The effects of co‐infection by different Potato virus Y (PVY) isolates on virus concentration in solanaceous hosts and efficiency of transmission

The influence of co‐infection on concentration and accumulation of genetically different isolates of Potato virus Y (PVY) in potato and tobacco plants and the efficiency of transmission by Myzus persicae of PVY isolates from doubly versus singly infected plants were evaluated. The vector ability to simultaneously transmit two virus isolates was examined. Eight PVY isolates represented three strain groups: PVY^O (pathotype and serotype O), PVY^NW (pathotype N and serotype O), and PVY^NTN (pathotype and serotype N). Different diagnostic methods, including DAS‐ELISA, multiplex RT‐PCR, aphid transmission tests and bioassays, were applied to detect the presence of PVY isolates in source and assay plants. Significant reductions in concentrations of certain PVY isolates during co‐infection with other isolates were found both in potato and tobacco plants. The observed effects were both isolate‐ and host‐dependent in form. The highest rates of virus transmission by single aphids were recorded with PVY^NTN isolates, and the lowest ones with PVY^O isolates. Individual aphids of M. persicae were able to simultaneously transmit two PVY isolates. The frequency of transmission was generally low, but it reached as high as 20% for one of the isolate combinations. The findings presented in the work provide proof for antagonistic within‐plant interactions between isolates of PVY, with some implications of these interactions for virus transmission by aphid vectors. Consequently, this research contributes to a better understanding of the epidemiology of the disease caused by PVY.     

Incidence of potato viruses and characterisation of <Emphasis Type="Italic">Potato virus Y</Emphasis> variability in late season planted potato crops in Northern Tunisia

Surveys were conducted of symptomatic potato plants in late season crops, from the major potato production regions in Northern Tunisia, for infection with six common potato viruses. The presence of Potato leafroll virus (PLRV), Potato virus Y (PVY), Potato virus X (PVX), Potato virus A (PVA), Potato virus S (PVS) and Potato virus M (PVM) was confirmed serologically with virus infection levels up to 5.4, 90.2, 4.3, 3.8, 7.1 and 4.8%, respectively. As PVY was prevalent in all seven surveyed regions, further biological, serological and molecular typing of 32 PVY isolates was undertaken. Only one isolate was shown to induce PVY^O-type symptoms following transmission to tobacco and to react only against anti-PVY^O-C antibodies. Typical vein necrosis symptoms were obtained from 31 samples, six of which reacted against both anti-PVY^N and anti-PVY^O-C antibodies showing they contained mixed isolates, while 25 of them reacted only with anti-PVY^N antibodies. An immunocapture RT-PCR molecular test using a PVY^NTN specific primer pair set in the 5’NTR/P1 genomic region and examination of recombinant points in three genomic regions (HC-Pro/P3, CI/NIa and CP/3’NTR) showed that all 25 serotype-N PVY isolates were PVY^NTN variants with similar recombinations to the standard PVYNTN-H isolate. This is the first report of the occurrence of the PVY^NTN variant and its high incidence in late season potatoes in Tunisia.     

A novel type of Potato virus Y recombinant genome,determined for the genetic strain PVYE

Two Potato virus Y (PVY) isolates collected in Brazil, PVY‐AGA and PVY‐MON, were identified as recombinants between two parent genomes, PVY^NTN and PVY‐NE‐11, with a novel type of genomic pattern. The new recombinants had an ordinary PVY^NTN genome structure for approximately 6·7‐kb from the 5′‐end of the genome whereas the 3′‐terminal 3·0‐kb segment had two fragments of NE‐11‐like sequence separated by another small PVY^NTN‐like fragment. PVY strains are defined based on the hypersensitive resistance (HR) response in potato indicators. Both PVY‐AGA and PVY‐MON isolates did not induce the HR in potato cultivars carrying Ny, Nc, or (putative) Nz genes and thus were able to overcome all known resistance genes to PVY. Only one of the two isolates, PVY‐AGA, induced a vein necrosis reaction in tobacco. The biological responses of the potato indicators and tobacco defined PVY‐MON as an isolate of the PVY^E strain. To distinguish PVY‐AGA and PVY‐MON from other PVY^NTN isolates, an RT‐PCR test was developed utilizing new specific primers from the capsid protein gene area and producing a characteristic 955‐bp band. Serological profiling of these PVY isolates with three monoclonal antibodies revealed an unusual reactivity, where one of the two commercial PVY^N‐specific monoclonal antibodies did not recognize PVY‐AGA. The ability of these new PVY recombinants to overcome resistance genes in potato producing mild or no symptoms, combined with the lack of serological reactivity towards at least one PVY^N‐specific antibody may present a significant threat posed by these isolates to seed potato production areas.     

Occurrence of Potato virus Y strain PVYNTN in foundation seed potatoes in Japan,and screening for symptoms in Japanese potato cultivars

In 2008 and 2009 seasons, a sudden increase in Potato virus Y (PVY) incidence was recorded in foundation seed potatoes in Hokkaido, northern Japan. This increase was obvious during the field inspection and the postharvest indexing. Molecular typing revealed that besides the previously reported strains of PVY^O and PVY^NA‐N, the most common strain identified was the recombinant PVY^NTN, with three characteristic recombinant junctions at the HC‐Pro, VPg and CP regions. No potato tuber necrotic ringspot disease (PTNRD) was observed in foundation seed potatoes in correlation with the presence of PVY^NTN. Moreover, an isolate with a typical PVY^NTN recombinant genome, namely Eu‐12Jp, did not induce PTNRD in 62 Japanese potato cultivars tested in both primarily and secondarily infected plants. Two cultivars carrying the extreme resistance gene Ry_chc were resistant to the infection with Eu‐12Jp, which presents potential sources of resistance to PVY^NTN. Eu‐12Jp induced systemic mottle in potato cultivars Desiree and King Edward carrying resistance genes Ny and Nc, respectively, but induced a hypersensitive reaction in potato cultivar Maris Bard, with the Nz hypothetical resistance gene typical of the PVY^Z strain group. Therefore, based on the genome structure and the reaction of the potato N resistance genes, Eu‐12Jp should be classified as PVY^Z‐NTN, as described for isolates from Idaho, USA recently. This is the first report of PVY^Z‐NTN in Japan and the sudden and increased occurrence of PVY^NTN/PVY^Z‐NTN represents a potential risk of PTNRD developing and increases the significance of PVY in Japan.     

Potato Virus Y from Petunia can cause Symptoms of Potato Tuber Necrotic Ringspot Disease (PTNRD)

A potyvirus known to be an important agent involved in causing a disease of trailing petunias, was identified as being a member of the necrotic strain of potato virus Y (PVY) using a number of monoclonal antibodies. The sequence of the coat protein gene for the PVY isolate was determined and when compared with sequences for other PVY strains it was shown to cluster closely with isolates of PVY^NTN and to have a recombination point present within the coat protein common with other isolates of PVY^NTN. When inoculated onto potato tuber necrotic ringspot disease (PTNRD) susceptible potato cultivars the petunia isolate was found to be capable of causing necrotic tuber symptoms, consistent with those caused by other isolates of PVY^NTN. Due to the number of similarities it is thought the petunia isolate belongs to the PVY^NTN group of isolates. Out of 24 species of bedding and pot plant crops tested, 19 were shown by mechanical inoculation to be susceptible to PVY, highlighting not only a clear risk to a number of commercially important plant species from PVY^NTN infected trailing petunias, but also other susceptible crops grown in these areas.     

The movement of potato virus Y (PVY) in the vascular system of potato plants

Potato virus Y (PVY) is responsible for major viral diseases in most potato seed areas. It is transmitted by aphids in a non-persistent manner, and it is spread in potato fields by the winged aphids flying from an infected source plant to a healthy one. Six different PVY strains groups affect potato crops: PVY^C, PVY^N, PVY^O, PVY^N:O, PVY^NTN, and PVY^N-Wi. Nowadays, PVY^NTN and PVY^N-Wi are the predominant strains in Europe and the USA. After the infection of the leaf and accumulation of the virus, the virus is translocated to the progeny tubers. It is known that PVY^N is better translocated than PVY^O, but little is known about the translocation of the other PVY strains. The translocation of PVY occurs faster in young plants than in old plants; this mature plant resistance is generally explained by a restriction of the cell-to-cell movement of the virus in the leaves. The mother tuber may play an important role in explaining mature plant resistance. PVY is able to pass from one stem to the other stems of the same plant through the vascular system of the mother tuber, but it is unknown whether this vascular link between stems is permanent during the whole life of the plant. Two greenhouse trials were set up to study the spread of PVY in the vascular system of the potato plant. The PVY-susceptible cultivar Charlotte was used for both trials. It was demonstrated that all stems growing from a PVY-infected tuber will become infected sooner or later, and that PVY^N-Wi translocates more efficiently to progeny tubers than PVY^NTN. It was also demonstrated that the progressive decay of the mother tuber in the soil reduces the possibility for virus particles to infect healthy stems through the vascular system of the mother tuber. This new element contributes to a better understanding of the mechanism of mature plant resistance.     

Effect of temperature on resistance to Potato virus Y in potato cultivars carrying the resistance gene Rychc

The effect of cultivation temperatures on the resistance reaction to three Potato virus Y strains (PVY^O, PVY^N and PVY^NTN) in potato cultivars carrying Ry_chc was examined. When potato plants carrying Ry_chc were cultivated at 22 °C, a few small necrotic spots developed on inoculated leaves by 5 days after mechanical inoculation (dpi), and systemic infection of a few symptomless plants was confirmed at 28 dpi by IC‐RT‐PCR. At 28 °C, distinct necrotic spots developed on inoculated leaves by 5 dpi, and systemic symptoms occasionally appeared at 28 dpi. Thus, high temperature weakens Ry_chc‐conferred resistance. However, the incidence of systemic infection and the titre of virus in resistant cultivars at 28 °C were lower than in a susceptible cultivar. In graft inoculation under high summer temperatures, some plants developed necrosis on the leaves and stem, but PVY was barely detected by RT‐PCR in leaves on potato carrying Ry_chc. When seedlings from progeny tubers of plants that were inoculated with PVY and grown in a greenhouse at >30 °C in the daytime were examined by ELISA and IC‐RT‐PCR, PVY was not detected in cultivars carrying Ry_chc. These results show that Ry_chc confers an extreme resistance to PVY strains occurring in Japan.     

Acquisition of potato virus YN by Myzus persicae from primarily infected ‘Bintje’ potato plants

When ‘Bintje’ potato plants were inoculated mechanically with potato virus Y^N (PVY^N),Myzus persicae acquired PVY^N from both the inoculated and non-inoculated leaves about one week earlier than when plants were inoculated byM. persicae. Only when young plants of about four weeks after planting were inoculated byM. persicae, this aphid acquired PVY^N from the non-inoculated top leaves within a fortnight. When plants later than four weeks after planting were inoculated byM. persicae it generally took at least four weeks for this aphid to acquire PVY^N from non-inoculated top and other leaves of such plants. A number of leaves situated on the potato stems near to the inoculated ones did not serve as a PVY^N-source forM. persicae within the experimental period of 38 days. The results indicate that it is possible that in seed potato growing areas primarily infected PVY^N-infected plants, not yet showing symptoms, can act as virus sources for further spread. This is especially true in the beginning of the season.     

Invasive weed facilitates incidence of Colorado potato beetle on potato crop

We examined whether the invasive silverleaf nightshade, Solanum elaeagnifolium (Cavanilles) can facilitate the infestation of potato (Solanum tuberosum (L.)) by the Colorado Potato Beetle, CPB, Leptinotarsa decemlineata (Say) in Greece, which would have important financial and pest management implications for the growing of potato crops. In laboratory tests, CPB from Lesvos could utilise S. elaeagnifolium if supplied with whole plants. In the field, however, CPB was only found on S. elaeagnifolium after the start of the spring potato harvest and the resulting loss of potato foliage, and no eggs were laid. This suggests that S. elaeagnifolium provides only a temporary food resource for adult beetles. One of 10 surveyed summer potato fields near the study population of S. elaeagnifolium was infested with CPB. It is likely that the presence of S. elaeagnifolium in the vicinity of spring and summer potato fields can help maintain CPB population viability in the immediate post harvest period of the spring potato crop, which may in turn facilitate the infestation of summer potato fields with CPB.     

Relation between concentration of potato virus YN and its availability to Myzus persicae

The ability ofMyzus persicae to transmit PVY^N from potato to tobacco is not influenced by the temperature at which aphids are reared. A positive correlation exists between the relative virus concentration of PVY^N in potato as determined by serology and A6-test, and its availability toM. persicae as indicated by transmission tests to tobacco.Samenvatting De Bokx en Piron (1977) vonden, dat de virusconcentratie van aardappelvirus Y^N (PVY^N) in Eersteling positief was gecorreleerd met de temperatuur, waarbij aardappelplanten werden geteeld. De vraag is nu of er een verband bestaat tussen de virusconcentratie in de waardplant en de verspreiding van PVY^N-virus door bladluizen (Myzus persicae). Myzus persicae gekweekt bij verschillende temperaturen (Tabel 1) werd gebruikt voor virusoverdracht uit aardappelplanten geteeld bij 22°C (=gelijke virusconcentratie), terwijl bladluizen gekweekt bij kamertemperatuur werden gebruikt voor overbrengen van PVY^N uit planten geteeld bij verschillende temperaturen (=verschillende virusconcentraties).De overdracht van PVY^N doorM. persicae, werd niet beïnvloed door de temperatuur waarbij de bladluizen werden gekweekt. Er was echter een positieve correlatie tussen de relatieve concentraties van PVY^N in aardappel, bepaald volgens de microprecipitatietoets en de A6-bladtoets, en de overdracht door bladluizen naar tabak (Fig. 1).     

Improving Potato virus Y strain nomenclature: lessons from comparing isolates obtained over a 73‐year period

Biological and whole genome properties were compared between eight historical European (1943–1984) and five Australian (2003–2012) Potato virus Y (PVY) isolates. Based on eliciting hypersensitivity genes Nc, Ny or Nz, the former belonged to biological strain groups PVY^C (CT, CRM1), PVY^O (CRN, KE, RS) or PVY^Z (CM2, CRM2, DS). The latter were inoculated to differential and other potato cultivars, tobacco and tomato. Two belonged to PVY^O (BL, DEL3), one to PVY^Z (ATL1), and one (KIP1) to suggested strain group (PVY^D) which elicited putative hypersensitivity gene Nd. Tomato isolate CN1 (and unsequenced CN2), which were poorly adapted to infect potato, were not grouped. Next‐generation sequencing (NGS) of samples containing all isolates except CN2, yielded 13 complete sequences of 9592–9700 nucleotides (nt), and one partial sequence of 9002 nt, none being recombinants. Comparing the former with 60 other PVY complete genomes, found one (CRM2) in phylogenetic subgroup Y^O, eight in Y^O5 (CM2, CRN, DS, KE, RS, ATL1, BL, DEL3), three in Y^C2 (CRM1, CT, KIP1) and one in Y^C1 (CN1). Thus, biologically defined PVY^O (5) and PVY^Z (4) isolates were within phylogenetic subgroups Y^O or Y^O5, biological PVY^C isolates (2) within Y^C2, biological PVY^D isolate KIP1 in Y^C2 and tomato isolate CN1 in Y^C1. NGS identified KIP1 and partial sequence KIP from mixed infection. KIP was in Y^O5. Grouping of four PVY^Z isolates within phylogenetic PVY^O, and the PVY^D isolate within phylogenetic PVY^C, reveals disagreement between current biological and phylogenetic PVY nomenclature systems. Using Latinized numerals for phylogenetic group names resolved this.     

Relative efficiency of a number of aphid species in the transmission of potato virus YN in the Netherlands

This paper reports the results of live-trapping winged aphids in an Ashby (1976) trap in potato crops in the Netherlands from 1983–1987. During this period, a total of 122 aphid species were trapped. Although only four of those species were able to colonise potato, 26 of them were able to transmit PVY^N from potato to potato test plants. The transmission rates and relative efficiency factors (REF's) of those transmitters were determined. Aphis sambuci, Cryptomyzus galeopsidis, Dysaphis spp.,Hyadaphis foeniculi, Hyalopterus pruni andMyzus cerasi were recorded for the first time as vectors of PVY^N in the Netherlands.The numbers of aphids per species caught per season differed very much, also the virus transmission results of some fluctuated from year to year, e.g.Brachycaudus helichrysi. The REF's in various reports differ greatly, thus the value of a universal REF is doubtful. Assessment of the rate of virus spread in a potato crop is discussed.Samenvatting Van de in Nederland voorkomende aardappelvirussen wordt het aardappelvirus Y^N (PVY^N) als het meest schadelijke bij de pootgoedproduktie beschouwd. Dit virus kan op non-persistente wijze door een aantal bladluissoorten worden overgebracht. De groene perzikluis,Myzus persicae wordt geacht dit het meest efficiënt te kunnen doen.In de periode 1983–1987 is onderzocht welke bladluissoorten, geregistreerd in aardappelpercelen te Wageningen, Y^N-virus kunnen overbengen. In deze periode werden 122 gevleugelde bladluissoorten met behulp van een Ashby-val levend gevangen en op hun vermogen om Y^N-virus van aardappel naar aardappel over te brengen getoetst. 26 soorten hiervan zijn potentiële Y^N-virusoverbrengers. De relatieve efficiëntiewaarden (REF) voor elk van deze soorten werden opnieuw berekend.De REF-waarde voorBrachycaudus helichrysi werd berekend op 0,21, in afwijking van de waarde (0,01) die Van Harten (1983) aan deze toekende na waarnemingen gedurende slechts één seizoen.40% van het totaal aantal geregistreerde bladluizen bestond uitCavariella aegopodii, M. persicae, Metopolophium dirhodum, Rhopalosiphum insertum, R. padi enSitobion avenae. Minder talrijk (6,5%) was de groepB. helichrysi, Brachycandus spp. enMacrosiphum euphorbiae. De groepAphis nasturtii, Myzus certus enPhorodon humuli had een aandeel van 3,2% in de totale vangst. BehalveC. aegopodii zijn de genoemde soorten in staat het PVY^N over te brengen van aardappel naar aardappel. Van de rest zijn echter ook een aantal soorten in staat PVY^N over te brengen maar die zijn slechts in geringe mate gevangen. A. nasturtii, Brachycaudus spp.,M. certus enM. persicae besmetten vaker dan andere soorten aardappelplantjes met PVY^N. Van de soortenAphis sambuci, Cryptomyzus galeopsidis, Dysaphis spp.,Hyadaphis foeniculi, Hyalopterus pruni andMyzus cerasi werd voor de eerste keer in Nederland overdracht van PVY geconstateerd.     

Problems associated with potyviruses in potato certification-field inspection and serological testing1

In UK, the tobacco veinal necrosis strain of potato virus Y (PVY^N), potato virus A (PVA) and potato virus V (PVV) each occur in the field only in limited ranges of potato cultivars in which they mostly cause mild symptoms or even symptomless infection; little is known about incidence of strain C of PVY (PVY^C). The ordinary strain of PVY (PVY°), however, is widespread causing symptoms ranging in severity from very severe through to very mild, depending on cultivar sensitivity/tolerance. During field inspections, very mild potyvirus symptoms may be missed, so inspectors are trained to be particularly vigilant when examining problem cultivars which react in this way. PVA is almost invariably treated, along with PVX, as a mild kind of virus infection, but infections with PVY°, PVY^N and PVV are treated as severe with stricter tolerances being applied for them (especially for PVY^N) regardless of symptom severity. Wide variation within the same cultivar in the behaviour of variants within the PVY° strain group also sometimes causes difficulties in interpretation at inspection. To detect PVY, PVA and PVV in routine serological testing on potato certification samples, it is necessary to employ specific antisera to each of them. PVY^N-specific monoclonal antibodies can be used in ELISA to distinguish PVY^N from PVY°.     

Aphid vectors of potato virus YN

BesideMyzus persicae a dozen other species were found to be vectors of potato virus Y^N. Eleven other species did not transmit the virus.White Burley tobacco and A6 potato are equally suitable as test plant to monitor the efficency ofRhopalosiphum padi as vector of PVY^N, but as PVY^N source tobacco is not suitable for this aphid species.Between some aphid species rather large differences exist in retention periods of PVY^N. WithR. insertum andAphis fabae transmission after a 1 h starvation period was still 50% of that without starvation. WithPhorodon humuli, M. certus andM. persicae this value was only 15, 30 and 30%, respectively.Samenvatting Van 12 bladluissoorten werd vastgesteld dat zij, evenalsMyzus persicae, vectoren van het aardappelvirus Y^N (PVY^N) zijn. Van 11 andere soorten kon dit niet worden vastgesteld. Nicotiana tabacum cv. White Burley enSolanum tuberosum cv. A6 bleken beide goed bruikbaar als toetsplant voor het vaststellen van de efficiëntie vanRhopalosiphum padi als vector van het PVY^N; voor deze bladluissoort is tabak ongeschikt als bron van PVY^N.De retentieperiode van het PVY^N lijkt bij verschillende bladluissoorten aanzienlijk te variëren. BijRhopalosiphum insertum enAphis fabae bracht één uur vasten na de acquisitie de overbrenging terug tot 50% van die welke zonder vasten werd verkregen. BijPhorodon humuli was de reductie in overbrenging na één uur vasten 85%, bijMyzus certus enM. persicae was deze 70%.     

Aggressive and mild Potato virus Y isolates trigger different specific responses in susceptible potato plants

Differences in the early responses of two potato cultivars, Igor and Nadine, to two isolates of Potato virus Y (PVY), the aggressive PVY^NTN and the mild PVY^N, were monitored. Microarray and quantitative real‐time PCR analyses were carried out to identify differentially expressed genes after inoculation with each virus isolate. Additionally, symptom severity and development was observed and the amount of virus isolate accumulated in systemically infected leaves was evaluated, where a significantly higher amount of PVY^NTN was detected. Microarray analysis revealed 572, 1288 and 1706 differentially expressed genes at 0·5, 12 and 48 h post‐inoculation, respectively in cv. Igor, with a similar pattern observed in cv. Nadine. Microarray and quantitative real‐time PCR results implied an earlier accumulation of sugars and lower photosynthesis in leaves inoculated with the aggressive isolate than in leaves inoculated with the mild isolate. The PVY^NTN isolate did not activate early differential expression of the Fe‐superoxide dismutase and pectin methylesterase inhibitor (PMEI) genes, indicating a delay in plant response relative to that following PVY^N inoculation. Differences in the expression of the β‐glucanase‐I gene were also observed in early plant responses to inoculation with each virus isolate.     

Determination of the infection pressure of potato virus YN

In 1976 consecutive series of plants ofNicotiana tabacum ‘White Burley’ replaced weekly, were used as bait plants to determine the infection pressure of potato virus Y^N (PVY^N) in a crop of ware potatoes in the centre of the Netherlands. The first PVY^N-infected tobacco plants were found mid May. The course of infection of the tobacco plants was not correlated with the flight ofMyzus persicae, which started towards the end of June. Aphid species other thanMyzus persicae presumably are responsible for the infection observed early.Rhopalosiphum padi andAcyrthosiphon pisum flew much earlier thanMyzus persicae and are vectors of PVY^N.     

Genetic variation and structure of Solanum elaeagnifolium in Australia analysed by amplified fragment length polymorphism markers

Solanum elaeagnifolium is a weed of national significance in Australia. However, the genetic diversity of S. elaeagnifolium is poorly understood. Four amplified fragment length polymorphism primer combinations were utilised to investigate the genetic variation and structure of 187 S. elaeagnifolium individuals collected from 94 locations in Australia. High genetic diversity was found, with an average Jaccard's genetic similarity at 0.26. Individuals were assigned to two genetic clusters or considered as admixed according to their membership coefficient value (q) calculated by Bayesian model‐based genetic structure analysis. This suggested that Australian S. elaeagnifolium may have originated from two distinct gene pools. These results were further supported by principal co‐ordinates analysis. Large spatial groups of individuals assigning to these two gene pools were found in western Victoria and south‐western New South Wales (NSW) and northern NSW, which correlated well with the early records of S. elaeagnifolium in both regions. The high genetic diversity found here could add difficulties to effective control of S. elaeagnifolium across regions.     

Characterization of potato Y potyvirus isolates from tomato crops in Islas Canarias (Spain)*

A disease caused by potato Y potyvirus (PVY) affects tomato plantations with variable severity in Tenerife Island. Affected plants show diverse symptoms such as necrotic lesions or mild to severe mosaic in leaves and whitish spots in green fruits that remain after ripening. Tomato PVY isolates and few potato and capsicum PVY isolates have been characterized on the basis of biological, serological and molecular criteria. All PVY isolates reacted positively to monoclonal antibodies specific for PVY^O/C or PVY^N strains, and nearly 50% of tomato PVY isolates were recognized by both. Differentiation of PVY strains according to the response of inoculated experimental plants was confusing due to the variability of viral aggressiveness and symptomatology induced. RFLP analysis of the CP gene and 3’untranslated region (UTR) revealed high variability. In addition to mixed infection by different PVY strains, the biological and molecular properties of those tomato PVY isolates that react to both monoclonal antibodies could be explained as the result of RNA recombination between distinct PVY strains which infect the same host plant.