Response of potato cultivars to North American isolates of PVYNTN

Potato cultivars were assessed for their sensitivity to potato tuber necrotic ringspot disease (PTNRD) induced by two North American isolates (Tu619 and Tu660) of the tobacco veinal necrosis strain of potato virus Y (PVY^N) and belonging to the tuber necrosis (PVY^NTN) sub-group. Several widely grown cultivars (e.g. Norchip and Ranger Russet) were found to be highly sensitive to PTNRD, while others (e.g. Russet Burbank, Russet Norkotah and Shepody) were not. The foliage symptoms induced by these isolates were distinct from those reported to be produced by European isolates of PVY^NTN, in that systemic necrotic rings and chlorophyll figures on the leaves were not produced. Instead, the symptoms were similar to those of normal strains of PVY^N. The apparent correlation between resistance to the PVY^O (common) strain group and cultivar sensitivity to PTNRD is discussed.     

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.     

Differential Spread of <Emphasis Type="Italic">Potato virus Y</Emphasis> (PVY) Strains O,N:O and NTN in the Field: Implications for the Rise of Recombinant PVY Strains in New Brunswick,Canada

Potato virus Y (PVY) is a major cause of yield and quality loss in potato crops worldwide. Recently, populations of PVY strains have shifted dramatically toward recombinant strains such as PVY^NTN and PVY^N:O. A 2010 to 2016 survey of PVY strains in commercial fields of New Brunswick (NB), Canada, and five field trials tracking PVY spread in NB and Manitoba, were conducted to study the current status of PVY strains and their relative rates of spread. In NB, PVY^O dropped from 82% of infections in 2010 to 14% in 2016, replaced mostly by PVY^NTN (64%) and PVY^N:O (22%).In field trials with Russet Burbank and Gold rush varieties, PVY^NTN spread most effectively compared to PVY^N:O and PVY^O. Strain-specific PVY spread varied with the potato variety, possibly due to selective PVY^O resistance in Goldrush, mostly expressed at the plant-to-plant transmission level with little difference in transduction to tubers in infected plants. Relevance of in-field differences in spread of strains to changes in regional PVY populations, and potential mechanisms responsible, are discussed.     

Incidence and Characterization of Potato virus Y in Seed Potatoes in Tunisia

Surveys during five consecutive years in four main seed potato-growing areas of Tunisia revealed large differences in Potato virus Y (PVY) incidence. Infection rates at harvest ranged from up to 19% in Cap Bon and in Jendouba, to 31% in Kairouan and 48% in Manouba. However, infection rates were very low across the country in 2003 (1–5%), except in Kairouan, and rather low in 2004 (4–10%). Secondary infections in plants growing from imported seed potatoes were assessed to be 1–6% depending on region and year. Serological analysis of a first set of 90 samples collected in 2004–2005 revealed dominance of the PVY^N group (90% of the total number of PVY positives). A second set of 44 isolates from samples collected in 2006 was further analyzed with a combination of serotyping, indexing on tobacco, and RT-PCR tests targeting four genomic regions (5′Ntr/P1, HcPro/P3, CI/NIa, CP). Thirty-five of these 44 isolates were typical PVY^NTN isolates, with three recombination junctions in HcPro/P3, CI/NIa, and CP regions, respectively, whereas no recombination junction was identified in the genome of five isolates belonging to the PVY^N group. One additional recombinant PVY^NTN isolate was recovered from a mixed infection with a PVY^O isolate. Only three PVY^O isolates were recovered from the samples analyzed.     

General resistance against potato virus Y introduced into a commercial potato cultivar by genetic transformation with PVYN coat protein gene

Summary Transgenic cv. Folva potato plants expressing the coat protein gene of potato virus Y strain N (PVY^N) were produced usingAgrobacterium tumefaciens mediated transformation. Forty independent transformants were selected for resistance screening. Four clones showed complete resistance to mechanical inoculation with all the five PVY isolates tested: the PVY^N isolate from which the coat protein gene was derived, two PVY^O isolates, and two PVY^NTN isolates. Two of the fully resistant clones contained only one copy of the transgene, demonstrating that it is possible by genetic engineering to obtain highly virus resistant potato clones that can also be useful in future breeding programmes.     

Census Report of the <Emphasis Type="BoldItalic">Potato Virus Y</Emphasis> (PVY) Population in Swiss Seed Potato Production in 2003 and 2008

The status of the Potato virus Y (PVY) in Swiss seed potato production was investigated in the years 2003 and 2008 by analysing 385 leaf samples of field-grown, suspicious potato plants collected in four representative seed control fields. Serological investigations by ELISA showed that in c. 84% of the PVY-positive samples in both years, viruses belonging to the PVY^N group were found. All 124 serologically positive PVY samples collected in 2003 and a selection of 81 isolates of 2008 were further typified by molecular tests and by biological assays on tobacco and potato plants. These tests largely confirmed the predominance of the PVY^N group and, within this group, the prevalence of recombinant PVY^NTN, with 81.4% and 70.4% in 2003 and 2008, respectively. The percentage of PVY^N-Wilga (PVY^N-Wi) increased from c. 6% to 17% between the two years. PVY^O was detected only in 10.5% and 4.9% of all molecularly analysed samples in 2003 and 2008, respectively. The persistent predominance of recombinant PVY^NTN in Swiss seed potatoes indicates that this strain group is now widespread, representing a considerable threat to Swiss seed potato production.     

Complete Genome Analysis of a PVY<Superscript>N-Wi</Superscript> Recombinant Isolate from <Emphasis Type="Italic">Solanum tuberosum</Emphasis> in China

The complete sequence of CF_YL21, a Potato virus Y (PVY) isolate from Solanum tuberosum in China, was determined to be 9718 nucleotides in length, excluding the 3′-terminal poly(A) tail. The viral genome had a single open reading frame of 9186 nucleotides encoding a polyprotein of 3061 amino acids. The polyprotein was predicted to be cleaved into ten functional proteins by three viral proteases. Sequence analyses indicated that CF_YL21 shared 97% nucleotide identity with Wilga5 (PVY^N-Wi), and two putative recombination signals were detected in the P1 and HC-Pro/P3 regions. Phylogenetic analyses, Bayesian Tip-association Significance (BaTS) test, and multiplex RT-PCR assay confirmed that the isolate had the similar molecular and genomic structure with PVY^N-Wi, a PVY strain formed by recombination between PVY^N and PVY^O. To our knowledge, this is the first report of the complete sequence structure of PVY^N-Wi strain from potato in China.     

Differentiating PVYNTN by unique single-restriction cleavage of PCR products

Summary A procedure for differentiating PVY^NTN from PVY^N is described and is based on the unique cleavage of their respective PCR products with strain specific restriction endonucleases. The PCR products corresponding to the 5′ end of the N and NTN strains of PVY were cloned and sequenced, and a restriction map was constructed which included common enzymes that were used for the differentiation of PVY^NTN. Unique, single cleavage of PCR products derived from the 5′ end of the PVY^NTN genome by Nco I, and that of the N-strain of PVY by Bgl II restriction endonuclease were demonstrated. The specific digestion patterns in polyacrylamide gel were used for the unequivocal differentiation between the N and NTN strains of the virus. Both single and mixed infections were detected in field samples of potatoes using this procedure.     

Evidence of <Emphasis Type="Italic">Potato virus Y</Emphasis> Spread through Post-Emergence Management Practices in Commercial Potato Fields

Potato virus Y (PVY) transmission was studied in six commercial potato fields in 2015 and 2016 in New Brunswick, Canada. Plants emerged from PVY-positive tubers, or PVY-free plants that were artificially inoculated with PVY shortly after emergence, were used as PVY inoculum plants in the study. In all trials, equal numbers of PVY inoculum plants from each of three strains common in the region, PVY^O, PVY^N:O and PVY^NTN, were used. PVY inoculated into tractor-traffic rows showed 2 to 7 times as much PVY spread to previously virus-free plants by the end of the season (up to 48.5% in one tractor row, compared to a maximum of 16.3% in a distant control row unaffected by tractor traffic). Evidence supporting a hypothesis that tractor traffic enhances PVY transmission through aphid disturbance was observed by PVY spread in both directions along the rows, not biased in the direction of tractor travel, and that the ratio of spread of the three strains was nearly indentical in control and tractor rows. However, the lack of spread to immediately adjacent rows, and statistically significant spatial pattern matching the circumference of tractor wheels specific to each field support the hypothesis that direct mechanical transmission of wounded plant sap could also be a factor in the enhanced PVY transmission.     

Soybean aphid,Aphis glycines Matsumura, a new vector ofPotato virus Y in potato

Soybean aphid (Aphis glycines Matsumura), an exotic species first discovered in the North Central region of the United States in 2000, is a competent vector of severalPotyviridae. Soybean aphid has high fecundity and produces alatae (winged morphs) readily, characteristics typical of proficient virus vectors. When soybean aphids were exposed toPotato virus Y (PVY)-infected potato plants and then clip-caged on healthy potato plants in groups of five or as single aphids, PVY transmission ranged from 14% to 75% across all experiments. PVY^o, PVYⁿ, and PVP^ntn strains were transmitted by soybean aphid.     

Protection against potato virus Y (PVY) in the field in potatoes transformed with the PVY P1 Gene

Lines of potato cv. Pito transformed with the P1 gene ofPotato virus Y (PVY^o) in sense or antisense orientation were evaluated for resistance to PVY in the field in 1997 and 1998. The transgenic resistance fully protected the crop from infection with PVY^o transmitted by aphids in both years. These plants were not resistant to the field isolates of the PVY^N strain group, which is in agreement with our greenhouse experiments. Consequently, several transgenic lines produced higher yields than the nontransgenic cv. Pito plants. These results showed that the P1 gene-mediated resistance provides significant benefits under conditions were the incidence of infections and damage by PVY^o are considerable.     

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.     

Mineral Oil Inhibits Movement of Potato Virus Y in Potato Plants in an Age-Dependent Manner

Potato Virus Y (PVY) is one of the most devastating pathogens threatening potato production worldwide. It is a RNA virus that is disseminated by aphids in a non-persistent manner. Regular application of mineral oil on potato fields is known to reduce the number of PVY-positive tubers in post-harvest testing. The mechanism of this phenomenon is not well understood, but it is hypothesized to influence the virus-vector-plant relationships. Here, we present data from greenhouse and field trials that shed light on the effect of mineral oil on local and systemic accumulation of PVY^O in susceptible Shepody and Russet Burbank. The data suggests that mineral oil did not influence PVY^O levels in mechanically-inoculated leaves nor tubers of plants with a secondary infection. However, a reduction in systemic PVY^O levels was observed in mineral oil-treated older plants but not in younger plants, suggesting that mineral oil inhibits PVY^O movement in an age-dependent manner.     

Host range,symptomology, and serology of isolates of potato virus Y (PVY) that share properties with both the PVYN and PVYO strain groups

In surveys for the tobacco veinal necrosis strain of potato virus Y (PVY^N) in potato, two isolates (I-136 and I-L56) were obtained that shared properties with both the PVY^N and the common (PVY^O) strain groups. The isolates produced veinal necrosis on tobacco (Nicotiana tabacum L.) and mild symptoms on potato (Solanum tuberosum L.) cv. Jemseg, typical of PVY^N but their symptoms on some other indicator species such asCapsicum frutescens L.,Chenopodium amaranticolor Coste & Reyn.),Physalis angulata L.,P. floridana Rybd. were more typical of PVY^O. Their serological properties were also more typical of PVY^O in that they reacted with a PVY^O-specific monoclonal antibody (MAb) in ELISA and they failed to react with four PVY^N-specific MAbs. The possible taxonomic position of these isolates is discussed.     

Evidence of Recombinant Isolates of Potato Virus Y (PVY) in Argentina

In order to make a first approach in the identification of the genetic diversity of Potato virus Y (PVY) in Argentina, 46 PVY isolates from different potato growing regions of Argentina were characterized both, biological and serologically. Five of them (ST11, RCA5a, RCA6, RCA14b and SSF6) were selected for further genomic analyses. Four genomic fragments containing hot spot regions of recombination (HSR) reported previously were sequenced in each isolate and compared to PVY^N (CS434575.1) and PVY^O (U09509.1) reference genomes looking for genomic recombinations. Isolates with one, two or three recombination points were identified among these, including the two strains considered typical PVY^N (RCA5b) and PVY^O (SSF18) used as controls. This is the first report of the presence of recombined PVY in Argentina using a combination of biological, serological and molecular tools that shed light on the genetic diversity of PVY viruses in this country.     

Potato plants transformed with a potato virus Y P1 gene sequence are resistant to PVYO

The cloned P1 sequence of PVY^O was transferred in sense orientation into the potato cultivar Pito usingAgrobacteriummediated transformation. Sixteen of the putatively transformed plants (NPTII positive) were assayed for PVY^O resistance. No PVY^O was detected in four plants, representing two lines, 21 days after two sap-inoculations and 35 days after graft-inoculation, and the plants remained symptomless, whereas other tested plants showed mosaic symptoms and had high PVY titers similar to those of the control plants. No line was resistant to PVY^N and potato viruses A and X. Southern analysis confirmed the presence of the transgene(s) in the two PVY^O-resistant and one susceptible line examined, but no signal was detected in nontransformed Pito. These results suggest a high level of protection against PVY^O in potato transformed with P1 sequence of PVY^O.     

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.     

New isolates of the necrotic strain of potato virus Y (PVYN) found recently in Poland

Summary In comparison to the previously known isolates of potato virus Y^N (PVY^N), some isolates found in Poland since 1984 are more infectious to potato plants, reach faster a higher concentration and induce milder disease symptoms. Potato cultivars resistant to the standard type of PVY_N may be susceptible to the new isolates whereas those that are extremely resistant to PVY remain extremely resistant to the new isolates. The potato cultivar Elipsa is suitable for the differentiation of PVY^N isolates.