Solanum demissum P.I. 230579, a True seed diagnostic host for potato virus Y

Seedlings ofSolarium demissum P.I. 230579 in the 5-to 10-leaf stage growing under 18-hour days and at 20 to 24°C developed dark, irregular, slightly elongated local lesions 3 to 5 days following inoculation with any one of three strains of potato virus Y (PVY). Local lesions did not develop after similar inoculations with any one of two or more strains each of potato viruses A, M, S, X, or spindle tuber and a single strain each of potato calico and potato yellow dwarf. Inoculations from diseased specimens infected with PVY plus potato viruses A, M, S, and X, singly or in some combinations did not affect the efficiency of P.I. 230579 in detecting PVY in the mixed infections. Plants of P.I. 230579 developed significant numbers of local lesions from PVY inoculations at temperatures from 16 to 27°C and at inoculum dilutions through 1:100. Excised individual leaves of P.I. 230579 can be used to detect PVY. Leaves of plants exposed to air pollutants are unsuitable for assay purposes.S. demissum P.I. 230579 is homozygous for the local reaction to isolates of PVY and is a valuable aid for indexing aphid or mechanically inoculated potato clones or seedlings for resistance to the virus. It is a superior diagnostic host for differentiating PVY from other viruses commonly found in potatoes in the United States.     

The effect of metribuzin interaction with potato viruses X and Y on potato foliage,yield and grade

Foliar treatment of potato (Solanum tuberosum L.) with metribuzin at 0.57–1.0 lb/A caused a necrotic reaction in leaflets of plants infected with potato virus Y (PVY), but not in plants infected with potato virus X (PVX) or potato leafroll virus. Necrosis symptoms resulting from metribuzin-PVY interaction were distinct from symptoms of either PVY infection or of metribuzin injury. This reaction was similar in the Russet Burbank, Lemhi Russet, and Pioneer cultivars. Russet Burbank infected with PVY and PVX, alone or in combination, was treated with metribuzin to study herbicidevirus interaction effects on yield. Secondary PVY infection alone caused a 57% yield reduction, and when combined with PVX caused a 71% yield reduction. Although leaflet necrosis was induced by the metribuzin-PVY combination, there was no significant yield interaction. Results suggest that post-emergence application of metribuzin can be used as an aid for detecting and removing potato virus Y infected plants from potato seed fields     

Maintenance,symptoms and distribution of potato viruses X,S, Y,A, M and leafroll in potato tissue culture plantlets

Maintaining potato viruses X, S, Y, A, M and leafroll in tissue culture plantlets is a convenient, cost and space effective alternative to the use of greenhouse plants. Of these six viruses, only certain strains of PVX induced symptoms in tissue culture plantlets. Nevertheless, all infected tissue culture plants were found to be more reliable than greenhouse grown plants as virus-infected controls in enzyme-linked immunosorbent assay (ELISA) testing. Another advantage of maintaining viruses in tissue culture plantlets was the elimination of contamination by other viruses or other pathogens. Leaves, stems, and roots of virus infected plantlets were tested separately for antigen levels by ELISA. In these tests, the stems and leaves of all but PVA infected tissue culture plants consistently gave positive ELISA values. In contrast, root tissue from PVY infected tissue culture plantlets was not reliable for PVY detection. In all cases, the viruses detected in the original source material were detected in the resulting tissue culture plantlets.     

Occurrence and Distribution of Potato Pests and Diseases in Kenya

Potato plays an important role in food security in Kenya but yields are low (<10 t/ha), and this is partly attributed to the lack of healthy planting material. This study is the first wide-scale survey to determine the occurrence and distribution of common potato pests and diseases in Kenyan seed (certified and quality declared) and ware crops. Potato crops growing on 101 farms in 21 districts were examined. Approximately 36% of plants in farmers’ fields sampled both during the long rains (main potato-growing season) and short rains seasons displayed virus-like disease symptoms. Six viruses (potato leafroll virus (PLRV), Potato virus A (PVA), potato virus M (PVM), potato virus S (PVS), potato virus X (PVX), and potato virus Y (PVY)) were detected using double antibody sandwich enzyme-linked immunosorbent assay in potato samples. Sequencing of polymerase chain reaction products from PVY-infected plants revealed the presence of recombinant strains of PVY (NTN and Wilga). Four aphid species, Macrosiphum euphorbiae, Aphis gossypii, Myzus persicae, and Aphis fabae, colonized potato in all districts, occurring in greater numbers west of the Great Rift Valley than to the east. There was a positive correlation between virus incidence and aphid numbers in the long rains (main) potato-growing season. PLRV, PVM, PVS, PVX, and PVY were detected in solanaceous weeds. Ralstonia solanacearum was detected in soils from 13 farms in 8 of the 18 districts surveyed. Approximately 38% of soil samples were infested with Meloidogyne spp. Phytophthora infestans isolates belonging to the US 1 and 2_A1 genotypes were identified. Although many economically important diseases are present in Kenya, the lower aphid incidence in districts east of the Great Rift Valley may indicate that these districts are more suitable for seed potato production.     

Use of the virus strain group concept to characterize the resistance to PVX and PVYo in the potato cv Allegany

Resistance to the potato viruses X (PVX), Y (PVY) and A (PVA) was examined in the potato cv Allegany released by Cornell University in 1990. Standard potato cultivars from North America and Europe were included for characterization of the resistance response. Allegany was extremely resistant to a strain group 3 isolate of PVX and reacted with hypersensitivity to an ordinary strain isolate of PVY (PVY^o). However, Allegany was susceptible to an isolate of PVY causing necrosis in tobacco (PVY^N), and to an isolate of PVA. No symptoms appeared following infection with PVY^N and PVA. Identification of existing virus strain groups in a geographic area is an important aspect of predicting cultivar response to inoculation in the field.     

Evaluation of the enzyme-amplified ELISA for the detection of potato viruses A,M, S,X, Y,and leafroll

Various parameters,e.g. types of microtiter plate for DAS-ELISA (double antibody sandwich-enzyme-linked immunosorbent assay), use of fresh or frozen amplifier solutions for enzyme-amplified-ELISA, and use of sodium diethyldithiocarbamate (NaDIECA) in sample buffer in cocktail-ELISA were evaluated for the detection of potato viruses A, M, S, X, Y and leafroll from potato foliage. Dynatech Immulon immunoplates provided higher readings for all viruses. Fresh amplifier solution in amplifed-ELISA was superior to frozen solutions. Amplified ELISA gave only marginal improvement in the sensitivity over the standard DAS-ELISA. Addition of NaDIECA in sample buffer did not improve the detection of viruses in DAS-, amplified-, or cocktail-ELISA. Cocktail-ELISA can reduce antigen incubation time to as short as 15 min for PVA, PVM and PVX; 1 hr for PVY and PLRV; and 2–4 hr for PVS using pre-coated plates. Although amplified-ELISA is slightly more sensitive than DAS-ELISA for certain potato viruses, it is not suitable for large-scale indexing of potato viruses in Seed Certification Laboratories, in view of the additional steps needed in carrying out this procedure.     

Symptom development onA6 following inoculation with various viruses

Summary A6 plants were not susceptible to cucumber mild mosaic virus, pea early browning virus and potato mop-top virus. They produced local and systemic symptoms after inoculation with potato virus A (3 isolates), potato virus Y, potato virus X (4 isolates), potato aucuba mosaic virus (1 isolate), tobacco ringspot virus, tomato spotted wilt virus, cucumber mosaic virus (1 isolate) and only local necrotic symptoms after inoculation with potato virus A (2 isolates), tobacco mosaic virus, tobacco necrosis virus, tobacco rattle virus (3 isolates), tobacco etch virus and tomato black ring virus.A6 plants became systemically infected without symptoms after inoculation with potato aucuba mosaic virus (5 isolates), potato virus S, potato virus M and potato virus X (1 isolate). Detached leaves of those plants could be used reliably for the detection of potato virus A and potato virus Y.

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Zusammenfassung Es wurde festgestellt (K?hler, 1953; de Bokx, 1964; Spire et al., 1969), dass auch andere Viren als PVA und PVYA6 infizieren k?nnen. Eine unbekannte Infektion vonA6 k?nnte die Ergebnisse des für die Untersuchung auf PVA und PVY verwendetenA6-Testes beeinflussen. Aus diesem Grunde wurden gesundeA6-Pflanzen mit 20 Viren (und deren Isolaten) inokuliert, um herauszufinden, ob sie gepflückte oder nicht gepflückte Bl?tter vonA6-Pflanzen infizieren und Symptome ausl?sen k?nnen (Tabelle 1). Ferner wurde geprüft ob infizierte, symptomloseA6-Bl?tter für die Routine-Untersuchungen auf PVA und PVY verwendet werden k?nnen. Gepflückte Bl?tter vonA6-Pflanzen, die in einem blattlausfreien Glashaus gezogen wurden, undA6-Pflanzen, die in Klimakammern (20°C; Beleuchtung w?hrend 13 Stunden/Tag, Licht-intensit?t 16.000 lx) aufwuchsen, wurden nach Verletzung durch Karborundpulver 500 Mesh mit zerquetschten, virushaltigen Bl?ttern inokuliert. Das Vorkommen eines Virus sowohl in inokulierten als auch in nicht inokulierten Bl?ttern vonA6-Pflanzen wurde serologisch oder mit Testpflanzen untersucht (Tabelle 1). A6-Pflanzen waren nicht anf?llig für das Pea early browning-Virus, das Kartoffel Mop-top-Virus und das milde Gurkenmosaikvirus InA6 wurden generell lokale Infektionen verursacht durch CMV, PVA, TEV, TMV, TNV, TRV und TBRV (Tabelle 2; Abb. 1, 2, 3, 4, 5, 6, 7 und 8). Eine lokale und systemische Reaktion inA6 erfolgte nach Inokulation mit PAMV, PVX, PVY und TRSV (Tabelle 3, Abb. 9, 10). A6-Pflanzen wurden symptomlos, systemisch infiziert nach Inokulation mit den Isolaten von PVS und PVM.A6, inokuliert mit einem starken Stamm des AMV, zeigte systemische gelbe Flecken.A6-Pflanzen, systemisch infiziert mit AMV, PMV, PVS oder PVX, k?nnen als Testpflanzen für PVA und PVY (Abb. 11) verwendet werden; es ist aber zu empfehlen, nur gesundeA6-Pflanzen zu benützen, da infizierteA6-Pflanzen als Virusquelle wirken k?nnen. Die Ergebnisse sind in Tabelle 4 zusammengefasst.

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Résumé On a observé (K?hler, 1953; de Bokx, 1964; Spire et al., 1969) que d'autres virus que PVA et PVY pouvaient infecterA6. Une infection ignorée pourrait donc fausser les résultats du testA6 utilisé pour la détection de PVA et PVY. On a inoculé 20 virus (et leurs isolats) à des plantesA6 saines pour voir s'ils pouvaient infecter des feuilles détachées et non détachées de plantesA6 et induire des sympt?ms (Tableau 1). On a également recherché si des feuillesA6 infectées mais ne montrant pas de sympt?me pouvaient être utilisées dans les détections en série de PVA et PVY. On a inoculé avec du feuillage broyé porteur de virus, après poudrage avec de la poudre de carborundum de grosseur 500, des feuilles détachées de plantesA6 poussées en serre à l'abri d'aphides ainsi que des plantesA6 développées en chambre climatisée (20°C, éclairage durant 13 h par jour et intensité lumineuse de 16.000 lx).On a examiné sérologiquement et par plantes-tests (Tableau 1) la présence d'un virus à la fois sur feuilles inoculées et non inoculées de plantesA6. Les plantesA6 ne se sont pas montrées susceptibles au virus du brunissement précoce du pois, au virus mop-top de la pomme de terre, et au virus de la mosa?que légère de concombre. Généralement une infection locale surA6 est causée par CMV, PVA, TEV, TMV, TNV, TRV et TBRV (Tableau 2, Fig. 1–8). Une infection locale et systémique surA6 a suivi l'inoculation avec PAMV, PVX, PVY et TRSV (Tableau 3, Fig. 9, 10); l'inoculation des isolats PVS et PVM a provoqué une infection systémique sans sympt?me. L'inoculation d'un strain sévère de AMV a développé des taches jaunes systémiques. Les plantesA6 atteintes d'une infection systémique avec AMV, PVM, FVS et PVX peuvent être utilisées comme plantes-tests pour PVA et PVY (Fig. 11); il est toutefois recommandé de ne cultiver que desA6 sains, étant donné que lesA6 infectés pourraient agir comme source de virus. Les résultats sont résumés dans le Tableau 4.

     

湖南省马铃薯主产区马铃薯病毒种类及流行分析

马铃薯是世界第四大粮食作物,其病毒病危害严重。2010年对湖南马铃薯主产区采集的66个病毒标样进行了RT-PCR检测,结果表明,检测出的马铃薯病毒有马铃薯Y病毒(PVY)、马铃薯卷叶病毒(PLRV)、马铃薯X病毒(PVX)、马铃薯S病毒(PVS)、马铃薯A病毒(PVA)和马铃薯纺锤块茎类病毒(PSTVd)。其中PVS的检出率最高,为54.5%,其次是PVX,检出率为45.5%,PVY的检出率为39.4%,PSTVd和PVA的检出率均为21.2%,PLRV的检出率为18.2%。2~4种病毒的复合侵染现象较为普遍。PVY中重组型PVY占85.7%。     

Extreme resistance to infection by potato virus Y and potato virus X in an advanced hybridSolanum phureja — S. Stenotomum diploid potato population

Samples of 4285 individuals from a hybridSolanum phureja Juz. et Buk. —S. stenotomum Juz. (Phu-Stn) and 105 individuals ofSolanum phureja (Phu) diploid potato populations were twice inoculated with potato virus Y strain “o” (PVY°) using the air brush technique. After discarding seedlings with PVY visual symptoms both in the greenhouse and in the field, 1508 seedlings were judged to be resistant to PVY° (35.2%). Only 5.7% of the Phu seedlings were PVY° resistant compared to 35.2% of the Phu-Stn seedlings. At harvest, a mild selection pressure for yield and tuber appearance was applied and 602 Phu-Stn clones were chosen for further evaluation. Selected clones were re-evaluated for PVY° resistance in the greenhouse. Clones were mechanically inoculated twice with PVY°. Clones susceptible to PVY° were determined by visual observations, ELISA (Enzyme-linked immunosorbent assay) tests, grafting of tobacco PVY° infected shoots on potato clones, and infectivity tests usingNicotiana tabacum as a PVY° plant indicator. In the process, 224 clones were found to be extreme resistant to infection by PVY°, with an overall frequency for PVY° extreme resistance of 5.2%. In another experiment, the first year Phu-Stn PVY° resistant clones (554 clones) were mechanically inoculated twice with “common” strain of potato virus X (PVX). Similarly, we discarded clones susceptible to PVX by combining visual evaluation and ELISA with PVX re-inoculation of negative clones and an infectivity test, usingGomphrena globosa as a PVX plant indicator. After this process, seven extreme resistant and eight resistant clones to infection by PVX were found; the overall frequency of PVX extreme resistance was 1.3%. Five clones were extreme resistant to both PVY° and PVX.     

应用RT-PCR技术检测马铃薯A病毒

参考GenBank中马铃薯A病毒(potato virus A,PVA)的保守序列,利用Primer6.0引物设计软件设计并合成了一对特异性引物PVAF、PVAR,以此引物利用RT-PCR方法对PVA保守序列基因进行了特异性扩增。结果表明:引物PVAF、PVAR能从已知的感染PVA病毒的植株中扩增出834bp的cDNA特异性片段;该RT-PCR的检测灵敏度为1pg的病毒核酸,特异性强,重复性好,可用于PVA病毒的快速检测。     

Elimination of viruses and hypersensitivity to potato virus Y (PVYo) in an important sudanese potato stock (Zalinge)

Viruses that infect naturally an important Sudanese potato stock Zalinge were detected using enzyme-linked immunosorbent assay, immunosorbent electron microscopy and sap-inoculation to test plants. All of the 19 plants of Zalinge tested were infected with potato leaf roll virus (PLRV) and potato virus S (P VS), and five plants also with potato virus X (PVX). No potato virus Y (PVY), A (PVA) nor M (PVM) were found. The viruses were eradicated with thermo and chemotherapy using standard procedures. The combination of both therapies did not result in any virus-free plants, but resulted in poor plant survival. Thermotherapy reduced the incidence of PLRV and PVS by 45% and 50%, respectively, and one virus-free plant was obtained. It grew vigorously in the greenhouse, was symptomless and had a significantly increased tuber yield compared to the virus-infected plants. Following sap-inoculation with PVY^O, Zalinge showed mosaic symptoms, developed necrosis in the leaves and stem and died 14 days post-inoculation. However, the plants of Zalinge infected with PVY^N remained symptomless, which suggested that hypersensitivity was specific to PVY^O. The fast development of lethal necrosis following infection with PVY^O may contribute to the low incidence of PVY in Zalinge in the field in Sudan.     

An hypothesis for selection of strains of potato leaf roll virus by passage through Physalis floridana

Symptomatically distinct isolates of potato leaf roll virus were transmitted from White Rose to Russet Burbank potato by aphids and by grafting, from White Rose potato toP. floridana by aphids, and fromP. floridana toP. floridana by aphids. Isolates from White Rose potato generally produced stable and characteristic secondary leaf roll symptoms (but not primary leaf roll symptoms) when transmitted to Russet Burbank potato but variable symptoms when transmitted toP. floridana. Isolates from two of the latter plants with moderate and severe symptoms generally incited stable and characteristic symptoms when transmitted to otherP. floridana plants. The stable symptoms produced by each of these isolates were not modified by infestingP. floridana plants with varying numbers of viruliferous aphids. The results obtained in this study corroborate previous evidence for the existence of strains of potato leaf roll virus. It is suggested that certain White Rose potato plants contained mixtures of leaf roll virus strains. Furthermore, it is postulated that these mixtures were at least partially separated by “natural” selection following transmission toP. floridana. This mechanism provides an explanation for the stable symptoms produced by isolates transmitted fromP. floridana toP. floridana.     

Biological factors influencing the transmission of potato leafroll virus by different aphid species

Summary Macrosiphum euphorbiae, collected in the field from potato plants infected with potato leafroll virus (PLRV), transmitted the virus to fewer potato plants in a field trial than did laboratory-rearedMyzus persicae. In the laboratory,M. persicae was the only efficient vector of PLRV fromPhysalis floridana seedlings, potato sprouts or excised leaves toP. floridana. Two clones ofM. euphorbiae and one clone ofAulacorthum solani transmitted PLRV from infected potato plants toNicotiana clevelandii as effeciently asM. persicae but a clone ofAphis gossypii was an inefficient PLRV vector. An isolate of PLRV, whichM. persicae transmitted inefficiently from potato toN. clevelandii, was also transmitted inefficiently byM. euphorbiae andA. solani.     

Solanum microdontum (PI 558098): A diagnostic host plant for potato virus A

Solanum microdontum (PI 558098) develops diagnostic symptoms when inoculated with plant sap containing potato virus A (PVA). The symptoms consist of local lesions (2–4 mm), followed by systemic necrosis of leaf veins, bronzing of leaf surface, and leaf drop. Symptoms develop in a temperature range of 20–29 C with 4 to 8 Klux light intensity of 14 h daylength. Local lesions are visible within 1 wk of inoculation and can be caused by PVA containing sap dilutions of up to 1:100. Plants multiplied by shoot cuttings as well as those produced from true potato seed are equally sensitive. True potato seed production occurs under normal greenhouse conditions in cross-pollinated plants.     

Evaluation of tuber-bearingSolanum species for symptomology,as diagnostic hosts,and sources of immunityto potato virus Y necrotic strain (PVYN)

A total of 258 Plant Introductions (PI) belonging to 69Solarium species were evaluated in the greenhouse for their reaction to the tobacco veinal necrosis strain of potato virus Y (PVY N). One hundred and thirty-one (50.7%) of the PI accessions produced mosaic symptoms ranging from mild to severe. Local lesion and veinal necrosis symptoms were observed in 19 PI accessions (7.3%) and a variety of other symptoms were observed in another 11 PI accessions (4.2%). Only 97 PI accessions (37.5%) were symptomless carriers of PVY^N. PI accession 473505 ofS. sparsipilum and PI accession 498021 ofS. brachycarpum developed local lesions and veinal necrosis with PVY^N, but necrotic spots and mosaic with PVY^o. Common mechanically-transmitted potato viruses A, S, M, and X did not interfere with PVY symptom development inS. sparsipilum andS. brachycarpum. Thus, PI 473505 and PI 498021 can be used as indicator plants for specific identification of PVY^N. PI accession 472819 ofS. chacoense developed local lesions with systemic spread in PVY^o, but without systemic spread in PVY^N. Thus, this can be used as a differential host plant for PVY strains. Two PI accessions ofS. stoloniferum, PI 160372 and 161171 were immune to PVY^N.     

Solanum berthaultii P.I. 265858; a possible source of protection against all strains of PVX

Summary A clone ofSolanum berthaultii P.I. 265858 was not systemically infected when manually inoculated with isolates of PVX groups 1 to 3 but developed top-necrosis when grafted-inoculated, demonstrating it is hypersensitive. Plants sap- or graft-inoculated with an isolated of PVX group 4 or with PVX_HB (a recently-discovered strain overcoming all reported sources of resistance to PVX) were infected systematically. The plants were extremely sensitive to these isolates, young leaves becoming necrotic and the plant dying within a few weeks and, whilst still alive, plants contained little virus and were a poor source of infection. These characteristics suggest that this clone could provide a means of breeding cultivars that would be protected against all known strains of PVX.     

Population Growth of Myzus persicae on Potato Plants Infected with Different Strains and Variants of Potato virus Y

Potato virus Y (PVY) is one of the most economically important viruses affecting the potato crop. Several strains of the virus, including PVY^O, PVY^N, recombinant isolates; PVY^N:O (PVY ^N-Wi) and PVY^NTN and several variants of PVY^O have been reported from North American potato-production areas. The green peach aphid, Myzus persicae Sulzer, is a colonizer of potatoes and is considered the most important vector of PVY. The objective of this study was to measure the population growth of M. persicae on potato plants infected with different strains and genetic variants of PVY. The initial population of ten winged adults of M. persicae was allowed to develop on a potted plant for 12 days. Results clearly indicated that infections by different strains and genetic variants of PVY did not influence the population growth of M. persicae on potato plants during this period.     

Improved control of potato virus Y by mineral oil plus the pyrethroid cypermethrin applied electrostatically

Field potato plants sprayed with a mixture of a pyrethroid (cypermethrin at 40 g a.i./ha) and a paraffinic oil (Sunoco Sunspray 7E at 7 $\text{l}$/ha) were colonized by fewer aphids and were less frequently diseased by potato virus Y (PVY) than unsprayed plants or plants sprayed with either cypermethrin or the oil alone; aphids given access to leaves of PVY-infected field plants sprayed with the mixture were also less likely to transmit PVY to laboratory test plants than aphids given access to leaves of plants sprayed with either chemical alone, or to untreated leaves. When alate aphids were confined above virus-infected and healthy plants, combined oil and cypermethrin treatment also gave greater protection against virus inoculation than either chemical alone. Electrostatic charging of spray droplets increased chemical deposits on field plants, especially when the plants were small and on upper leaves. These greater deposits improved control of PVY by the oil and the cypermethrin, individually or in combination, and enhanced the aphicidal action of the cypermethrin.     

A Reexamination of the Effectiveness of Ribavirin on Eradication of Viruses in Potato Plantlets in vitro Using ELISA and Quantitative RT-PCR

The potato plantlets singly infected by PVA, PLRV, PVS, PVX and PVY and mix-infected by PVM, PVS and PVY were cultured on MS medium with different concentration of ribavirin. The effects of ribavirin on growth of the plantlets and efficiency of virus elimination were investigated. Results showed that the plant height and fresh weight obviously decreased with increase of ribavirin concentration from 0 mg/L to 150 mg/L, and most of the plantlets could not survive when the concentration reached 200 mg/L. According to the ELISA tests, ribavirin was more efficient for eradicating PVA, PVM, PVS and PVX than PVY and PLRV, and healthy plantlets could be obtained with high frequency (up to 100 %) by culturing with 75?~?150 mg/L ribavirin after 2?~?3 subcultures. Whereas, only 33?~?66 % PVY and PLRV infected plantlets were found to be virus-free after 3 subcultures with 75?~?150 mg/L ribavirin. The results of quantitative RT-PCR (qPCR) indicated that ribavirin could obviously reduce virus content in the plantlets. Except PLRV was detected positive after 3 subcultures with ribavirin, the healthy seedlings were obtained from infected stocks at the first or end of propagation and no viruses could be detected at the post-eradication stage. No apparent difference of genetic variation resulted from ribavirin treatment was found by SSR analysis between the control and the treated plantlets. All of these results above proved that ribavirin treatment in vitro was an effective method to eliminate viruses in the propagation of potato.