Subscribe Now Subscribe Today
Research Article
 

Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya



J.W. Muthomi, J.N. Nyaga, F.M. Olubayo, J.H. Nderitu, J.N. Kabira, S.M. Kiretai, J.A. Aura and M. Wakahiu
 
Facebook Twitter Digg Reddit Linkedin StumbleUpon E-mail
ABSTRACT

Field studies were carried out in farmer-based seed potato production to determine the incidence of potato aphids and potato aphid-transmitted viruses in two potato-producing areas of Kenya. Parameters determined included aphid population, virus disease incidence and tuber yield. Aphid population was monitored on leaves and in water-pan traps. Virus infection was determined based on symptoms and the viruses were identified in tubers sprouts by DAS-ELISA. Tuber yield was determined for plants showing virus symptoms and healthy-looking plants. Five aphid species were identified, with the most abundant being M. euphorbiae and A. gossypii on leaves and M. persicae and A. gossypii in water traps. The average aphid population was between 1.4 and 4.2 aphids per three leaves and 4.68 and 9.64 aphids per water pan trap. Farms with higher population of M. persicae had higher virus disease incidence. The most prevalent viruses were PVS, PLRV and PVM. Healthy looking plants had a latent infection rate 57.2% compared to 76.6% for symptomatic plants. Virus infection reduced the number and weight of tubers by 74 and 62.7%, respectively. However, virus infection increased the number and weight of the chats grade. The results indicated that aphid infestation and virus disease incidence were higher than the recommended for seed potato production. Therefore, there is need to create awareness among the farmers on aphid and virus symptom recognition and use of clean certified seed potato.

Services
Related Articles in ASCI
Search in Google Scholar
View Citation
Report Citation

 
  How to cite this article:

J.W. Muthomi, J.N. Nyaga, F.M. Olubayo, J.H. Nderitu, J.N. Kabira, S.M. Kiretai, J.A. Aura and M. Wakahiu, 2009. Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya. Asian Journal of Plant Sciences, 8: 166-171.

DOI: 10.3923/ajps.2009.166.171

URL: https://scialert.net/abstract/?doi=ajps.2009.166.171
 

INTRODUCTION

Potato (Solanum tuberosum L.) is the second most important food crop after maize in Kenya (Ministry of Agriculture, 2006). The national average yield is about 9.1 t ha-1 compared to 40 t ha-1 under research conditions (Ministry of Agriculture, 2007a; Kabira et al., 2006). The low productivity has been attributed to low quality seeds and diseases, especially bacterial wilt and viruses in smallholder farms. The informal sub sector contributes 99% of the seed potato in Kenya. This sector involves seed potato production without going through the certification processes and it includes unregistered growers and suppliers of seed mainly in their immediate localities (Kenya Agricultural Research Institute, 2006). It encompasses seed potato production with involvement of Non-Governmental Organizations (NGOs) and Community Based Organizations (CBOs), seed private growers and individual farmers through seed plot technologies and positive selection. However, the seed potato production rules require that the proportion of plants infected with virus, bacterial wilt and nematodes should not exceed 10, 3 and 3%, respectively (Ministry of Agriculture, 2006).

The low potato productivity is partially attributed to high losses due to pests and diseases, lack of adequate quantities of healthy planting materials and inadequate use of farm inputs (Ministry of Agriculture, 2006). Seed potato of most popular potato varieties has degenerated and need to be cleaned, multiplied and distributed to the industry. Due to lack of adequate and affordable certified seed potato, farmers generally recycle seed from previous harvests. This leads to further spread of tuber borne viruses and other diseases. About 40 viruses have been reported to affect potato (Bostan et al., 2006). Most of these viruses, such as potato leaf roll virus, potato virus X and potato virus Yare spread by aphids and tubers. The potato spindle tuber viroid is transmitted by aphids only together with potato leafroll virus. The green peach aphid (Myzus persicae) is the most efficient vector in transmission of virus diseases and is known to transmit over 100 viruses in different plants (Braedle, 2006; Ming et al., 2007; Raman, 1985). However, the capacity of farmers to recognize and manage aphids and potato virus diseases is very low (International Potato Centre, 2006; Kabira et al., 2006; Kibaru, 2003; Machangi, 2003). This lack of awareness about the damage caused by aphids and aphid-transmitted virus diseases in the informal seed potato production system contributes to seed degeneration and low yields.

The study was carried out to determine the levels of aphids and viruses in farmer-based seed potato production system and the associated yield losses.

MATERIALS AND METHODS

Experimental site and design: The study was carried out on smallholder farmers’ fields in Njabini, Nyadarua south and Limuru, Kiambu west districts of Kenya over two seasons between November 2006 and February 2008. It targeted individual farmers or farmer groups involved in seed potato production. Four seed-potato producing farms were selected in each district. The experimental design was randomized complete block design. In each farm, 0.25 ha potato plot was selected and divided into four equal portions, which acted as replicates while the farm acted as a block.

Assessment of aphid population: Aphid population was monitored on leaves and by use of yellow water pan traps. Leaves were used to monitor wingless (apteral) aphids while the yellow water pan traps were used to monitor winged (alate) aphids. Sampling of leaves was done weekly from second week after emergence to maturity. Ten potato plants were randomly selected from each plot and three leaves were picked from top, middle and bottom of each plant. The leaves from each plant were put in separate labelled paper bags and kept in cool box until aphids were counted and identified. The yellow water pan traps consisted of yellow basins 3/4 filled with water and a few drops of liquid detergent added to break surface tension to make the trapped insects sink to the bottom. Five water pan traps were placed in each 0.25 ha plot. The traps were replaced weekly and aphid counts taken. The aphids were counted and identified to species level under a stereo telescopic dissecting microscope (x40 magnification). Aphid identification was based on aphids colour in life, lateral abdominal spiracles, antennal tubercles, shape of siphunculi and dorsal abdominal pigmentation.

Assessment of virus disease and effect on tuber yield: In each 0.25 ha portion of the farm, 100 plants were selected at random and observed for virus symptoms. Incidence of virus infection was determined as the proportion of plants showing leaf roll and mosaic symptoms. Virus incidence was determined weekly from the second week after emergence to crop maturity. At harvest, the tubers were tested for presence of potato leaf roll virus (PLRV), potato virus X (PVX), potato virus S (PVS), potato virus M (PVM), potato virus Y (PVY) and potato virus A (PVA) by double antibody sandwich-enzyme linked immunosorbent assay (DAS-ELISA) method as described by International Potato Centre (2007) and Clark and Adams (1977).

At flowering 10 healthy-looking and 10 symptomatic plants were tagged in each 1/4 portion of the 0.25 ha potato plot such that a total of 40 healthy-looking and 40 symptomatic plants were tagged. At maturity, tubers from each plant were harvested separately and graded into ware (>55 mm), seed (55-25 mm) and chats ( < 25 mm). The number and weight of tubers in each grade were determined and percentage yield reduction calculated as the percentage difference between the number or weight of tubers from the healthy-looking and the symptomatic plants.

Data analysis: All data were subjected to analysis of variance (ANOVA) using Genstat software and differences among the treatment means compared using Fisher’s Protected LSD test at 5% probability level.

RESULTS

Aphid species identified were Macrosiphum euphorbiae, Aphis gossypii, Aphis fabae, Myzus persicae and Rhopalosiphum maidis. The most abundant species on leaves were A. gossypii and M. euphorbiae while the least abundant was R. maidis (Table 1). The most abundant species in water pan traps were M. euphorbiae and M. persicae in Njabini but higher populations of A. gossypii and R. maydis were caught in Limuru during both growing seasons. However, high levels of A. fabae were traped in water pan traps in Limuru only during the short rain season. The farms differed in population and distribution of different aphid species. Aphid population was higher and more widely distributed in Limuru than in Njabini. However, higher numbers of Myzus persicae were caught in water traps in Njabini than in Limuru. But more of this species was found on potato leaves in Limuru than in Njabini. The aphid population on leaves generally increased with growth of the crop, then decreased towards maturity, with peak population being between 6th to 8th weeks after crop emergence (Table 2). There was a higher total aphid population during the short rain season compared to the long rain season.

Table 1: Mean population of different aphid species per 3 leaves and per water pan trap during short and long rain seasons in two potato-producing areas of Kenya
Image for - Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya
Me = M. euphorbiae, Ag = A. gossypii, Af = A. fabae, Mp = M. persicae, Rm = R. maidis

Table 2: Mean aphid population trend per 3 leaves and per water pan trap during short and long rain seasons in two potato-producing areas of Kenya
Image for - Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya
ns: Non significant

Image for - Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya
Fig. 1: Mean percent virus disease incidence on potato plants in Njabini and Limuru, Kenya, during the short and long rain seasons

Potato virus incidence was found at higher levels (2-30%) in Njabini than in Limuru (10-21%) (Fig. 1). Farms with higher population of M. persicae had higher virus disease incidences. Potato viruses detected in tubers were potato leaf roll virus (PLRV), potato virus M (PVM), potato virus X (PVX), potato virus Y (PVY), potato virus S (PVS) and potato virus A (PVA). The most prevalent virus was PVS (100%) followed by PLRV (92.5%) and PVM (90.3%) while the least prevalent was PVY (16.9%) (Fig. 2a, b). Healthy-looking plants had a lower incidence of latent infection of 57.2% compared to 76.6% for plants showing virus symptoms. A higher virus load was detected in tubers during the short rains and tubers from Njabini had higher virus titre in tubers from both the healthy-looking and symptomatic compared to Limuru (Fig. 2).

Image for - Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya
Image for - Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya
Fig. 2: Types and amounts of viruses infecting healthy looking and symptomatic potato plants in farmer-based seed potato production units in (a) Limuru and (b) Njabini, Kenya in the 2007 season

The viruses detected in highest amounts were PVS, PVM and PLRV over the two seasons. Virus infection reduced the number of tubers by between 9.6 and 35.5%. The number of tubers from healthy-looking plants was significantly (p ≤ 0.05) higher than from symptomatic plants during the two seasons (Table 3). In Njabini, the reduction in number of potato tubers was by 9.6 and 29.7% during the short and long rains, respectively, while in Limuru, reduction was by 17.7 and 35.5% during the long and short rains, respectively (Table 3). The reduction in the number of tubers was also significant (p ≤ 0.05) among the potato grades during the two seasons. The reduction in number of tubers was greatest (46-74%) for the ware grade but the symptomatic plants produced higher number of chats compared to the healthy-looking plants. Tuber weight was significantly (p ≤ 0.05) reduced by between 36.4 and 62.7% (Table 4). The highest weight reduction was in the ware grade at 65 and 47% in Njabini and 62.7 and 36.8% in Limuru during the short and long rains, respectively. However, symptomatic plants had higher weight of chats grade during the two seasons.

Table 3: Mean number of tubers per 10 plants for different grades harvested from healthy-looking and symptomatic potato plants during short and long rain seasons in two potato-producing areas of Kenya
Image for - Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya
Ware >55 mm, Seed 25-55 mm, Chats < 25 mm, H: Healthy, S: Symptomatic, ns: Not significant

Table 4: Mean weight of tubers per 10 plants in kg for different potato grades harvested from healthy-looking and symptomatic plants during short and long rain seasons in two potato-producing areas of Kenya
Image for - Incidence of Aphid-Transmitted Viruses in Farmer-Based Seed Potato Production in Kenya
Ware > 55 mm; Seed 25-55 mm; Chats < 25 mm; H: Healthy; S: Symptomatic, ns: Not significant

DISCUSSION

Potato aphids were prevalent in small-scale potato farms, with an average of 1.4 and 4.2 aphids per three leaves in Njabini and Limuru, respectively. This translates to 46.6 and 140 aphids per 100 leaves, respectively, which is higher than the threshold of between 3 and 10 aphids per 100 leaves recommended for seed potato production (Capinera, 2001; Thomas, 2002). Among the five species identified, M. persicae is the most efficient vector of potato viruses (Bostan et al., 2006; Bunt, 2001; Braedle, 2006; Ming et al., 2007). The population of this species was higher during the short rains than during the long rains. This can be explained by the warm conditions that favour aphids during the short rain season (De Temmerman et al., 2002). Factors that affect aphid population include weather conditions, nutrition and presence of predators (Handizi and Legorburu, 2002). Temperatures of less than 17.8 °C greatly restrict the number of M. persicae (Radcliffe, 1982) while vegetation around potato plots play a critical role in aphid population dynamics (Handizi and Legorburu, 2002). Therefore, the differences in population of the different aphid species in the two potato-growing regions could be due to variation in types of other crops grown and the type of vegetation in the vicinity of potato fields.

The results indicate that aphid control measures, such as chemical spray and rouging of infected plants, are required to reduce virus spread in the seed potato (Struik and Wiersema, 1999; Thomas, 2002; Walingo et al., 2004). Peak aphid infestation was observed at the seventh and eighth week after crop emergence, which agrees with an earlier study by KARI (Kabira et al., 2006). Therefore, the control measures should start early in the season, preferably immediately after crop emergence by rouging infected plants. Chemical control is applied when aphid population reaches economic threshold of between three to ten aphids per 100 leaves irrespective of the stage of growth (Capinera, 2001; Thomas, 2002). Apterae aphids have been found to transmit more viruses into a potato plant than alate aphids (Yvon et al., 2000) and there is a positive correlation between aphid population and viral load in tubers suggesting that aphids are directly responsible for the virus contamination of the tubers (De Temmerman et al., 2002; Powell et al., 2006; Verheggen et al., 2007). Therefore, reducing aphid population would have a direct effect of reducing infection and increase potato yields (Walingo et al., 2004; Paola et al., 2005).

The study showed that the average virus disease incidence was between 12.4 and 17.7%, which is higher than the maximum threshold of 10% recommended in seed potato production (Machangi et al., 2004; Ministry of Agriculture, 2006; Nderitu and Mueke, 1986). Disease incidence is a measure of seed purity and is used in seed potato certification under the formal seed potato production system (Ministry of Agriculture, 2007b). The high incidence can be attributed to lack of aphid control measures by the farmers who have no capacity to recognize aphids and they are not aware of the dangers posed by aphids in seed potato production (Kabira et al., 2006). Recycling of seed due to unavailability of clean, certified seed is also common. Infected seed potato tubers transmit viruses to the germinating plants and therefore the virus load continues to increase in successive seasons until the yield is diminished (Ming et al., 2007; Paola et al., 2005; Syller, 2001). Healthy-looking potato plants were latently infected with viruses but the virus load was lower than for symptomatic plants. This is consistent with other studies done elsewhere (Birch et al., 1999; De Temmerman et al., 2002; Jonathan and Alison, 2008; Bostan et al., 2006). Therefore, farmers could be advised to rogue symptomatic plants early after crop emergence to leave healthier plants and thereby increase production (Kabira et al., 2006). Visual disease incidence was correlated to viral load of PVY and PLRV suggesting that cumulative vector intensity and disease incidence could be used to predict virus threat to seed potato (Basky, 2002).

Reduction in the number of tubers in symptomatic plants ranged between 9.6 and 35.5% while the reduction in tuber weight was between 36.8 and 62.7%. The results indicate that virus infection has a direct effect on potato yield and therefore the income the farmer would get from the crop (Khurana, 2000; Saucke and Doring, 2004). Virus infection reduced the number and weight of ware and seed grades but increased the yield of the chats grade. The chat grade has no economic benefit to the farmer both in terms of food or seed. This indicates that even with good agronomic practices, the farmers would not attain desired yields unless virus diseases management is incorporated in the production system (Robert and Bourdin, 2000; Hoffmann et al., 2001; De Temmerman et al., 2002; Kabira et al., 2006; Kenya Agricultural Research Institute, 2007; Yvon et al., 2000). Therefore, there is need to create awareness on importance of aphid management and use of certified seed in farmer-based seed potato production systems in Kenya.

ACKNOWLEDGMENT

This research was conducted with financial support from the Commission for Higher Education (CHE), Kenya (CHE 500-673-002).

REFERENCES

1:  Basky, Z., 2002. The relationship between aphid dynamics and two prominent potato viruses (PVY and PLRV) in seed potatoes in Hungary. Crop Protect. J., 21: 823-827.
CrossRef  |  

2:  Birch, A.N.E., I.E. Geoghegan, M.E.N. Majerus, J.W. McNicol, C.A. Hackett, A.M.R. Gatehouse and J.A. Gatehouse, 1999. Tri-trophic interactions involving pest aphids, predatory 2-spot ladybirds and transgenic potatoes expressing snowdrop lectin for aphid resistance. J. Mol. Breed., 5: 75-83.
CrossRef  |  Direct Link  |  

3:  Braedle, C., G.K. Davis, J.A. Brisson and D.L. Stern, 2006. Wing dimorphism in aphids. J. Heredity, 97: 192-199.
CrossRef  |  

4:  Bunt, A.A., 2001. The Main Viruses Infecting Potato Crops. In: Viruses and Virus-Like Diseases of Potatoes and Production of Seed Potatoes, Lobenstein, G., P.H. Berger, A.A. Brunt and R.H. Lawson (Eds.). Kluwer Academic Publishers, Dordrecht, pp: 65-67

5:  Capinera, J., 2001. Handbook of Vegetable Pests. 1st Edn., Academic Press, New York, ISBN-13: 978-0121588618, pp: 2700

6:  International Potato Centre (CIP), 2006. Select the Best: Positive Seed Potato Selections Field Aid. CIP, Sub-Saharan Africa region, Nairobi, Kenya

7:  International Potato Centre (CIP), 2007. DAS-ELISA Kit for Potato Viruses Detection Instruction Manual. CIP, Lima, Peru

8:  Clark, M.F. and A.N. Adams, 1977. Characteristics of the microplate method of enzyme-linked immunosorbent assay for the detection of plant viruses. J. Gen. Virol., 34: 475-483.
CrossRef  |  PubMed  |  Direct Link  |  

9:  De Temmerman, L., A. Hacour and M. Guns, 2002. Changing climate and potential impacts on potato yield and quality CHIP: Introduction, aims and methodology. Eur. J. Agron., 17: 233-242.
Direct Link  |  

10:  Handizi, J. and F.T. Legorburu, 2002. Escaping from potato virus Y: Aphid repellants and planting dates. Proceedings of the 8th International Plant Viruses Epidemiology Symposium: Aschersleben, May 12-17, Germany.

11:  Bostan, H., C. Guclu, E. Ozturk Isil Ozdemir and H. Ilbagi, 2006. Influence of aphids on the epidemiology of potato virus diseases (PVY, PVS and PLRV) in the high altitude areas of Turkey. Pak. J. Biol. Sci., 9: 759-765.
CrossRef  |  Direct Link  |  

12:  Hoffmann, K., M. Verbeek, A. Romano, A.M. Dullemans, J. van den Heuvel and F. van der Wilk, 2001. Mechanical transmission of poleroviruses. J. Virol. Methods, 91: 197-201.
Direct Link  |  

13:  Jonathan, R.L. and K.W. Allison, 2008. Transcomplementation and synergism in plants: Implications for viral transgenes. Mol. Plant Pathol., 9: 85-103.
CrossRef  |  

14:  Kabira, J.N, M. Wakahiu, W. Wagoire, P. Gildemacher and B. Lemaga, 2006. Guidelines for Production of Healthy Seed Potatoes in East and Central Africa. Kenya Agricultural Research Institute, Nairobi

15:  Kenya Agricultural Research Institute, 2007. Annual report 2006. National Potato Research Centre, Tigoni, Kenya.

16:  Khurana, S.M.P., 2000. Potato viruses: Detection and management African potato association. Proceedings of the 5th Triennial Congress of the African Potato Association, May 29-June 2, 2000, Kampala, Uganda, pp: 257-269

17:  Kibaru, A., 2003. Management of aphids and aphid transmitted viruses in seed potato production in Kenya. M.Sc. Thesis. University of Nairobi, Kenya.

18:  Machangi, J.M., 2003. Occurrence of viruses and aphid vectors in small-scale potato seed production systems and their effect on yield in Kenya. M.Sc. Thesis, University of Nairobi, Kenya.

19:  Machangi, J.M., F.M. Olubayo, R.W. Njeru, J.H. Nderitu, El-Badewy, D.M. Yobera and J.A. Aura, 2004. Occurrence of four major potato viruses in three main potato growing areas in Kenya. Proceedings of the 6th Triennial Conference of the African Potato Association, April 5-10, 2004, Agadir, Morocco, pp: 273-281

20:  Ming, G.F., C. Chun, S. Su-W, Y. Sheng, H.S. Zhi-C and C. Xue-X, 2007. Aphid dispersal flight disseminates fungal pathogens and parasitoids as natural control agents of aphids. J. Ecol. Entomol., 32: 97-104.
CrossRef  |  Direct Link  |  

21:  Ministry of Agriculture, 2006. Ministry of Agriculture-National Policy on Potato Industry: Policy and Reforms to Improve Production, Research, Marketing and Regulatory Framework. Government Press, Nairobi, Kenya

22:  Ministry of Agriculture, 2007. Annual reports 2005 and 2006. Ministry of Agriculture, Nairobi, Kenya.

23:  Ministry of Agriculture, 2007. The crop production and livestock (Seed and ware potato production and marketing standards) rules. Kenya Gazette Supplement No. 38, Legal Notice No. 44, 2005, Government Press, Nairobi, Kenya.

24:  Nderitu, J.H. and J.M. Mueke, 1986. Aphid infestation on eight potato cultivars (Solanum tuberosum L.) in Kenya. Int. J. Trop. Insect Sci., 7: 667-682.
CrossRef  |  Direct Link  |  

25:  Paola, I., E. Lahoz, F. Porrone, C.B.A. Carella and R. Contillo, 2005. Use of light mineral oil to reduce the severity of PVY and CMV infections on tobacco in Italy. J. Tobacco, 2: 57-61.
Direct Link  |  

26:  Powell, G., C.R. Tosh and J. Hardie, 2006. Host plant selection by aphids: Behavioral, evolutionary and applied perspectives. Annu. Rev. Entomol., 51: 309-330.
CrossRef  |  Direct Link  |  

27:  Radcliffe, E.F., 1982. Insect pests of potato. Annu. Rev. Entomol., 27: 173-204.
CrossRef  |  

28:  Raman, K.V., 1985. Transmission of Potato Viruses by Aphids International Potato Center (CIP) Technical Information Bulletin 2. 3rd Edn., Lima, Peru, pp: 23

29:  Robert, Y. and D. Bourdin, 2000. Aphid Transmission of Potato Viruses. In: Virus and Virus-Like Diseases of Potatoes and Production of Seed Potatoes, Lobenstein, G., P.H. Berger, A.A. Brunt and R.H. Lawson (Eds.). Kluwer Academic Publishers, Dordrecht, pp: 195-225

30:  Saucke, H. and T.F. Doring, 2004. Potato virus Y reduction by straw mulch in organic potatoes. Ann. Applied Biol. J., 144: 347-355.
Direct Link  |  

31:  Struik, P.C. and S.G. Wiersema, 1999. Seed and Potato Technology. Purdue University Press, The Netherlands, pp: 382

32:  Syller, M., 2001. Potato leafroll virus-assisted aphid transmission of potato spindle tuber viroid to potato leafroll virus-resistant potato. J. Phytopathol., 149: 195-201.
Direct Link  |  

33:  Thomas, C., 2002. Bug vs. Bug-Crop Scouting. Integrated Pest Management Programm. Pennsylvania Department of Agriculture, Harrisburg, Pensylvania

34:  Verheggen, F.J., Q. Fagel, S. Heuskin, G. Lognay, F. Francis and E. Haubruge, 2007. Electrophysiological and behavioral responses of the multicolored Asian lady beetle, Harmonia axyridis Pallas, to sesquiterpene semiochemicals. J. Chemical Ecol., 33: 2148-2155.
Direct Link  |  

35:  Walingo, A., C. Lungaho, N. Nganga, P.M. Kinyae and J.N. Kabira, 2004. Potato marketing, storage, processing and utilization in Kenya. Proceedings of the 6th Triennial Congress of the African Potato Association, April 5-10, 2004, Agadir, Morocco, pp: 389-403

36:  Robert, Y., J.A.T. Woodford and D.G. Ducray-Bourdin, 2000. Some epidemiological approaches to the control of aphid-borne virus diseases in seed potato crops in Northern Europe. J. Virus Res., 71: 33-47.
CrossRef  |  

©  2022 Science Alert. All Rights Reserved