop-mediated isothermal amplification (LAMP) that reduces the time and cost of detection (Ju, 2011). Despite these advances in detection, field detection of tuber infection continues to be dependent on farmer experience in recognizing visual symptoms.
Improved propagation techniques
Potato viruses are present in the vascular ring of tubers that connect all buds but is absent in the meristematic tip of the growing vegetative bud. Thus meristem tip culture has been widely used to eliminate viruses in new planting material (Salazar, 1996). By this method Al-Taleb et al., were able to completely eliminate PVY infection and obtain clean plantlets (Al-Taleb, Hassawi,
& Abu-Romman, 2011). Thermotherapy, where tubers are treated with hot air (37 o
C) for 3-6 weeks can also eradicate certain viruses (such as PLRV) from tubers (Kaiser, 1980; Kassanis,
1949). Further, with the two methods in combination, it is possible to obtain virus-free mother plants from infected potato plants (Biniam & Tadesse, 2010; Ranalli, 1997). Such disease free plantlets can then be propagated for large scale seed production by an array of techniques including single node cuttings, stem cuttings and microtubers produced in beds or with soilless aeroponic or hydroponic systems (Mbiyu et al., 2012; Ranalli, 1997; Tsoka, Demo, Nyende, &
Ngamau, 2012). Such systems are available in many developing countries although the extent of their impact in large scale use of virus-free tubers and disease epidemics is unclear. Of particular concern is that the use of in vitro propagated material could result in the evolution to high-titer virus strains (van den Bosch et al., 2007).
Yet another method of propagating disease free potatoes is using the TPS technology. Launched by the International Potato Center in late 1970s, this method had many advantages including lower seed bulkiness, fewer disease (soil and seed borne) transmissions and limited need for cold storage facilities (Almekinders et al., 2009). The method however requires modifications in all aspects of potato production and has had limited success in a few developing countries
(Almekinders et al., 2009).
Literature cited:
Al-Taleb, M. M., Hassawi, D. S., & Abu-Romman, S. M. (2011). Production of virus free potato plants using meristem culture from cultivars grown under Jordanian environment. American-Eurasian
journal of agricultural and environmental science, 11(4), 467–472.
Almekinders, C. J. M., Chujoy, E., & Thiele, G. (2009). The use of true potato seed as pro-poor technology: the efforts of an international agricultural research institute to innovating potato production. Potato Research, 52(4), 275–293.
Bertschinger Lukas, Keller Ernst R., & Gessler, C. (1995). Characterization of the Virus x Temperature
Interaction in Secondarily Infected Potato Plants Using EPIVIT. Phytopathology, 85(7), 815–819.

Biniam, T., & Tadesse, M. (2010). A survey of viral status on potatoes grown in Eritrea and in vitro virus elimination of a local variety “Tsaeda embaba.” African Journal of Biotechnology, 7(4). Retrieved from http://www.ajol.info/index.php/ajb/article/view/58436
Boukhris-B, S., Rouze-Joua, J., Souissi, R., Glais, L., & Hulle, M. (2011). Transmission Efficiency of the
Strain PVYNTN by Commonly Captured Aphids in Tunisian Potato Fields. Plant Pathology Journal,
10(1), 22–28. doi:10.3923/ppj.2011.22.28
Davis, J. A., Radcliffe, E. B., & Ragsdale, D. W. (2009). Planter skips and impaired stand favors Potato virus Y spread in potato. American journal of potato research, 86(3), 203–208.
Dessureault, M., Prasad, R., Meberg, H., & Teasdale, C. (2011). Controlling aphid-vectored viruses for organic seed potato production: Literature and knowledge review. Retrieved from http://www.certifiedorganic.bc.ca/programs/osdp/I-122_Virus_Lit_Review_Final.pdf
Fankhauser, C. (1999). Main diseases affecting seed degeneration in Ecuador: New perspectives for seed production in the Andes. In European Association for Potato Research (pp. 50–51). Sorrento
Italy.: EAPR.
Fiers, M., Edel-Hermann, V., Chatot, C., Le Hingrat, Y., Alabouvette, C., & Steinberg, C. (2012). Potato soil-borne diseases. A review. Agronomy for sustainable development, 32(1), 93–132.
Fuglie, K. O. (2007). Priorities for potato research in developing countries: results of a survey. American
Journal of Potato Research, 84(5), 353–365.
Gildemacher, P. R., Demo, P., Barker, I., Kaguongo, W., Woldegiorgis, G., Wagoire, W. W., … Struik, P. C.
(2009). A description of seed potato systems in Kenya, Uganda and Ethiopia. American journal of
potato research, 86(5), 373–382.
Gildemacher, P. R., Kaguongo, W., Ortiz, O., Tesfaye, A., Woldegiorgis, G., Wagoire, W. W., … Struik, P. C.
(2009). Improving potato production in Kenya, Uganda and Ethiopia: A system diagnosis. Potato
Research, 52(2), 173–205.

Hossain, M., Ali, M. S., & Rashid, M. M. (1994). Effect of inoculum levels of potato virus Y (PVY) on yield and subsequent spread of the disease under insecticide spray and unsprayed condition
[Bangladesh]. Bangladesh Journal of Botany, 23. Retrieved from http://agris.fao.org/agrissearch/search/display.do?f=1995/BD/BD95002.xml;BD9425026
Ioannou, N. (1989). Production of seed potatoes in Cyprus: the effects of roguing and planting date on the spread of potato leaf roll virus, tuber yield, and infestation by potato tuber moth. Potato
research, 32(3), 331–339.
Iritani, W. M. (1968). Factors affecting physiological aging (degeneration) of potato tubers used as seed.
American Journal of Potato Research, 45(3), 111–116.
Jaffee, S., & Srivastava, J. (1994). The roles of the private and public sectors in enhancing the performance of seed systems. The World Bank Research Observer, 9(1), 97–117.
Ju, H.-J. (2011). Simple and rapid detection of potato leafroll virus by reverse transcription loopmediated isothermal amplification, 27(4), 385–389.
Kaiser, W. J. (1980). Use of thermotherapy to free potato tubers of alfalfa mosaic, potato leaf roll, and tomato black ring viruses. Phytopathology, 70(11), 1119–1122.
Kassanis, B. (1949). Potato tubers freed from leaf-roll virus by heat. Nature, 4177, 881.
Kawakami, K. (1962). The physiological degeneration of potato seed tubers and its control. Potato
Research, 5(1), 40–49.
Lutaladio, N. B., & Castaldi, L. (2009). Potato: The hidden treasure. Journal of Food Composition and
Analysis, 22(6), 491–493.
Maoka, T., Sugiyama, S., Maruta, Y., & Hataya, T. (2010). Application of cDNA macroarray for simultaneous detection of 12 potato viruses. Plant Disease, 94(10), 1248–1254.

Mbiyu, M. W., Muthoni, J., Kabira, J., Elmar, G., Muchira, C., Pwaipwa, P., … Onditi, J. (2012). Use of aeroponics technique for potato (Solanum tuberosum) minitubers production in Kenya. Journal
of horticultural forestry, 4(11), 172–177.
McKey, D., Elias, M., Pujol, B., & Duputié, A. (2010). The evolutionary ecology of clonally propagated domesticated plants. New Phytologist, 186(2), 318–332. doi:10.1111/j.1469-8137.2010.03210.x
Monares, A. (1988). The Social Sciences at CIP. International Potato Center.
Nolte, P., Whitworth, J. L., Thornton, M. K., & McIntosh, C. S. (2004). Effect of seedborne Potato virus Y on performance of Russet Burbank, Russet Norkotah, and Shepody potato. Plant disease, 88(3),
248–252.
Radcliffe, E. B., & Ragsdale, D. W. (2002). Aphid-transmitted potato viruses: the importance of understanding vector biology. American Journal of Potato Research, 79(5), 353–386.
Rahman, M. S., Akanda, A. M., Mian, I. H., Bhuian, M. K. A., & Karim, M. R. (2010a). Growth and yield performance of different generations of seed potato as affected by PVY and PLRV. Bangladesh
Journal of Agricultural Research, 35(1), 37–50.
Rahman, M. S., Akanda, A. M., Mian, I. H., Bhuian, M. K. A., & Karim, M. R. (2010b). Growth and yield performance of different generations of seed potato as affected by PVY and PLRV. Bangladesh
Journal of Agricultural Research, 35(1), 37–50.
Ranalli, P. (1997). Innovative propagation methods in seed tuber multiplication programmes. Potato
Research, 40, 439–453.
Robert, Y., Woodford, J. A., & Ducray-Bourdin, D. G. (2000). Some epidemiological approaches to the control of aphid-borne virus diseases in seed potato crops in northern Europe. Virus Research,
71(1), 33–47.
Salazar, L. F. (1996). Potato Viruses and Their Control. International Potato Center.

Saucke, H., & Döring, T. F. (2004). Potato virus Y reduction by straw mulch in organic potatoes. Annals of
Applied Biology, 144(3), 347–355.
Schulte-Geldermann, E., Gildemacher, P. R., & Struik, P. C. (2012). Improving seed health and seed performance by positive selection in three Kenyan potato varieties. American Journal of Potato
Research, 89, 429–437.
Sisterson, M. S., & Stenger, D. C. (2013). Roguing with replacement in perennial crops: conditions for successful disease management. Phytopathology, 103(117), 128.
Solomon-Blackburn, R. N., & Barker, H. (2001). Breeding virus resistant potatoes (Solanum tuberosum): a review of traditional and molecular approaches. Heredity, 86, 17–35.
Thiele, G. (1999). Informal potato seed systems in the Andes: Why are they important and what should we do with them? World development, 27(1), 83–99.
Thresh, J. M., & Cooter, R. J. (2005). Strategies for controlling cassava mosaic virus disease in Africa.
Plant pathology, 54(5), 587–614.
Tsoka, O., Demo, P., Nyende, A. B., & Ngamau, K. (2012). Potato seed tuber production from in vitro and apical stem cutting under aeroponic system. African Journal of Biotechnology, 11(63), 12612–
12618.
Van den Bosch, F., Jeger, M. J., & Gilligan, C. A. (2007). Disease control and its selection for damaging plant virus strains in vegetatively propagated staple food crops; a theoretical assessment.
Proceedings: Biological Sciences, 274, 11–18.
Whitehead, T. (1930). A STUDY OF THE DEGENERATION OF CERTAIN POTATO STOCKS. Annals of Applied
Biology, 17(3), 452–486. doi:10.1111/j.1744-7348.1930.tb07226.x
Whitworth, J. L., Nolte, P., McIntosh, C., & Davidson, R. (2006). Effect of Potato virus Y on yield of three potato cultivars grown under different nitrogen levels. Plant disease, 90(1), 73–76.