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.     

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.     

Potato Cultivar and Seed Type Affect the Development of Systemic <Emphasis Type="Italic">Potato virus Y</Emphasis> (PVY<Superscript>N-Wi</Superscript>) Infection

Potato virus Y (PVY) infection is one of the greatest challenges to seed potato production in the United States. To determine how cultivar and seed type affect the development of systemic PVY infection, Russet Burbank and Russet Norkotah Colorado 3 cultivars were grown from two types of pre-nuclear seed (i.e., plantlets and minitubers) and Generation 3 (G3) tubers and challenged with PVY strain Wilga (PVY^N-Wi). Systemic PVY infection was measured by assaying spread of virus from the inoculation site to upper non-inoculated leaves. The Burbank cultivar had a lower incidence of systemic PVY infection compared to the incidence of systemic PVY that developed in the Colorado 3 cultivar. Furthermore, Burbank plants grown from G3 tubers had a lower incidence of systemic PVY infection, as compared to Burbank plants grown from plantlets. Together our results indicate that both cultivar and seed type affect the development of systemic PVY^N-Wi infections post-inoculation.     

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.     

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.     

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.     

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.     

Priming Potato with Thiamin to Control Potato Virus Y

Potato virus Y (PVY) is a major potato pathogen affecting potato yields worldwide. Thiamin, a water-soluble B vitamin (vitamin B₁) has been shown to boost the plant’s immunity, thereby increasing resistance against pathogens. In this study, we tested different concentrations of thiamin (1 mM, 10 mM, 50 mM, 100 mM) and multiple thiamin applications (once, biweekly and monthly) on potato resistance to PVY in Ranger Russet potatoes. Plants were mechanically inoculated with PVY^N:O. This PVY strain is known for causing well-defined foliar symptoms. We collected leaflets weekly through April and May 2015 and tested them with an enzyme-linked immunosorbent assay specific to PVY as well as by real time quantitative RT-PCR. These assays allowed us to determine the presence and level of PVY in different parts of the plants. We found that the highest thiamin concentration treatment (100 mM) produced the lowest virus level in potatoes across all dates and leaflet samples. Also, it was found that multiple applications of thiamin had a positive effect on reducing virus level, especially when thiamin was applied every four weeks.     

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.     

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.     

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.     

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.     

Effectiveness of Combined Use of Mineral Oil and Insecticide Spray in Reducing <Emphasis Type="Italic">Potato Virus Y</Emphasis> (PVY) Spread under Field Conditions in New Brunswick,Canada

In the 2014 and 2015 crop seasons, the efficacies of different types, rates and combinations of mineral oil and insecticide foliar sprays for reducing Potato virus Y (PVY) spread were tested in controlled field trials in New Brunswick (NB), Canada. Experimental plots were planted with certified PVY-free Goldrush, supplemented with known virus-infected seed to raise PVY inoculum to 2.3% and 3% at the beginning of the 2014 and 2015 seasons, respectively. Treatments consisted of mineral oil-only sprays at different application rates, insecticide-only sprays of differing numbers, and several combined mineral oil and insecticide spray regimes, all compared to a no-spray control treatment. PVY spread to 18% (2014) and 22% (2015) of initially virus-free plants in no-spray control plots, with significant reductions observed in PVY spread in several treatments. Greatest PVY reductions, as low as 4% (2014) and 12% (2015), were in combined mineral oil and insecticide spray treatments, followed by oil-only sprays; while insecticide-only sprays did not significantly reduce PVY spread. As well as measuring PVY spread to marked test plants and randomly collected post-harvest tuber sample from the plots, exhibited similar treatment pattern for PVY incidence. Multiple logistic regression modeling confirmed the relative efficacy of combined oil and insecticide sprays for reducing PVY spread, while accounting for variable inoculum and aphid factors. Modeling also highlighted the importance of planting low-PVY seed initially, and of early application of foliar sprays. Local best management practice recommendations for reduction of in-field PVY spread were discussed.     

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.     

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.     

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

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

Inheritance of extreme resistance to potato virus Y in potato

Summary Segregation for extreme resistance to PVY was evaluated in progenies derived from crossing two extremely resistant potato clones with parents differing in resistance. Resistance was evaluated after mechanical inoculation with PVY^O and PVY^N, and after graft inoculation with PVY^O. Biological and serological tests (ELISA) were used for virus detection. The extreme resistance is governed by a single dominant gene, but observed segregations deviated from the expected ratios. Considerable modifying effects were detectable, depending on the potato genotype and virus isolate, for a significant excess of susceptible genotypes was observed in some progenies. Moreover, genotypes with non-parental types of resistance to PVY were observed.     

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.     

Rate of spread of PVYn by alateMyzus persicae (Sulzer) from infected to healthy plants under laboratory conditions

Summary Alate green peach aphids,Myzus persicae (Sulzer), tested in a flight chamber during their maiden flight period displayed behaviours ranging from repeated trivial flights to settling on the plants. The interaction of alate vector density and PVYⁿ spread was dichotomous, virus spread was significantly related to vector density in some trials but virus spread was nil or limited and not significantly dependent on vector density in others. The green peach aphid colony used in these experiments provided a mixture of active and highly active alate populations. Results suggest that inactive and active vectors came from the active and highly active alate populations, respectively. Therefore, winged aphids within a species cannot all be attributed the same vector efficiency unless known to originate from the same population. At a 15% inoculum level the intercept for the regression model for the spread of PVYⁿ was 5.03% indicating that there is a significant probability of propagation at aphid densities as low as one. However, over the range of aphid densities tested, the rate of spread per aphid was low, 0.08%, suggesting that reinfection of newly infected plants or movement interference between aphid vectors rapidly became important factors negatively affecting virus spread. Although these results cannot be directly transferred to field conditions they provide confirmation that lowM. persicae numbers can transmit unacceptable levels of mosaic and that low inoculum levels are required to decrease the risk of transmission by the small aphid numbers which cannot be realistically controlled.