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1'; /$ --C// PRODUCTION OF SEED POTATOES

IN CYPRUS

Nicos J. Vakis --driU -4- - --.-q

AGRICULTURAL RESEARCH INSTITUTE

MINISTRY OF AGRICULTURE AND NATURAL RESOURCES

NICOSIA

N O V E M B E R 1 9 8 0

CYPRUS

PRODUCTION OF SEED POTATOES IN CYPRUS

Nicos J. Vakis

SUMMARY

Locally produced seed of the potato varieties Arran Banner, Spunta, Cara and Up-teDate performed equally well as importcd certified seed. T h e original grade of the seed used (foundation vs certified seed) did not have ang consistent significant effect on the performance of the seed produced. Seed produced a t tuvo sites (low vs high altitude) produced ware crops of equal health standard and yield. Ware potatoes p o ~ r m from locally produced seed were earlier hecause of the more advanced phpiological state of the wed tubers. Yields of ware potatom declined as the time lapse from removal of seed from cold storage to planting increased from three to nine weeks. Under Cyprus conditions 50% of the local requirements for seed potatoes can be met from a single multiplication of imported seed stocks (foundation or certified A seed).

INTRODUCTION

The potato industry has long been an integral part of the economic structure of Cyprus and in recent years i t has made significant contri- butions to the strengthening of the overall economy of the island. A high percentage of the foreign exchange fetched in Cyprus is earned from ware potato exports, which result to more than CS'l5 million annually; potato exports from the 1980 spring crop exceeded 155,000 metric tons.

At present all the seed potatoes required for the spring crop (normally planted during December - January) are imported from various European countries, mainly the Netherlands, the United Kingdom and Ireland. Annual imports of seed potatoes range from 8,500 - 11,000 tons, which, depending on world prices, cost C£ 1.0 - 1.5 million.

Elaborate seed production programmes have been developed in many countries in order to provide seed of the desired variety and quality with regard to size, physiological state and freedom from diseases (fungal, bacterial, virus). A complete seed production pro- gramme consists of two parts, i.e. production of basic seed (foundation, elite seed) and production of certified seed (van der Zaag, 1972). The work described in this paper was underaken in 1976, on the assumption that a complete seed production programme would not be feasible in Cmrus at this stace; only the simplest form of seed production whereby a seed stock of known quality is multiplied once was

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considered feasible from both the economic and technical point of view; thus the main target set was to study the feasibility of produc- ing locally half of the seed potatoes required, in order to save foreign exchange.

Several factors affect the performance of seed potatoes. Svensson (1966) (cited by Wurr, 1978) showed that the coefficient of variation of yield of plants grown from individual seed tubers varied from 20 to 50%. Location, temperature, altitude, time of planting, time of defoliation and harvesting, stem density and soil type affect the quality of seed produced and its subsequent performance (Beukema and van der Zaag, 1979; Hutchinson, 1978a; Wurr, 1978; Shepard and Chaflin, 1975; de Bokx and Mooi, 1974; Bodlaender, 1973; Burton, 1966; Proud- foot and McCallum, 1961). De Bokx and Mooi (1974) found that yield from seed grown at low altitude was higher than that from seed grown a t higher altitude. However, Staikov and Chavdarov (1971) (cited by Wurr, 1978) showed that yield increased progressively with the altitude where the seed was produced up to 939m but further increase in altitude resulted in reduced yield. The physiological age (the physiological state of the tuber at a given time) of the seed tuber affects the subse- quent growth and yield of the crop (Allen and Scott 1980; O'Brien and Allen, 1978; Iritani, 1968a; Toosey 1964). According to Kawakami (1963) there is an optimum physiolo,@cal age for maximum (final) yield and if this is exceeded total yield is reduced. Of the storage factors temperature ex,erts the largest effect on seed ,performance through its influence on the physiological age of the seed (O'Brien and Allen, 1978; Hutchinson, 1978b; Iritani, 1968b; Murphy et al., 1967). Other conditions being equal lower temperature gives physio- logically younger tubers than higher temperature.

To assess the feasibility of producing part of the seed require- ments, work was conducted during the past 5 years on the major factors affecting seed quality under Cyprus conditions. The work included studies on the effects of site of seed production, on the initial grade of the seed stock used for multiplication and on several agro- nomic aspects and storage conditions. Finally comparisons were made of the performance of local and imported seed with regard to cost, yield, healthiness and earliness.

MATERIALS AND METHODS Production of the seed

The varieties Arran Banner, Spunta and Cara were used through- out the trials.

The main sites of production were: Akhelia (elevation 50m), Palekhori (700m) and Agros (1010m). Athalassa Farm (140m) was used only a t the initial stages. Field trials under grawers' conditions were also carried out a t Mandria (Paphos), Timi, Agros, Kyperounda and Palekhori.

4

For each variety two grades of imported seed were used, namely certified (A) and foundation (FS or E) stocks. These stocks were multiplied once. In a separate trial seed produced from virus tested stem cuttings (VTSC seed) was used for two successive multiplica- tions.

Seed was planted whole (cut seed is mainly used in Cyprus for ware production) in rows 60 cm apart; spacing within rows was 20 cm. Seed rates per donum (one donum = 1339m2) ranged from 400 to 600 kg depending on seed size and variety. Work on the effects of seed rate (seed size x spacing) on the yield of seed potatoes is in progress. Planting was done either by hand (Palekhori, Agros, Ky- perounda) or by seed planters (Paphos). Planting was done during January at Paphos, and February to early April a t Pitsilia (PBlekhori: Kyperounda and Agros).

Cultural practices were similar to those used for ware potato pro- duction except where differential treatment was necessary. Ferti- lizer rates were lower than those used for ware potato production (Krentos and Orphanos, 1979). Six to eight sprays against blight were made in combination with pirimicarb or methomyl for controlling aphids.

Soon after emergence all plots were inspected regularly (every week) for records and roguing; the rogued plants and tubers were removed from the field and destroyed. Air temperatures were re- corded from planting through harvesting by thermoscripts.

Haulm destruction (burning) was done by chemicals except in the field trials at Palekhori, Agros and I<yperounda where the foliage was destroyed mechanically. Depending on weather conditions haulm destruction at Paphos area was done 85 - 90 days after planting for Spunta, 90 - 95 days for Arrsn Banner and 100 - 105 days for Cara ;

more days for each variety were allowed a t Pitsilia area. At the time of haulm destruction the tubers were of an acceptable seed size (28 -60 mm). Harvesting was done about 10 days after haulm des- truction in order to allow skin setting.

Storage of seed potatoes

After lifting, the seed potatoes were packed in either bruce boxes or jute bags and were transported to Nicosia for cold storage. The temperature was reduced gradually to 2' - 3'C over a period of 15 days; the seed potatoes were kepi at this temperature for 4.5-6 months, depending on the site of production and the date of lifting. In general the seed kept in a very good condition.

Seed was removed to ambient about 3 - 4 weeks before planting and was allowed to sprout under light (sunlight) conditions in order to develop sturdy sprouts. In trials initiated in 1979 seed was removed

at 0 , 1 , 2 , 3 , 5 , 7 or 9 weeks before planting in order to det'ermine the best time of removal before planting and its effects on yield and earliness.

Testing of local seed

Subsequent performance of ware crops was the major criterion used to assess the quality of locally produced seed. Yield trials with local and imported seed were carried out at Xylophagou, Akhelia and Athalassa. In spring 1980 a small quantity of local seed was given to growers in the Kokkinochoria area (the main potato growing area) for observation purposes. Two liftings were made for each experiment in order to determine defferences in earliness. Records were taken on the rate of plant emergence, tuberization, incidence of pests and diseases (particularly viruses), exportable yield, dry matter content, and tuber size.

RESULTS AND DISCUSSION

Seed yields and multiplication rates

Yields of seed potatoes obtained at Paphos, Palekhori and Agros are shown in Table 1. There were practically no differences in yield between foundation and certified seed, therefore the data presented are means for both classes. Arran Banner, Spunta and Up-to-Date gave similar yields at all sites tested; Cara performed satisfactorily at Paphos but yielded less at Palekhori and especially at Agro's. This was attributed to microclimatic conditions and the fact that planting in these areas has to be done late (March to April). Depending on variety, longer days and higher temperatures favour more the development of plants (foliage) rather than tubers. Indeed haulm growth of Cara at Agros was very vigorous.

Multiplication rates (kilos of seed potatoes produced per kilo of moth,er stock planted) obtained in the trials ranged from 7 - 10; similar, and in some cases better, rates were obtained b~y co-operating growers in semi-commercial trials. Such rates must b'e regarded as satisfactory and are generally comparable to those obtained in countries producing seed potatoes. According to van der Zaag (1972) multiplication rates ot 5 - 7 are common in many places; in the Netherlands these rates range from 7 - 15.

TABLE 1. Yield ( tons /donum) of seed potatoes of four varieties g rown at three sites.

Variety Site Paphos Palekhori Agros

Up-to-Date 4.0 3.4 - Arran Banner 3.4 3.2 3.4 Spunta 4.2 4.4 4.9 Cara 4.4 3.6 2.5

--

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Incidence of virus diseases

The inadence of virus diseases in the experimental plots for seed production varied from season to season but it was generally low (Table 2). Differences between certified and foundation seed were not consistent except for Cara in which no virus infected plants were observed in plots grown from basic stocks. Differences among sites were inconsistent and insignificant. This was also evidenced by the subsequent performance of the seed.

Virus infection in ware crops grown from local seed was generally low and much below accepted tolerances. The effect of original seed grade on the incidence of viruses was small and varied with variety and season. In most cases seed derived from basic stocks had a lower percentage of virus infected plants than seed from certified stocks. Differences between local seed and imported certified seed were small and inconsistent. When the ware crop was grown at Paphos a higher percentage of infected plants was observed than at Kokkinochoria, probably due to later planting.

Production of virus-free seed is essential for satisfactory yields of ware potatoes (Reestman, 1970, 1972). These results suggest that good quality seed can be produced in Cyprus at all sites used in these trials. If basic seed can be obtained at relatively reasonable prices, it should be preferred to certified seed. However, sound seed pro- duction management (spraying, roguing etc) seem to be more im- portant than the grade of the initial stock, at least under the conditions of Cyprus and for a single multiplication.

The main virus diseases observed in these trials were leaf rd l , leafdrop streak, mosaics (mild, severe and rugose) and a disorder of 'Cara resembling purple top wilt (attributed to a mycoplasma). Further detailed studies on virus diseases and their vectors are in progress.

TABLE 2. Virus infection in potato seed crops at Palekhori.

Variety and seed grade' % --- Virus infectedplants ~- 1977 1.978 197.9 1.980

Arran Banner - Certifipd seed 0.8 0.3 0.7 0 - Foundation seed 0.7 0 0.2 0

Up-to-Date - Certified seed 1.0 0 0.7 0 - Foundation seed 0 0 1 .O 0

Spunta - Certified seed 3.8 0.7 0 0.5 - Foundation seed 1.5 0 0.5 0.8

Cara - Certified seed 0.3 0.2 0.2 0 - Foundation seed 0 0 0 0

* Grade refers to the initial stock used for t,lultiplication in Cyprus.

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Performance of locally produced seed

Locally produced seed was not inferior to imported certified seed. On the contrary in most cases and especially with earlier liftings local seed gave higher yields (Table 3). Similar results were obtained at both test locations for ware production, i.e. Xylophagou (Table 3) and Paphos (data not pesented) although yields at Paphos were generally lower. In addition to the experimental data, results from semi-commercial trials by growers who used local seed indicated that locally produced seed performed equally well as imported certi- fied seed; the effect of local seed on earliness was also demonstrated in these trials. TABLE 3. Performance of local and impor ted seed o f f o u r po ta to varieties

a t Xylophagou (spr ing crops). -

Grade and site Exportable yield (tonsldonum) Variety of seedproduction* -- 7778 1 98b-

96days'" 107days 701 days 722 days Up-to-Date Certified - Imported 4.48bW'* 5.33a - -

- Palekhori 4.66ab 5.03a - - - Paphos 5.37a 5.64a - -

Foundation - Palekhori 5.43a 5.74a - - - Paphos 5.21a 5.64a - -

Arran Banner Certified - Imported 4.80b 5.50a - 4.57a - Palekhor~ 5.57a 6.10a 3.33a 4.91 a - Paphos 5.72a 5.97a 3.29a 4.47a - Agros - - 3.50a 4.75a

Foundation - Palekhori 5.35ab 6.38a 3.43a 4.43a - Paphos 5.51a 5.57a 3.46a 4.63a

Spunta Certified - Imported 5.2913 6.71 b 3.9513 6.84a - Palekhor~ 6.52a 7.20ab 3.80b 5.89b - Paphos 6.36a 7.37a 3.95b 5.691, - Agros - - 4.19ab 6.54ab

Foundation - Palekhori 6.99a 7.02ab 3.98b 5.84b - Paphos 6.96a 6.90ab 4.46a 6.00ab

Cara Certified - Imported 4.1 1 b 5 . 8 9 ~ 2.58b 5.551, - Palekhor~ 6.48a 6.84b 3.46a 5.91ab - Paphos 6.21a 7.87a 3.77a 6.31 ab - Agros - 3.45a 5.84ab

Foundation - Palekhori - 3.25a 6.27ab - Paphos - 3.71 a 6.63a

* See footnote to Table 2. Days from planting to liking.

" * ' Means within columns and varieties bearing lhe samc lcttcr do not differ significantly at P = 0.05.

Effect of seed grade on subsequent yield of ware patatoes Yields obtained from local seed derived from single multiplication

of basic and certified stocks are also shown in Table 4; yields from imported certified seed are also shown for comparison. No signi-

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ficant differences were obtained between seed derived from FS or certified stocks; the results were not consistent with either type of seed; in some cases FS resulted in higher yi'elds and vice-versa. Compared to imported seed, local seed performed equally well or, in most cases, better.

These results demonstrate that single multiplication of either FS or certified A stocks can give good quality seed for ware production. The determining factor in both cases appears to be the conditions under which the seed is produced. Wurr (1978) suggested that th'ere is nothing specific about the source of seed, as this represents the combination of many varying factors, which may or may not combine to influence se'ed performance. TABLE 4. Effect o f seed gradeon yield of w a r e potatoes grown atxylophagou

(spring crops). --

Variety Seedgrad2 Exportable yield (tonsldonurn) 1979 1980

Arran Banner Certified - Imported - Local

Foundation - Local

Spunta Certified - Imported 6.72a 5.39a - Local 7.11a 5.01 a

Foundation - Local - 5.07a

Cara Certified - Imported 5.30b 5.55b - Local 5.98a 6.02ab

Foundation - Local 5.95a 6.45a

^ See footnote to Tabk 2. * * Means within columns and varieties bearing the same lctter do not differ significantly

at P = 0.05.

Effect of site of seed prodhetion on subsequent yield

Differences in yield of ware potatoes grown from seed produced at Paphos and Palekhori were inoonsistent and not significant except in one case with Cara (Tables 5 and 6). Site of seed production had practically no influence on subsequent performance of the seed. Diffe- rences in yield b'etween local and imported seed were attributed to the physiological state of the seed; imported seed was physiologically younger at the time of planting the ware crop. Seed produced at Agros gave similar ~esul ts as seed from Paphos and Palekhori.

These results indicate that under Cyprus conditions the effect of site of seed production is exerted mainly through the physiological state of the seed. Wurr (1978, 1980), Bodlaender (1973) and other workers attributed differences in seed performance to different temperature regimes. Warmer temperatures during seed production result in physiologically older seed. Temperatures at Akhelia and Agros (elevation 50m and lOlOm respectively) were generally the

9

same during the period of seed production and especially during the latter part of the development of the seed crop (Fig. I), This might esplain why the quality of seed produced at both sites was practically the same; data on aphid populations seem to support this although further work is required. Reports on the effects of altitude on sub- sequent seed performance are conflicting (De Bokx and Mooi, 1974; Staikov and Chavdarov, 1971).

TABLE 5. E f fec t of s i te o f seed c rop o n expor tab le yield ( tons ldonum) o f wa re potatoes g r o w n a t Xylophagou (spr ing c rop 1978).

Variety Site o f seed crop Days to lifting Qfi m7

Up-to-Date N. Ireland Palekhori Paphos

Arran Banner N. Ireland Palekhori Paphos

Spunta Holland Palekhori Paphos

Cara Ireland 4.11b 5 . 8 9 ~ Palekhori 6.48a 6.8413 Paphos 6.21 a 7.87a

" Means witllili colnmns and varieties bearing the same letler do not differ significantly at P = 0.05.

TABLE 6. Ef fec t o f s i te o f seed c rop o n yield o f wa re potatoes g r o w n a t Xylophagou (spr ing c rop 1979).

--

Variety Site of seed crop Exportable vieid (tons ldonumi

Arran Rannar N Ireland . . . . . - . -. . - Palekhori Paphos

Spunta Holland Palekhori Paphos

Cara Ireland Palekhori Paphos

0 L Jon. Feb Mar Apr Moy June

Fig. 1. Mean maximum and minimum air temperature at Akhelia and Agros (1979).

Effect of local seed om earliness

Locally produced seed gave earlier ware crops than imported seed, although final yields were similar. (Table 7). The advanced maturity of ware crops grown from local seed was also reflected in the dry matter content of the tubers. Tubers from local seed had higher dry matter content than tubers from imported seed, particularly during the first lifting. The increase in dry matter content between two liftings was higher in tubers from imported seed, indicating that these tubers matured later (Tables 7 and 8). It was also observed that sprouting, tuberization and senescence were earlier in plots grown from local seed. Although in local seed the number of stems per bud was higher, development of canopy was restricted compared to that of imported seed. These effects were attributed to the advanced phy- sialogical state of the local seed. Semilar effects were reported by other workers (Allen and Scott, 1980; Wurr, 19'18; Burton, 1966; Toosey, 1964).

Earliness of the ware crop is of prime importance for the marke- ting of Cyprus potatoes. The present results showed that production of local seed would positively contribute to the efforts to produce early crops for export. Lifting of the seed potatoes during May - July and storage through October - November would result in the required physiological state of the seed for planting in November - December.

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TABLE 7. Inf luence of seed source on t h e increase in yield (%) o f ware potatoes between t w o l i f t ings a t Xylophagou (spr ing crop 1979).

%increase in yield between Variety Seed source two liftings

Arran Banner N. Ireland Palekhori Paphos

Spunta N. Ireland Palekhori Paphos

Cara Ireland Palekhori Paphos

TABLE 8. Dry mat te r con ten t of wa re potatoes a t two l i f t ings as af fected by seed source. Xylophagou (spr ing c rop 1980).

Variety Seed source % dry matter % increase 1st liffing 2nd lifting

z r a n Banner N. Ireland 15.8 18.6 18.0 Palekhori 17.3 19.3 11.4 Paphos 17.7 19.3 9.0

Spunta Holland 15.7 17.6 12.4 Palekhori 16.0 17.3 8.1 Paphos 16.1 17.8 10.4

Cara Ireland 15.8 18.1 14.4 Palekhori 16.7 18.3 10.1 Pa~hos 16.7 18.2 9.1

Handling of seed potatoes following storage The length of storage varied according to season and site at which

the seed was produced. Seed from Paphos was stored for 5f - 6 months, from Palekhori 5 - 5f months and from Agros 43 - 5 months. The cost of cold storage may represent 30 - 35% of total cost of the seed; there- fore work was initiated to find out how long before planting could seed potatoes be removed from storage in order to reduce this cost. The results are shown in Figs. 2 and 3. As the time lapse between removal from cold storage and planting increased beyond three weeks final yields of Cara, Spunta and Arran Banner decreased. This can be attributed to differences in physiological ageing of the seed. As suggested by O'Brien and Allen (1978) early growth (and tuber yield) may be related to a scale of accumulated day-degrees above a base temperature experienced by the seed from the onset of sprout growth to planting. Similar findings were reported by Wurr (1980) and other workers.

Although more detailed work on these aspects is in progress, it appears that the maximum saving in storage time, without adverse effects on yield is about 3-4 weeks. These results also show that presprouting of local seed is not required.

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a CARA

r SPUNTA . ARRAN BANNER

0 0 1 3 5 7 9

WEEKS A T AMBIENT FOLLO'NING STSACGE

Fig. 2. Effect at ageing of potato seed on yleld of ware potatoes. The seed was gmwn at Paphcs (1979) and the ware crop a1 Athalassa (spring 1980).

o CARA

r SPUNTA

., W

3 -

2 - 0 L 0 I 3 5 S

WEEKS AT ANBlENT FOLLOWING STORAGE

Fig. 3. Effect of ageing of potato seed on yield of ware potatoes. The seed was grown at Agros (1979) and the ware crop at Athalassa (spring 1980).

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CONCLUSIONS AND RECOMMENDATIONS

Work carried out over a period of 5 years has shown that under Cyprus conditions the production of good quality seed potatoes is both feasible and advantageous. From the results presented in this paper the following conclusions could be drawn:

a. Locally produced seed performed equally satisfactorily as imported seed; ware crops grown from local seed were as healthy as crops grown from imported seed, and gave similar or, in some cases, better yields.

b. The physiological state of local seed at planting time appears to be such as to allow the production of early crops with satisfactory yields. This is very important to Cyprus because of the necessity to grow early potatoes for export.

c. The grade of the initial stock used for multiplication in Cyprus did not significantly affect the subsequent performance of the seed. For a single multiplication, management (roguing, preventive sprays, time of defoliation) seems to be more important than the grade of the mother stock. Both certified (A) seed and foundation (FIS) seed can be used for seed production in Cyprus, depending on the avail- ability and cost of each type.

d. Both Paphos (low altitude) and Pitsilia (high altitude) pro- duced equally good seed. Healthiness of ware crops produced from seed grown at either site was similar and no significant differences in yield or earliness were observed. Incidence of virus diseases in ware crops was much below accepted tolerances irrespective of the site of seed production.

e. Yields of seed potatoes for Spunta and Arran Banner were comparable a t both sites, but those of Cara were lower at Pitsilia than at Paphos. This was due to the fact that Cara when planted late (March - April) has virogorous growth but restricted tuherization. The quality of seed, however, was equal at both sites.

f . Following storage at 2-3'C, for 4.5-6 months, seed can be removed to ambient for 3-4 weeks before planting without adverse effects on yield. Removal of seed for more than 4 weeks before planting results in decreased yields.

I t appears that under Cyprus conditions the best approach to produce part of the requirements of seed potatoes is at present a single inultiplication of imported stocks (certified A or FS). This can result in a significant reduction of seed imports and to considerable savings in foreign exchange. With regard to site of production, both Paphos and Pitsilia have certain advantages and disadvantages. At Paphos the cost of production is lower than at Pitsilia due to large scale operations and mechanization; however, storage costs are higher

14

because of the longer time the seed has to be kept before planting; the opposite is true for Pitsilia. In 1980, cost of locally produced seed was lower compared to that of imported seed of Cara and Spunta. Cost of local seed of Arran Banner was similar to that of imported seed. Although cost of seed is an important consideration in potato seed production it must be pointed out that production of local seed will minimize the dependency on foreign suppliers and will save foreign exchange.

Further work is required on specific aspects of seed production including detailed studies of the factors affecting the physiological quality and health of the seed. Seed certification should be an integral part of any sound seed production pragramme. Adoption of regulations and standards for inspection and seed certification to meet local requirement is also necessary. Sites of production must be defined. In the long term, production of basic seed must be considered, depend- ing on the success and development of seed production as suggested by this work.

ACKNOWLEDGEMENTS

The author greatfully acknowledges the technical assistance of N. Onoufriou, Th. Demosthenous and S. Gregoriou in field and labo- ratory work. The cooperation of the Cyprus Potato Marketing Board is also acknowledged.

REFERENCES

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Bodlaender, K.B.A. 1973. Influence of telnperature during growth of reed tubers and their seed value. Proceeding 5th Triennial Conference, European Association for Potato Research.

Bokx. JA. de, and J.C. Mooi. 1974. Methods of quality assessment of seed potatoes. Potato Research, 17, 410--33.

Burton, W.G. 1966. T h e Potato. H. Yeenman 8i %onen NT', Waeenineerl. 382 p. .. .. Hutchinson, R.W. 1978a. The dormancy of seed potatoes. I , The effect of tilue of haulrn

destruction and harvesting. Potato Research, 21, 23-65, Hotchinson, R.W. 1978b. The dorlnanq of seed patalaes. 2. The eifect of storage temperature.

Potato Research, 21, 267-75. Iritani, 1 V . M . ISF8a. Factors affecting physiological ageing (degeneration) of potato tubrrs

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Burhank seed. American Potato Journal, 45, 322-26. Karrakami, K. 1963. Age of potato seed tubers affects growth and yield. Ameriw Potato

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O'Brien, P.J. and J.E. Allen. 1978. The relationship het~reen storagc temperature OF seed tubers and tuber yields. Al,strarts of the 7th Triennial Conference, European Association far Potato Rercarch.

Prodfoot, K.G. and J. hlcCallurn. 1961. Secd potato production in Northern Ireland. E t ~ r n pean Pataw Journal, 4, 330-40.

Reestman, A.J. 1970. Importance of the degree of virus infestation for the production o I ware potatoes. Potato Research, 13, 248-68.

Reestman, .4.J. 1972. Incidence of infection In commercial crops and consequent losses. In Virus diseases and seed potato production. (rd. Bokx, J.A. dele pp 152-157, PUnOC. \Vageningen. - -

Shepard. J.F. and L.F. Chaflin. 1975. Critical analyws of the principles of seed certification. Annual Review of Phytopathology, 13, 251-99.

Staikov. G. and Ch. Cha~daro?. 1971. Rasteniev dni Nauki, 8, 45-53.

Toosey. R.D. 1964. The pre-sprollting of sen1 potatoes. Factors affecting sprout gronrth and stzbsequent development. Field Cmp Abstracts, 17, 161-8.

h a g , D.E. van der. 1972. Dutch techniques oE vow in^ secd potatoes.. In Vims diseases and seed potato production. (ed. Bokx, J.A. d c ) pp 188-205. PUDOC, Wageningen.

Wurr, D.C.E. 1980. Physiological quality of potato seed tubers. ADAS Quatcrly Review, 36, 27-39.

, C E 1978. Seed tuber production and management. In T h e potato mop: the scientific hasis for "npro>oncnt. (ed. Harris, P.14.). pp 327-54. Lonrlon: Chaprrtan

and Hall.