Potato Tuber Necrotic Ringspot Disease Research Articles

Antagonism and Synergism Characterize the Interactions between Four North American Potato Virus Y Strains

Potato virus Y (PVY) is one of the most important constraints to potato production worldwide. There is an increasing occurrence of recombinant PVY strains PVYNTN and PVYN-Wi and a decline in the incidence of the nonrecombinant PVYO. We hypothesized that this may be due to the ability of these recombinant strains to antagonize and/or outcompete PVYO in mixed infections. To determine this, we investigated interactions between PVYO and three recombinant PVY strains common in North America: PVYNTN, PVYN-Wi, and PVYN:O. Overall, our study showed that these interactions are tissue-dependent. Specifically, PVYNTN, the main causal agent of potato tuber necrotic ringspot disease (PTNRD), was found to be more adaptable than PVYO, especially in potato leaves due, at least in part, to the Ny gene that confers hypersensitive resistance (HR) to PVYO. Furthermore, PVYN-Wi was found to repress PVYO in potato tubers but act synergistically in potato leaves. The PVYO-induced foliage necrosis in cultivar ‘Ranger Russet’ was observed to be more severe in plants co-infected by PVYN-Wi and PVYN:O, respectively, resulting in plant death. Strikingly, this PVYO -induced necrosis was suppressed by PVYNTN in doubly infected plants. These interactions may, at least partially, explain the decreasing incidence of PVYO in United States potato production regions, especially given that many cultivars contain the Ny gene, which likely limits PVYO enabling PVYNTN and PVYN-Wi to outcompete. We also found that replication and cell-to-cell movement of these PVY strains in tubers at 4 °C was similar to levels at ambient temperature.

Management of Potato virus Y (PVY–NTN) causing potato tuber necrotic ringspot disease (PTNRD) in potato by prior treatment with a mild PVY strain

Management of Potato virus Y (PVY–NTN) causing potato tuber necrotic ringspot disease (PTNRD) in potato by prior treatment with a mild PVY strain

Prevalence of Recombinant Strains of Potato Virus Y in Seed Potato Planted in Idaho and Washington States Between 2011 and 2021.

Potato virus Y (PVY) has emerged as the main reason for potato seed lot rejections, seriously affecting seed potato production in the United States throughout the past 20 years. The dynamics of PVY strain abundance and composition in various potato growing areas of the United States has not been well documented or understood up to now. The objective of this study was to find out the prevalence of PVY strains in potato fields in the Pacific Northwest (PNW), including seed potato production systems in the State of Idaho and commercial potato fields in the Columbia Basin of Washington State between 2011 and 2021. Based on the testing of >10,000 foliar samples during Idaho seed certification winter grow-out evaluations of seed potato lots and seed lot trials in Washington State, a dramatic shift in the PVY strain composition was revealed in the PNW between 2011 and 2016. During this time period, the prevalence of the ordinary, PVYO strain in seed potato dropped 8- to 10-fold, concomitantly with the rise of recombinant strains PVYN-Wi and PVYNTNa, which together accounted for 98% of all PVY positives by 2021. In Idaho seed potato, PVYNTNa strain associated with the potato tuber necrotic ringspot disease (PTNRD) was found to increase threefold between 2011 and 2019, accounting for 24% of all PVY positives in 2019. Mild foliar symptoms induced by recombinant PVY strains may be partially responsible for the proliferation of PVYN-Wi and PVYNTNa in potato crops. A spike of another PTNRD-associated recombinant, PVY-NE11, was recorded in the PNW between 2012 and 2016, but after reaching a 7 to 10% level in 2012 to 2013 this recombinant disappeared from the PNW potato by 2019. Whole genome sequence analysis of the PVY-NE11 suggested this recombinant was introduced in the United States at least three times. The data on PVY strain abundance in the PNW potato crops suggest that virus management strategies must consider the current dominance of the two recombinant PVY strains, PVYN-Wi and PVYNTNa.

Molecular and Biological Characterization of Recombinant Isolates of Potato virus Y Circulating in Potato Fields in Mexico.

Potato virus Y (PVY) is a significant threat to potato (Solanum tuberosum) production in Mexico. The presence of recombinant strains of PVY circulating in potato has been reported in the country, but no systematic study on the genetic diversity of PVY in potato and prevalence of PVY strains has been conducted yet. We report on a series of surveys in seed potato production areas in two states in Mexico, namely, Chihuahua and Jalisco, between 2011 and 2019. PVY was detected through the period of nine years in multiple potato cultivars in both states, often remaining asymptomatic in the most popular cultivars, such as 'Fianna' and 'Agata'. When typed to strain, all PVY samples studied were found to have N-serotype, and were all identified molecularly as isolates of the same recombinant strain, PVYNTN. Five of these PVY isolates were tested on tobacco (Nicotiana tabacum), where they induced vein necrosis supporting the molecular typing. This identification was also confirmed biologically on differential potato cultivars, where one PVYNTN isolate from the 2013 survey triggered the hypersensitive resistance conferred by the Nztbr gene in the cv. Maris Bard. Seven of these Mexican PVYNTN isolates, collected between 2013 and 2019, including two PVY isolates from potato tubers exhibiting potato tuber necrotic ringspot disease, were subjected to whole genome sequencing and found to show a typical PVYNTNa recombinant structure. When subjected to phylogenetic analysis, Mexican PVYNTN sequences clustered in more than three separate clades, suggesting multiple introductions of PVYNTN in the country. The wide circulation of the PVYNTN strain in Mexican potato should be considered by potato producers, to develop mitigation strategies for this PVY strain associated with tuber necrotic symptoms.

Effects of the Age-Related Resistance to Potato virus Y in Potato on the Systemic Spread of the Virus, Incidence of the Potato Tuber Necrotic Ringspot Disease, Tuber Yield, and Translocation Rates Into Progeny Tubers.

The recombinant strain of potato virus Y (PVY), PVYNTN, is the main cause of the potato tuber necrotic ringspot disease (PTNRD) in susceptible potato cultivars, which reduces the quality of potato tubers, in addition to the yield loss. Control of PVY has been the main challenge in most potato-producing areas. Here, the effects of the age-related resistance (ARR) were investigated in transplants of a potato cultivar Yukon Gold to the infection with PVYNTN strain in greenhouse experiments. Within the first 3 weeks after transplanting into soil (week 1 [W1] to W3), Yukon Gold plants developed ARR that impaired the systemic movement of PVYNTN into upper noninoculated leaves and concomitant translocation into progeny tubers starting from W4 after transplanting. The yield and quality of tubers from PVY-infected plants with the established ARR (W5 to W8) were comparable with the healthy controls, suggesting that late PVY infection would not significantly affect commercial potato production. Plants inoculated early (W1 to W2), before the establishment of the ARR, exhibited a 100% primary systemic infection with PVYNTN and produced fewer tubers of smaller sizes, exhibiting PTNRD; this resulted ≤70% yield reduction compared with plants inoculated later in the season (W5 to W8). This ARR greatly restricted the systemic movement of PVYNTN in the foliage and resulted in very limited translocation rates of the virus into tested progeny tubers: 7.8 and 4.1% in 2017 and 2018, respectively, of all plants inoculated later in the season (W5 to W8). This study suggests that PVYNTN management programs in Yukon Gold seed potato should focus more on the early stages of the potato development before the onset of the ARR.

Genetic Diversity of Potato virus Y in Potato Production Areas in Northeast China

In 2011-2014, ELISA or nucleic acid spot hybridization (NASH) testing for common potato viruses or Potato spindle tuber viroid (PSTVd) was performed on 500 leaf samples collected in potato fields in the northeast provinces Heilongjiang and Inner Mongolia, China. The results revealed that 38.4% (Heilongjiang) and 27.7% (Inner Mongolia) were positive for Potato virus Y (PVY). To unveil the strain composition and population structure of PVY in the region, the multiplex RT-PCR described by Chikh-Ali et al. was performed on all of the ELISA-PVY-positive samples. Of the 158 samples whose PVY strain scenarios could be determined, PVYNTN-NW-SYR-II and PVYN-Wi were the most abundant strains, occurring in 58.9 and 47.5% samples, followed by PVYNTN-NW-SYR-I (31.0%), PVYN:O (19.6%), Eu-PVYNTN (7.6%), NA-PVYN (1.3%), and PVYO (0.6%). In the 84 single-strain-infected samples, PVYN-Wi accounted for 41.7%, PVYNTN-NW-SYR-II for 40.5%, PVYNTN-NW-SYR-I for 14.3%, and PVYN:O and Eu-PVYNTN for 3.6% each. Seven isolates representing PVYNTN-NW-SYR-I (HLJ-6-1 and HLJ-9-4), PVYNTN-NW-SYR-II (INM-W-369-12 and SC-1-1-2), PVYN:O (HLJ-30-2), and PVYN-Wi (HLJ-BDH-2 and HLJ-C-429) were sequenced and analyzed molecularly. Whereas the sequence identities for isolates belonging to the same strain group were >98.5%, they fell for isolates belonging to different strain groups to 92.7-98.1% at the genome level and 96.1-98.4% at the polyprotein level. Interestingly, the exact location of the recombination events varied among isolates within a strain group. Phylogenetic analysis of all 42 full length PVY sequences from China indicated that most clustered to various recombinant groups, despite the fact that the PVY isolates were isolated from at least five host species. Pathological analysis of four isolates representing PVYN:O, PVYN-Wi, PVYNTN-NW-SYR-I, and PVYNTN-NW-SYR-II revealed that the PVYNTN-NW-SYR-II isolate incited the most severe symptoms on potato cultivar Kexin 13, followed by PVYNTN-NW-SYR-I, PVYN:O and PVYN-Wi. The PVYNTN-NW-SYR-I and PVYNTN-NW-SYR-II isolates also caused necrotic ringspots on the tubers of Kexin 13, indicating their ability to induce the potato tuber necrotic ringspot disease in potato.

Occurrence of Diverse Recombinant Strains of Potato virus Y Circulating in Potato Fields in Egypt.

Potato is one of the staple crops in Egypt, grown under irrigation almost continuously year-round. Potato virus Y (PVY) has been reported as one of the main viruses affecting potatoes in Egypt, but limited information is available on PVY strains circulating in potato fields in the country. From 2014 to 2016, virus surveys were conducted in several potato-growing governorates of Egypt, and PVY-positive samples were found to represent at least five distinct recombinant PVY strains, including PVYNTN and PVYN-Wi. Whole genome sequences were determined for four isolates representing strains PVY-SYR-III (Egypt7), PVY-261-4 (Egypt11), PVYNTNa (Egypt35), and a novel recombinant named Egypt24 that combined molecular properties of strains PVY-261-4 and PVY-Wilga156var. At least three recombinants found in Egypt in potato were previously found associated with potato tuber necrotic ringspot disease (PTNRD). The identification of multiple recombinant types of PVY in potato in Egypt, including the novel recombinant Egypt24, suggests a wide presence of PTNRD-inducing virus strains in the country.

Strain-Specific Resistance to Potato virus Y (PVY) in Potato and Its Effect on the Relative Abundance of PVY Strains in Commercial Potato Fields.

Potato virus Y (PVY) is a serious threat to potato production due to effects on tuber yield and quality, in particular, due to induction of potato tuber necrotic ringspot disease (PTNRD), typically associated with recombinant strains of PVY. These recombinant strains have been spreading in the United States for the past several years, although the reasons for this continuing spread remained unclear. To document and assess this spread between 2011 and 2015, strain composition of PVY isolates circulating in the Columbia Basin potato production area was determined from hundreds of seed lots of various cultivars. The proportion of nonrecombinant PVYO isolates circulating in Columbia Basin potato dropped ninefold during this period, from 63% of all PVY-positive plants in 2011 to less than 7% in 2015. This drop in PVYO was concomitant with the rise of the recombinant PVYN-Wi strain incidence, from less than 27% of all PVY-positive plants in 2011 to 53% in 2015. The proportion of the PVYNTN recombinant strain, associated with PTNRD symptoms in susceptible cultivars, increased from 7% in 2011 to approximately 24% in 2015. To further address the shift in strain abundance, screenhouse experiments were conducted and revealed that three of the four most popular potato cultivars grown in the Columbia Basin exhibited strain-specific resistance against PVYO. Reduced levels of systemic movement of PVYO in such cultivars would favor spread of recombinant strains in the field. The negative selection against the nonrecombinant PVYO strain is likely caused by the presence of the Nytbr gene identified in potato cultivars in laboratory experiments. Presence of strain-specific resistance genes in potato cultivars may represent the driving force changing PVY strain composition to predominantly recombinant strains in potato production areas.

Molecular characterization of potato virus y inducing potato tuber necrotic ringspot disease in Serbia

The Potato virus Y (PVY) is the most important limiting factor for potato seed production in Serbia. Currently, PVY is a major concern for the potato seed growers. Initially, serological (ELISA) tests were carried out on 100 potato seed tubers from each of the seven potato cv. during 2013. The infection rates with the PVYN was between 5 and 36%. A complete genome sequencing of the most common Serbian isolate of PVY (3D), followed by molecular characterization and phylogenetic analysis has been performed to show what group it belongs to. Our isolate?s complete genome sequence (KJ946936) showed that the Serbian PVY isolate (3D) is 99.7% identical at nt level, with other tuber necrosis strain group (PVYNTN) from Europe. Phylogenetic analysis revealed three consistent lineages of isolates, showing that our isolate was clustered with the isolates from Europe and North America in the PVYN lineage which induces potato tuber necrotic ringspot disease (PTNRD). The Serbian isolate of PVYNTN together with the isolates from Europe was clustered in the branch of European sublineage, with a high bootstrap support and no genetic diversity. This is the first study in Serbia demonstrating phylogenetic distinction between our isolate and other isolates of PVY.

Occurrence and Molecular Characterization of Recombinant Potato virus YNTN Isolates from Sulawesi, Indonesia.

The importance of potato has increased dramatically in Indonesia over the last three decades. During this period, 'Granola', a potato cultivar originally from Germany, has become the most common cultivar for fresh consumption in Indonesia. In August 2014, a survey was conducted in Sulawesi, where potato fields cultivated with Granola and its selection, 'Super John', were sampled for Potato virus Y (PVY) presence. PVY was found in Sulawesi for the first time. Samples determined to be positive for PVY were subsequently typed to strain using reverse-transcription polymerase chain reaction assays. All PVY isolates sampled were identified as PVYNTN recombinants, with three recombination junctions in P3, VPg, and CP regions of the genome. Three local PVY isolates were subjected to whole-genome sequencing and subsequent sequence analysis. The whole genomes of the Indonesian PVYNTN isolates I-6, I-16, and I-17 were found to be closely related to the European PVYNTN-A. This recombinant type was shown previously to cause potato tuber necrotic ringspot disease (PTNRD) in susceptible potato cultivars. The dependence of potato farmers on mostly a single cultivar, Granola, may have given a competitive advantage to PVYNTN over other PVY strains, resulting in the predominance of the PVYNTN recombinant. The dominance of PVYNTN in Sulawesi, and possibly in Indonesia as a whole, represents a potential risk to any newly introduced potato cultivar to the country, especially cultivars susceptible to PTNRD.

Molecular Characterization of Recombinant Strains of Potato virus Y From Saudi Arabia.

Potato virus Y (PVY) exists as a complex of strains, many of which are recombinants. The practical importance of PVY recombinant strains has increased due to their ability to induce potato tuber necrotic ring spot disease (PTNRD) that seriously affects tuber quality. In Saudi Arabia, potato production has increased fivefold during the last three decades, reaching 460,000 tons per year. Although PVY has been reported as one of the main viruses affecting potatoes, no information is available on PVY strains circulating in the country. In August 2014, a survey was conducted in a seed potato field at Al-Jouf, Saudi Arabia. PVY-positive samples selected based on visual symptoms and serological reactivity were subjected to strain typing using multiplex RT-PCR assays and were determined to represent recombinant PVY strains. Whole genome sequences were determined for two representative isolates, S2 and S9, through direct sequencing of a series of overlapping RT-PCR fragments for each isolate, and found to represent strains PVY-NE11 and PVYZ (SYR-III), respectively. One of the recombinant types, SYR-III, was previously found in nearby Syria and Jordan, but the second recombinant, PVY-NE11, was found before only in the United States. Both recombinants, PVY-NE11 and SYR-III, were previously found associated with PTNRD and thought to be rare. The current identification of PVY-NE11 and SYR-III in seed potato in a new geographic region suggests that these recombinants may not be as rare as previously believed. This is the first report on the occurrence of recombinant strains of PVY in potato in Saudi Arabia, and the first report on the PVY-NE11 strain of PVY found in potato outside of the United States.

First report of potato tuber necrotic ringspot disease associated with PVY recombinant strains in Ireland

Potato tuber necrotic ringspot disease (PTNRD) as described by Kus et al., (1992) is found worldwide but has not previously been identified in Ireland. In recent years necrotic symptoms, veinal necrosis, spots, mottling, mosaics and…

First Report of Recombinant Potato virus Y Strains in Potato in Jalisco, Mexico.

Potato virus Y (PVY) is a serious problem for potato production worldwide. The virus reduces both tuber yield and quality, and recent spread of recombinant strains of PVY in potato production areas is largely credited with the spread of potato tuber necrotic ringspot disease (PTNRD) (1). In Mexico, recombinant strains of PVY were reported in at least two states, Chihuahua (4) and the State of Mexico (3); however, no surveys have been conducted in other potato-producing areas, and the spectrum of PVY isolates circulating in the country has remained uncharacterized. In October 2011, a small-scale survey of seed potato was conducted in the state of Jalisco, Mexico, to identify PVY isolates present in fields. Twelve seed potato fields were inspected visually. These represented various generations of seed potato, from nuclear to G2. Leaf samples were collected from plants displaying mosaic, crinkling, and yellowing symptoms, and were tested for PVY. Fifty samples were collected from cultivars Fabula, Mondial, Fianna, Gigant, Caesar, and Adora. Of the 50 leaf samples collected, seven were PVY-positive using the Immuno-strip Kit (Agdia, Elkhart, IN), and six of these were determined to have a N-serotype according to the typing by the Pocket Diagnostics lateral flow kit (Forsite Diagnostics, Ltd., York, UK). PVY-positive samples came from cultivars Fabula (2 with N serotype), Mondial (4 with N serotype), and Fianna (1 with O serotype). Extracts of the seven PVY-positive leaf samples were applied to Whatman FTA cards (Sigma, St. Louis, MO), dried, and transported to the Plant Virology Laboratory at the University of Idaho for further characterization. All samples immobilized on FTA cards were subjected to RNA extraction and standard reverse transcriptase (RT)-PCR typing using a set of PVY-specific primers (2) to determine the strain type. All PVY isolates were recombinant. The six N-serotype samples were found to contain recombinant PVYNTN isolates and produced characteristic bands of 181 and 452 bp in RT-PCR, which indicated the presence of two recombination junctions in the HC-Pro/P3 and VPg regions typical of European PVYNTN isolates. The one O-serotype sample was identified as a recombinant PVYN-Wi/N:O isolate, and produced 181 and 689 bp bands in RT-PCR, which indicated the presence of one recombination junction in the HC-Pro/P3 region. Sequence analysis of RT-PCR products amplified from five samples with N serotype identified them as PVYNTN isolates, and from the one with O serotype identified it as PVYN-Wi/N:O isolate. Sequence comparisons confirmed that N serotype samples contained PVY isolates most closely related to typical PVYNTN sequences (Accession No. EF026075), while the O serotype sample contained the PVY isolate most closely related to PVYN-Wi from Europe (HE608963). The data obtained suggest the presence of two different types of PVY recombinants, PVYNTN and PVYN-Wi, in seed potato in Jalisco. Additional surveillance for these recombinant isolates may be needed, as well as a survey of their effects on tuber quality in production areas. This is the first report of recombinant isolates of PVY often associated with PTNRD circulating in seed potato in Jalisco, Mexico.

Assessment of SNaPshot and single step RT-qPCR methods for discriminating Potato virus Y (PVY) subgroups

Potato virus Y (PVY) is the most important virus infecting potato (Solanum tuberosum), causing potato tuber necrotic ringspot disease (PTNRD), with a great impact on seed potato production. Numerous PVY strain groups with different pathogenicity and economical impact are distributed worldwide. Tools for accurate and reliable detection and discrimination of PVY strain groups are therefore essential for successful disease management.Two state of the art characterization tools based on detecting molecular markers – RT-qPCR (Kogovsek et al., 2008) and SNaPshot (Rolland et al., 2008) – were assessed for their ability to assign PVY accurately to the correct group. The results were validated by bioassay, ELISA and in silico sequence analysis. The spectrum of PVY strain groups distinguished by SNaPshot is broader than that by RT-qPCR. However, the latter was more reliable in discriminating the PVYNTN group members, known for their ability to induce PTNRD on selected potato cultivars. The difference in discrimination precision was due to different molecular markers being targeted by RT-qPCR and SNaPshot. Both tools use genotypic markers for detecting PVYNTN strain groups. Future development, however, should be focused on identifying the genomic determinants of the tuber necrosis property. Until then, the RT-qPCR and SNaPshot methods remain the most powerful diagnostic tools for detecting the PVY subgroup isolates found in Europe.

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 PVYO and PVYNA‐N, the most common strain identified was the recombinant PVYNTN, 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 PVYNTN. Moreover, an isolate with a typical PVYNTN 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 Rychc were resistant to the infection with Eu‐12Jp, which presents potential sources of resistance to PVYNTN. 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 PVYZ strain group. Therefore, based on the genome structure and the reaction of the potato N resistance genes, Eu‐12Jp should be classified as PVYZ‐NTN, as described for isolates from Idaho, USA recently. This is the first report of PVYZ‐NTN in Japan and the sudden and increased occurrence of PVYNTN/PVYZ‐NTN represents a potential risk of PTNRD developing and increases the significance of PVY in Japan.

Epitope Mapping for Monoclonal Antibodies Recognizing Tuber Necrotic Isolates of Potato Virus Y

Potato virus Y (PVY) is an important viral pathogen of potato responsible for reducing tuber yield and quality across the globe. The PVYN and PVYNTN strains, the latter of which induces potato tuber necrotic ringspot disease (PTNRD), are regulated for international potato trade, and have been routinely detected using monoclonal antibodies (MAbs) that discriminate between PVYN and PVYO serotypes. Here, we identify the distinct binding sites in the capsid protein of PVY for three of the four main PVYN-specific MAbs, Bioreba-N, SASA-N, and Neogen-N, available commercially. These binding domains were mapped through a combination of TAS-ELISA testing of MAbs on multiple reference isolates of PVY, sequence analysis, heterologous expression of capsid protein fragments, and synthetic peptide binding experiments. All three MAbs were found to bind linear epitopes located within the first 31 N-terminal amino acids of the capsid protein. Bioreba-N MAb epitope spanned aa 1-17 and included three positions, aa 1, aa 11, and aa 17, which differ between PVYN and PVYO serotypes. Both SASA-N and Neogen-N epitopes spanned aa 22-30, and included two positions, aa 24 and aa 29, which differ between PVYN and PVYO serotypes. Epitopes for SASA-N and Neogen-N MAbs are likely to be identical or overlapping. Examination of available sequences for tuber necrotic isolates of PVY that do not react with PVYN-specific MAbs SASA-N and Neogen-N indicated possible selection for substitutions in corresponding epitopes leading to the loss of reactivity towards these antibodies. The data obtained suggested that testing with more than one PVYN serotype-specific MAb could assure a reliable serological identification of a PVYN or PVYNTN isolate.

An infectious full-length cDNA clone of potato virus YNTN-NW, a recently reported strain of PVY that causes potato tuber necrotic ringspot disease

A full-length infectious cDNA clone of potato virus Y (PVY) was constructed based on an isolate of the PVY(NTN-NW) strain, SYR-II-2-8, which is able to cause potato tuber necrotic ringspot disease (PTNRD). Silent point mutations were introduced to modify sequences similar to the prokaryotic promoter elements detected at the 5' terminus of the P3 coding region of SYR-II-2-8. This, along with modification of the growing conditions of E. coli, led to the successful construction of a stable full-length infectious cDNA clone, named p2-8C3. The biological properties of p2-8C3 were identical to those of SYR-II-2-8, both of which induced PTNRD in potato, veinal necrosis in tobacco, and similar symptoms in the potato cultivars inoculated.

PVYNTN‐NW, a novel recombinant strain of Potato virus Y predominating in potato fields in Syria

Detailed characterization of a number of isolates of PVYSYR, a novel recombinant strain of Potato virus Y (PVY) from Syria, was conducted to elucidate their origin, assess their significance and achieve a final classification of PVYSYR. Recombination analysis grouped isolates of PVYSYR into three recombination patterns, SYR‐I, SYR‐II and SYR‐III, which varied in the first 700 nucleotides of their genomes, with the second recombination pattern, SYR‐II, the most frequent. PVYSYR isolates shared highest genomic identity and close phylogenetic relationships with PVYNTN and PVYNW isolates from Syria, suggesting a common origin and local emergence of these isolates in Syria. All PVYSYR isolates (total of 20) induced tobacco veinal necrosis, but reacted to a PVYO monoclonal antibody, typical characteristics of the previously reported PVYNW (or PVYN:O). In potato, however, four isolates tested (one of SYR‐I and three of SYR‐II) induced potato tuber necrotic ringspot disease (PTNRD), which is the characteristic phenotype of PVYNTN. Given the shared properties of SYR‐I and SYR‐II isolates with PVYNTN and PVYNW, it was decided that they represent a new recombinant strain of the PVYN strain group, with the proposed name PVYNTN‐NW. The classification of SYR‐III will be possible only after testing the phenotype in potato. The high prevalence of PVYNTN‐NW in potatoes and weeds, as well as its ability to induce PTNRD, demonstrates its importance and the necessity for its control.

The simultaneous differentiation of Potato virus Y strains including the newly described strain PVY NTN-NW by multiplex PCR assay

New recombinant strain and genotype of PVY, designated as PVY NTN-NW and SYR-III, respectively, shared properties with PVY NTN and PVY NW has been reported recently. PVY NTN-NW predominated in potato fields in Syria and was able to induce potato tuber necrotic ringspot disease (PTNRD). Due to the rapid spread of the recombinant strains of PVY which might be the case of PVY NTN-NW, a specific and reliable detection method is an essential step to control this strain and minimize its spread. The shared properties of PVY NTN-NW and SYR-III with PVY NTN and PVY NW, however, complicate their identification involving multiple detection methods. Therefore, a multiplex polymerase chain reaction (PCR), that relies on a combination of previously published and newly designed primers was developed for the detection and identification of PVY NTN-NW and SYR-III in single or mixed infections with the main PVY strains, PVY O, PVY N, PVY NTN and PVY NW. In addition, the present PCR assay was able to detect the recombination points in the P1 region enabling the differentiation of the variable genotypes of the recombinant strains PVY NTN-NW, PVY NTN and PVY NW. The reliability of this PCR assay was confirmed using a significant number of well characterized PVY isolates collected from Syria and Japan including those of PVY NTN-NW, SYR-III, PVY O, NA-PVY N, PVY NW and PVY NTN.

Occurrence of Potato Tuber Necrotic Isolates of Potato virus Y in a Commercial Tobacco Field in Southern Ontario, Canada.

Most strains of Potato virus Y (PVY) can infect tobacco plants (Nicotiana tabacum) and cause vein clearing followed by leaf mottling, except the PVYN strain, which induces severe vein necrosis. Some isolates within the PVYN strain also cause potato necrotic tuber ringspot disease, but these have not been reported from Canadian tobacco fields. PVYNTN isolates include European (EU) and North American (NA) types that are serologically identical to PVYN, but can be distinguished by nucleic acid-based assays and potato bioassay (1,2). Some PVY isolates, PVYN-Wi or PVYN:O, resulting from a recombination between RNA molecules of PVYN and the common strain, PVYO, are identified as PVYO in serological assays, but induce necrosis in tobacco (2). In August of 2007, two samples of tobacco (N. tabacum, unknown cultivar) leaves showing necrotic symptoms resembling those induced by PVYN, PVYNTN, or PVYN-Wi were collected from a tobacco field in southern Ontario, Canada and submitted to the Charlottetown Laboratory, Canadian Food Inspection Agency, Charlottetown, PE. Virus in both samples (PVY-204 and PVY-205) reacted with PVYN-specific antibodies 1F5 and 4E7 (3) and induced vein necrosis in tobacco (N. tabacum cv. Samsun). A multiplex reverse transcription (RT)-PCR assay (1) for the simultaneous detection and differentiation of various PVY strains amplified two fragments (181 and 452 bp) associated with EU-PVYNTN isolates. Restriction fragment length polymorphism (RFLP) analysis targeting the P1 and NIb gene (3) also indicated that PVY-204 and PVY-205 were EU-PVYNTN isolates. Known isolates of PVYO, PVYN, and NA-PVYNTN were used in all evaluations as references (3). Furthermore, the nucleotide sequences of the P1 and NIb genes of PVY-204 and PVY-205 determined by automated cycle sequencing (3) and subjected to phylogenetic analysis indicated that the nucleotide and deduced amino acid sequences of both isolates were 96 and 95% identical, respectively, to NA-PVYNTN isolates reported from Canada, but 99% identical (both nucleotide and amino acid) to EU-PVYNTN isolates from Europe and Mexico (3). Potato (cv. Yukon Gold) plants mechanically inoculated with leaf sap from tobacco (N. tabacum cv. Samsun) infected with PVY-204 and PVY-205 developed various leaf symptoms including severe local and systemic necrotic lesions, leaf wilting, and leaf death in 3 to 5 weeks postinoculation under greenhouse conditions. The infected plants recovered in 5 to 6 weeks. Potato (cv. Yukon Gold) plants inoculated with leaf sap from tobacco (N. tabacum cv. Samsun) infected with a PVYNTN isolate (HX8) (3) and healthy tobacco leaf sap were used as positive and negative controls. The number and yield of the tubers harvested from infected plants were significantly reduced (50%), and PVY-204 and PVY-205 induced typical potato tuber necrotic ringspot disease in 52.6% of the progeny tubers with an average disease index of 0.364 (C. Kerlan and K. Charlet-Ramage, EAPR Virology Meeting Proceedings, 1998). PVYNTN was detected by RT-PCR and RFLP in all necrotic tubers and 66.7% of the asymptomatic tubers. Some tubers (15.8%) harvested from the infected plants were negative in RT-PCR targeting either P1 protein gene or NIb gene and showed neither external nor internal necrotic symptom. To my knowledge, this is the first evidence of the occurrence of PVYNTN isolates in field-grown tobacco plants in Canada.