Transmission of potato leafroll virus to or from tuber slices by aphids feeding through a membrane

Summary Eye-bearing slices, cut from healthy potato tubers and placed between Parafilm membranes, were inoculated with potato leafroll virus (PLRV) byMyzus persicae. PLRV was detected by ELISA and by transmission tests in tuber slices and in plants grown from the slices of the susceptible cv. Désirée, but not in those of the resistant cv. Arkula. These results suggest that PLRV replication and transport within tuber phloem is controlled by specific mechanisms of resistance.M. persicae was also able to acquire and transmit PLRV toPl floridana from slices cut from tubers of infected plants. The aphids effectively transmitted PLRV from slices cut from the sprouting rose end but they failed to transmit it from slices cut from the heel end of tubers.     

Acquisition of potato leafroll virus byMyzus persicae from secondarily-infected potato plants of different genotypes

Summary The acquisition of potato leafroll virus (PLRV) byMyzus persicae nymphs from the top leaves of potato plants was studied throughout a growing season in relation to the antigen titre in those leaves and the feeding behaviour of the aphid. Secondarily-infected plants of eight potato genotypes with different levels of field resistance served as virus sources. Early in the growing season, plants were efficient sources for virus acquisition. The amount of viral antigen detected inM. persicae nymphs fed on the top leaves was strongly correlated with the titres of viral antigen in these leaves. Virus acquisition from the top leaves of older potato plants was markedly impaired and could not be correlated with their virus titre. With increasing age of the potato plants and the development of virus symptoms, the virus titre in the leaves declined and the initial weak correlation between the virus titre and field resistance ratings disappeared. Thus, screening secondarily-infected potato plants for field resistance to PLRV based on the concentration of viral antigen in leaves or in aphids fed on them should be avoided later in the growing season. The feeding rate ofM. persicae, measured by the number of honeydew droplets excreted, did not account for the reduced uptake of virus from older plants since it was not influenced by the age of the plant. Throughout the growing season, the feeding rate ofM. persicae nymphs on PLRV-infected plants was higher on genotypes with low levels of field resistance to PLRV than on genotypes with high ones.     

Resistance to potato leaf roll virus inSolanum brevidens

Summary Plants ofSolanum brevidens graft-inoculated with potato leaf roll virus (PLRV) grew vigorously and had a normal healthy appearance. Although presence of the virus was confirmed in all inoculated plants by graft tests to potato and/orDatura stramonium, recovery of PLRV fromS. brevidens PI 218228 usingPhysalis floridana and the aphidMyzus persicae was erratic and only few test seedlings became infected. Ease of recovery of the virus using aphids was not influenced by presence of a continuous graft union with infected potato. Testing ofS. brevidens PI 245763 withM. persicae was not possible due to poor aphid survival on plants of this accession.     

Domyzus (nectarosiphon) species other thanm persicae pose a threat to the idaho potato crop?

Green peach aphids (Myzus persicae (Sulzer) ) pose a threat to the Idaho potato (Solanum tuberosum L.) crop primarily because they transmit potato leafroll virus (PLRV). Only colonizing vectors are epidemiologically significant because PLRV is persistently transmitted. Additionally, Idaho winters are severe enough that most permanently anholocyclic aphid species do not establish. It is now accepted that the taxon that has been known asMyzus (Nectarosiphon)persicae (Sulzer) is in fact at least three species (M. persicae, Myzus nicotianae Blackman andMyzus antirrhinii (Macchiati) ). It was not known whether the newly designated species had the potential to damage the Idaho potato crop. The objectives of our research were to determine whetherMyzus (Nectarosiphon) spp. other thanM. persicae occurred naturally in Idaho, whether they could colonize potato and transmit PLRV, and whether they were potentially holocyclic. NoMyzus (Nectarosiphon) spp. other thanM. persicae were found in Idaho in trap nurseries; howeverMyzus ascalonicus Doncaster recently has been found on stored bulbs and in suction trap collections in north Idaho. Laboratory bioassays for PLRV vector potential usingPhysalis floridana Rydb. indicator plants show thatM. nicotianae can transmit PLRV as efficiently asM. persicae. Similarly,M. nicotianae colonized potato as well asM. persicae in a replicated laboratory experiment comparing two clones ofM. nicotianae and one clone ofM. persicae. Our observations indicate that North AmericanM. nicotianae may be capable of a limited facultative holocycle. Thus, in laboratory experiments,M. nicotianae is as damaging asM. persicae with respect to PLRV vector potential; however,M. nicotianae may not occur naturally in areas like Idaho that have no tobacco production. A summary of allMyzus (Nectarosiphon) spp. with respect to PLRV epidemiology is provided.     

Transmission Efficiency of <Emphasis Type="Italic">Potato Leafroll Virus</Emphasis> by Four Potato Colonizing Aphid Species in Tunisian Potato Fields

This study investigated the transmission efficiency of Potato leafroll virus (PLRV) by four potato colonizing aphid species, Myzus persicae, Macrosiphum euphorbiae, Aphis gossypii and Aphis fabae, reported from leaves and yellow water trap. Physalis floridana was used as a test plant for virus transmission. DAS-ELISA was used for virus screening of samples as well as virus detection on the test plant after transmission experiment. A 2-h period was sufficient for the tested aphids to acquire PLRV virions. However, a difference in the transmission potential occurred according to the aphid species. The highest potential was recorded for M. persicae and M. euphorbiae, at 90 and 80%, respectively. For the first time, the study revealed the PLRV transmission efficiency of A. fabae, estimated at 50%. The lowest potential rate of 30% was recorded for A. gossypii. This study highlights the PLRV transmission capacities of four potato colonizing aphids suspected to play a key role in the spread of PLRV in potato seed production sites.     

Naturally occurring viral infections inSolanum brevidens andS. fernandezianum

Summary Two species of wild potato,S. brevidens andS. fernandezianum, were surveyed for six potato-infecting viruses in their natural habitats in the Puerto Montt Region and Robinson Crusoe Island of Chile, respectively. Potato viruses S (PVS), M (PVM) and X (PVX) and potato yellowing virus (PYV) were found in some clones of the Chilean Potato Germplasm Collection, whereas only one population ofS. brevidens out of six was virus-infected, namely by PVS. The cultivated potatoes on Robinson Crusoe Island were infected by PVS, PVM and PVX and infested byAulacorthum solani andMyzus persicae, whereasS. fernandezianum was infected by PYV and infested byA. circumflexum, A. solani andAphis spp.     

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.     

The infection pressure of potato leafroll virus and potato virus Y in relation to aphid populations in Cyprus

Summary The infection pressure of two viruses, potato leafroll (PLRV) and potato virus Y (PVY), both common in seed potatoes grown in Cyprus, was determined in three experiments in 1982–83. Virus-free bait plants, of potato and four other species, were exposed weekly to field infection during the growing season (March–June), and then returned to an aphid-free glasshouse for symptom expression. Only tobacco plants produced clear symptoms enabling reliable assessment of PVY infection pressure. When assessed with ELISA or by tuber indexing, the potato plants were efficient baits for both viruses whose infection period commenced at emergence (mid March to early April) and ended within 6–7 weeks. The seasonal trend of aphid populations, determined with Moericke traps or 100-leaf counts, correspond to that of virus spread. Correlation and regression analysis of aphid and virus data implicated the alate form ofMyzus persicae as the principal vector of both viruses.     

Epidémiologie de l’Enroulement de la Pomme de terre: capacité vectrice de stades et de formes des puceronsAulacorthum solani Kltb.Macrosiphum euphorbiae Thomas etMyzus persicae Sulz

Résumé L’étude comparée de la capacité vectrice de larves du 4è stade d’aptères, de nymphes des 3è et 4è stades d’ailés, d’adultes virginipares aptères et ailés d’une souche d’Aulacorthum solani, deMacrosiphum euphorbiae et deMyzus persicae a été réalisée surSolanum tuberosum var.Claudia. Il ressort que les catégories les plus efficaces sont les nymphes et les adultes aptères d’A. solani et les larves du 4è stade d’aptères deM. euphorbiae et deM. persicae; les catégories les moins efficaces sont respectivement les larves du 4è stade d’aptères d’A. solani, les nymphes deM. euphorbiae et les adultes ailés deM. persicae.

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Summary During a study of the transmission of leaf roll comparative tests were made of the vector ability of larval and adult stages of apterous and alate forms ofAulacorthum solani, Macrosiphum euphorbiae andMyzus persicae. The experiments were carried out at 18°C.; the source of the virus was sprouted tubers ofBintje and the indicator plants were sprouted half tubers ofClaudia which were replanted in the field after inoculation. Comparisons could be made from results obtained by the multiple transfer method. The results were, in general, homogeneous and showed both large differences in the vector ability of stages and forms of a single clone and that the most effective stages and forms are not necessarily the same for all species of aphid. In the case of apterous forms (Table 1), the most effective stage of As₅ is the adult whereas that ofM. euphorbiae andM. persicae is the larval. In the case of alate forms (Table 2) the most effective stage is the adult byM. euphorbiae whereas it is the larval byA. solani andM. persicae. The ascending order of vector capacity, calculated for an individual of each of the categories of the three species of aphid, is as follows: Aulacorthum solani: 4th stage larva of apterae (2,12%), alatae virginoparae (15,13%), 3rd or 4th stage larva of alatae (25,35%) and apterae virginoparae (29,43%). Macrosiphum euphorbiae: 3rd or 4th stage larva of alatae (0,84%), alatae virginoparae (4,70%), apterae virginoparae (6,61%) and 4th stage larva of apterae (12,83%). Myzus persicae: alatae virginoparae (49,75%), apterae virginoparae (58,03%), 3rd or 4th stage larva of alatae (66,78%) and 4th stage larva of apterae (70,94%). In the discussion stress is laid on two points arising from these observations: in the laboratory it is necessary to use more than one form or stage when investigating all aphid species liable to transmit a virus, otherwise, correct interpretation of a negative result is impossible; finally, it is essential to distinguish between the intrinsic vector ability of an aphid and its real possibilities of field dissemination of a virus. In this respect, its active behaviour for searching food may compensate eventually its deficiency in vector ability: the example of alatae has been chosen.

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Zusammenfassung In einer Studie über die Uebertragung des Blattrollvirus wurden die Uebertragungsf?higkeit von Larven oder ausgewachsenen L?usen, von ungeflügelten oder geflügelten Formen vonAulacorthum solani, Macrosiphum euphorbiae und vonMyzus persicae vergleichsweise geprüft. Die Versuche wurden bei einer Temperatur von 18°C durchgeführt; als Virusquelle dienten gekeimte Knollen der SorteBintje. Als Testpflanzen wurden halbe gekeimte Knollen der SorteClaudia, die anschliessend ins Freie gepflanzt wurden, verwendet. Die Vergleiche konnten anhand der Resultate, die sich durch die Methode mehrfacher Uebertragungen ergaben, angestellt werden. Die im allgemeinen gleichartigen Ergebnisse zeigen einerseits grosse Unterschiede in der Uebertragungsf?higkeit der Stadien und Formen der einen, gleichen Blattlausrasse; anderseits sind nicht bei allen Blattlausarten die gleichen Stadien und Formen an übertragungsf?higsten. Bei den ungeflügelten Formen (Tabelle 1) von As₅ ist das ausgewachsene Stadium das übertragungsf?higste, w?hrend dies beiM. euphorbiae undM. persicae auf das Larvenstadium zutrifft. Bei den geflügelten Formen (Tabelle 2) vonM. euphorbiae ist das ausgewachsene Stadium am übertragungsf?higsten, w?hrend es beiA. solani undM. persicae das Larvenstadium ist. Die Reihenfolge in der zunehmenden übertragungsf?higkeit, berechnet pro Individuum jeder Kategorie der drei Blattlausarten, ist wie folgt: Aulacorthum solani: Larve des 4. Stadiums der Ungeflügelten (2,12%), virginopare Geflügelte (15,13%), Nymphe im 3. oder 4. Stadium der Geflügelten (25,35%) und virginopare Ungeflügelte (29,43%). Macrosiphum euphorbiae: Nymphe im 3. oder 4. Stadium der Geflügelten (0.84%), virginopare Geflügelte (4,70%), virginopare Ungeflügelte (6,61%) und Larve des 4. Stadiums der Ungeflügelten (12,83%). Myzus persicae: virginopare Geflügelte (49,75%), virginopare Ungeflügelte (58,03%), Nymphe des 3. oder 4. Stadiums der Geflügelten (66,78%) und Larve des 4. Stadiums der Ungeflügelten (70,94%). In der Diskussion liegt die Betonung auf zwei Punkten, die sich aus diesen Beobachtungen ergeben: im Laboratorium ist es notwendig, mehr als nur eine Blattlausform oder ein Blattlausstadium zu verwenden, wenn man alle der Virus-übertragung verd?chtigen Blattlausarten untersucht, da sonst ein negatives Ergebnis nicht richtig interpretiert werden kann. Es ist schliesslich notwendig, zwischen der absoluten Uebertragungsf?higkeit einer Blattlausart und ihrer wirklichen Potenz zur Virusübertragung im Feld zu unterscheiden. Im Feld kann das allgemeine Benehmen bei der Nahrungssuche gelegentlich einen Mangel an Kapazit?t als Virusvektor kompensieren. Zur Erl?uterung wurde das Beispiel der Geflügelten herangezogen.

     

Aspects of insecticide resistance and reproductive biology ofAphis gossypii glover on seed potatoes

Summary For several years the aphid population patterns and the incidence of the main potato virus diseases have been monitored in Italian potato-growing areas. Starting from 1992, an increase inAphis gossypii, as compared toMyzus persicae, Macrosiphum euphorbiae andAphis fabae, was observed on seed potato fields in the north of Italy. Laboratory tests revealed thatA. gossypii was insensitive to the pesticide pirimicarb, even at the maximum dose tested (2400 μg/ml). Moreover, in the presence of pirimicarb the reproductive capacity increased up to 30% as compared with the control, and the new-born aphids began to appear 2 days earlier in the treated group. By contrast,M. persicae was controlled at a lower dose (300 μg/ml). Lambda-cyhalothrin and imidacloprid were very effective against both aphids.     

Henbane mosaic potyvirus pathogenic to wild and cultivated potato

Summary A virus isolated from severely diseased wild plants ofDatura stramonium in Hungary was identified as a strain of henbane mosaic potyvirus (HeMV) by its host range, vector transmissibility, electron microscopy and serology. This W/H isolate of HeMV induced necrotic local lesions onChenopodium quinoa and characteristic systemic symptoms in species ofDatura, Nicotiana andPhysalis but it was apathogenic to species ofCapsicum, Cucumis andPhaseolus. The virus is transmissible by aphids,Myzus persicae, in a non-persistent manner. Electron microscopy showed that it consisted of flexuous filaments, c. 180×12 nm. In plants ofDatura andNicotiana spontaneously infected by and artificially inoculated with HeMV-W/H, pinwheel structures typical of the potyvirus group were found. Serological examinations confirmed the identity of HeMV-W/H with the standard strain of the virus, HeMV-R. Plants of the potato cultivars Désirée and Gracia, graft-inoculated with HeMV-W/H, were latently infected. Inoculated plants of wildSolanum species responded to the virus with systemic symptoms.     

Soil survival and thiabendazole sensitivity ofHelminthosporium solani isolates from Alberta,Canada

Summary Survival of the pathogen causing silver scurf of potato (Helminthosporium solani) in Alberta soils was studied by evaluatingH. solani infection in the progeny ofH. solani-free nuclear seed potato tubers planted in fields where potatoes had never been grown or were grown 1, 2, 3, or 4 years previously. Daughter tubers from all the fields developed silver scurf lesions, andH. solani was isolated from infected tubers. This is the first report of survival ofH. solani in Alberta soils. Soil-borne inoculum appears to have a role in the epidemiology of the disease and in the introduction of the pathogen into silver scurf-free potato seed stock. Of 31 plant species tested, only potato was found to be a host ofH. solani. Most of theH. solani isolates from north central Alberta were more sensitive to thiabendazole than those from southern Alberta, where thiabendazole is much more commonly used.     

The relation between aphid flights and the spread of potato virus YN (PVYN) in the Netherlands

Summary In the Netherlands early haulm destruction is the main method of preventing excessive spread of virus diseases in seed potato crops. When potato leaf roll virus was the principal problem, the time of haulm destruction was determined by the flight ofMyzus persicae (Sulzer), studied with the aid of Moericke yellow water traps. As potato virus Y^N became a problem and other aphid species interfered, the interpretation of aphid flights became difficult. An attempt is made to quantify total aphid flight activity in terms of risk to the crop. By attributing relative efficiency factors to 9 vector species and considering their flights as recorded with suction traps, values of vector-pressure are obtained that correlate well with weekly infection of bait plants. If accumulated vector pressures are compared with the flights ofM. persicae as recorded with Moericke traps during 1970–1979, it appears that during 6 of these years the critical periods of both systems coincide. However, in 1974 and 1976 much potential vector threat occurred before the start of the flight ofM. persicae. Suggestions are made as to how to apply the method in practice. A semi-popular account of this paper has appeared in Dutch inGewasbescherming 12 (1981) 57–71.     

Transmission of potato leaf roll virus byMyzus persicae (Sulz.) from leaves and plants of different ages

Summary Young potato plants were a better source of potato leaf roll virus (PLRV) for aphids,Myzus persicae (Sulz.), than old ones. For plants 6, 7.5 and 9 weeks old, the best sources of PLRV were the lower, middle and upper leaves, respectively. The frequency of PLRV transmission from upper leaves did not change much with increasing age of plants nor did it change with different leaflets from the same leaf.     

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.     

The development of mature plant resistance in four potato cultivars against aphid-inoculated potato virus YO and YN in four potato cultivars

Summary Field-grown potato plants of cvs King Edward, Record, Maris Piper and Désirée were inoculated on seven different dates during the growing season of 1987 and 1988 with either potato virus Y^O (PVY^O) or PVY^N, using three viruliferous peach-potato aphids (Myzus persicae) per plant. In each cultivar, the proportion of progeny tubers infected with PVY^O or PVY^N was high in plants inoculated during the four weeks following emergence, the proportion declining to zero or close to zero in the subsequent 4–6 wks.     

Transmission of potato virus Y by cereal aphids

Summary The cereal aphidRhopalosiphum padi transmitted PVY^O and PVY^N from and to both tobacco and potato. However, it transmitted much less frequently thanMyzus persicae, even when aphids making single probes were compared.R. padi andM. persicae retained PVY for a similar duration. Both alatae and apterae ofR. padi transmitted infrequently, as also did progenies of individual aphids that had transmitted PVY.Metopolophium dirhodum also transmitted PVY infequently butSitobion avenae did not transmit.

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Zusammenfassung In Laborversuchen übertrugen ca, 1% der Getreideblattl?useRhopalosiphum padi undMetopolophium dirhodum, nicht jedochSitobion avenae das Kartoffel-Y-Virus (PVY). 65% vonMyzus persicae übertrugen PVY.R. padi übertrugauch dann weniger h?ufig alsM. persicae, wenn die Aphiden bei Einzelentnahmen untersucht wurden.R. padi übertrug PVY^O und PVY^N von und auf die Tabaksorte White Burley und die Kartoffelsorte King Edward. Zwei oder zehn Minuten Einwirkung waren optimal fürM. persicae, um PVY zu erlangen, w?hrendR. padi PVY h?ufiger nach l?ngeren Entwirkungszeiten übertrug (Tabelle 1).R. padi undM. persicae beinhalteten PVY für eine ?hnliche Zeitdauer (Tabelle 2). Sowohl alatae als auch apterae vonR. padi übertrugen unregelm?ssig, ebenso Nachkommen von individuellen Aphiden, welche PVY übertragen hatten.

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Résumé En tests de laboratoire, environ 1% des pucerons des céréalesRhopalosiphum padi etMetopolophium dirhodum, aucun deSitobion avenae et environ 65% deMyzus persicae transmettent PVY.R. padi transmet également moins fréquemment queM. persicae lorsque les pucerons sont mis en comparaison par piq?re unique.R. padi acquiert PVY^O et PVY^N à partir des variétés de tabac White Burley et de pomme de terre King Edward et les transmet à ces deux mêmes variétés. Deux ou 10 minutes d'acquisition sont optimum pour queM. persicae acquiert PVY tandis queR. padi transmet PVY plus fréquemment après une plus longue période d'acquisition (tableau 1).R. padi etM. persicae conservent PVY dans les mêmes délais (tableau 2). Les ailés et les aptères deR. padi transmettent rarement PVY comme les descendants de pucerons ayant transmis PVY.

     

Epidemiology of potato leaf roll virus in the Fraser River delta of British Columbia

The spread of potato leaf roll virus (PLRV) and the resulting phloem necrosis in the tubers has been a limiting factor in the production of the Netted Gem (Russet Burbank) variety in southwestern British Columbia, Canada. Steckling sugar beets are an important overwintering host for nymphs and adults of the green peach aphid,Myzus persicae (Sulzer). By June, aphids are widely disseminated to the florets and new leaves of sugar beet and other hosts, including potato. Colonies with alates are produced on summer hosts which are not treated with insecticides. The peak of the flight of alates from the colonies on the summer hosts occurs during the first half of August. Those alates that were produced on PLRV-infected potato and which travel to other potatoes transmit the virus as soon as they feed. By the end of August, the sugar beet seed crops and the early and mid-season potato crops have been defoliated or harvested. Thus many colonies ofM. persicae are destroyed and the threat of PLRV spread diminishes. Crop and weed plants which serve as overwintering hosts become infested during late summer with alate aphids. Their offspring survive the winter in numbers which are determined by the weather and survival of host plants.     

Volunteer potatoes as a source of potato leafroll virus and potato virus X

Volunteer potatoes were investigated as infection sources for potato leafroll virus (PLRV) and potato virus X (PVX) in a high elevation seed potato growing area of eastern Idaho. Population densities ofMyzus persicae were assessed. Percentage of PLRV and PVX infection of the volunteers and seed potato crops was determined, as well as density of volunteers and certain parameters of volunteer growth and reproduction. Volunteers apparently harbored no more PLRV than the potato crop from which they originated. But they were found to be an important reservoir of PVX with the infection increasing as much as 12.43% in one year. No aphids capable of transmitting PLRV were found although one species that can transmit potato virus Y was recorded. The mean density of volunteers varied from 0 to 84,880 stems/ha. The number of tubers remaining in the field after harvest and winter weather conditions appeared to be the only factors affecting volunteer density. Volunteer plants arising from seed pieces at an average depth of 6.1 cm were found to set an average of 2.1 new tubers per plant at an average depth of 4.0 cm. These results suggest that volunteer potatoes are a significant source of PVX infection in subsequent seed potato crops.