Transmission Efficiency of Potato virus Y strains PVYO and PVYN-Wi by Five Aphid Species

Development of an attenuated potato virus Y mutant carrying multiple mutations in helper-component protease for cross-protection

Tobacco (Nicotiana tabacum) is one of the major cash crops in China. Potato virus Y (PVY), a representative member of the genus Potyvirus, greatly reduces the quality and yield of tobacco leaves by inducing veinal necrosis. Mild strain-mediated cross-protection is an attractive method of controlling diseases caused by PVY. Currently, there is a lack of effective and stable attenuated PVY mutants. Potyviral helper component-protease (HC-Pro) is a likely target for the development of mild strains. Our previous studies showed that the residues lysine at positions 124 and 182 (K124 and K182) in HC-Pro were involved in PVY virulence, and the conserved KITC motif in HC-Pro was involved in aphid transmission. In this study, to improve the stability of PVY mild strains, K at position 50 (K50) in KITC motif, K124, and K182 were separately substituted with glutamic acid (E), leucine (L), and arginine (R), resulting in a triple-mutant PVY-HCELR. The mutant PVY-HCELR had attenuated virulence and did not induce leaf veinal necrosis symptoms in tobacco plants and could not be transmitted by Myzus persicae. Furthermore, PVY-HCELR mutant was genetically stable after six serial passages, and only caused mild mosaic symptoms in tobacco plants even at 90 days post inoculation. The tobacco plants cross-protected by PVY-HCELR mutant showed high resistance to the wild-type PVY. This study showed that PVY-HCELR mutant was a promising mild mutant for cross-protection to control PVY.

Infection Dynamics of Potato Virus Y Isolate Combinations in Three Potato Cultivars

The United States potato industry has recently experienced a strain shift; recombinant potato virus Y (PVY) strains (e.g., PVY^NTN) have emerged as the predominant strains over the long dominant ordinary strain (PVY^O), yet both are often found as single infections within the same field and as mixed infections within individual plants. To understand mixed infection dynamics in potato plants and in daughter tubers, three potato varieties varying for PVY resistance, 'Red Maria', 'CalWhite', and 'Pike', were mechanically inoculated either at the pre- or postflowering stage with all possible heterologous isolate combinations of two PVY^O and two PVY^NTN isolates. Virus titer was determined from leaves collected at different positions on the plant at different times, and tuber-borne infection was determined for two successive generations. PVY^NTN accumulated to higher levels than PVY^O at nearly all sampling time points in 'Pike' potato. However, both virus strains accumulated to similar amounts in 'Red Maria' and 'CalWhite' potato early in the infection when inoculated preflowering; however, PVY^NTN dominated at later stages and in plants inoculated postflowering. Regardless of inoculation time, both virus strains were transmitted to daughter plants raised from the tubers for most isolate combinations. The relative titer of PVY^NTN and PVY^O isolates at the later stages of mother plant development was indicative of what was found in the daughter plants. Although virus titer differed among cultivars depending on their genetics and virus isolates, it did not change the strain outcome in tuber-borne infection in subsequent generations. Differential virus accumulation in these cultivars suggests isolate-specific resistance to PVY accumulation.

Comparison of Mineral Oil, Insecticidal, and Biopesticide Spraying Regimes for Reducing Spread of Three Potato virus Y Strains in Potato Crops

In-field management of Potato virus Y (PVY) faces challenges caused by the changing availability and environmental acceptability of chemical agents to control aphid vectors of the virus and by proliferation of PVY strains with different symptoms and rates of spread. From 2018 to 2020, foliar spray treatments were compared in field experiments in New Brunswick, Canada, to measure effectiveness at reducing spread of PVY^O, PVY^N:O, and PVY^NTN strains. Mineral oil, insecticide, combined oil and insecticide spray, and a biopesticide (i.e., LifeGard WG) were compared. Insecticide-only and mineral oil-only treatments were not effective, but several combined oil and insecticide treatments and biopesticide treatments significantly reduced PVY spread. The biopesticide was proportionately more effective with recombinant PVY^N:O and PVY^NTN strains, possibly by exciting the plant's hypersensitive resistance response, caused naturally only in cultivar 'Goldrush' by PVY^O. Pesticide residue analysis showed that mineral oil increased the retention of pyrethroid insecticide in the potato foliage longer than with insecticide applied alone, which may explain the beneficial synergistic effect of combined sprays for reducing PVY spread. Tuber yields were generally unchanged in chemical insecticide treatments but were slightly lower in biopesticide treatment. The cost per PVY treatment was competitive across all effective treatments, including biopesticide; however, there was some revenue loss from lower yield with the biopesticide. This biopesticide is certified organic, however, and thus a small premium on the price for organic production could offset this yield deficit.

Virus and helper component interactions favour the transmission of recombinant potato virus Y strains

In recent years, several recombinant strains of potato virus Y, notably PVYNTN and PVYN:O have displaced the ordinary strain, PVYO, and emerged as the predominant strains affecting the USA potato crop. Previously we reported that recombinant strains were transmitted more efficiently than PVYO when they were acquired sequentially, regardless of acquisition order. In another recent study, we showed that PVYNTN binds preferentially to the aphid stylet over PVYO when aphids feed on a mixture of PVYO and PVYNTN. To understand the mechanism of this transmission bias as well as preferential virus binding, we separated virus and active helper component proteins (HC), mixed them in homologous and heterologous combinations, and then fed them to aphids using Parafilm sachets. Mixtures of PVYO HC with either PVYN:O or PVYNTN resulted in efficient transmission. PVYN:O HC also facilitated the transmission of PVYO and PVYNTN, albeit with reduced efficiency. PVYNTN HC failed to facilitate transmission of either PVYO or PVYN:O. When PVYO HC or PVYN:O HC was mixed with equal amounts of the two viruses, both viruses in all combinations were transmitted at high efficiencies. In contrast, no transmission occurred when combinations of viruses were mixed with PVYNTN HC. Further study evaluated transmission using serial dilutions of purified virus mixed with HCs. While PVYNTN HC only facilitated the transmission of the homologous virus, the HCs of PVYO and PVYN:O facilitated the transmission of all strains tested. This phenomenon has likely contributed to the increase in the recombinant strains affecting the USA potato crop.

Different potato virus Y strains frequently co-localize in single epidermal leaf cells and in the aphid stylet

Single aphids can simultaneously or sequentially acquire and transmit multiple potato virus Y (PVY) strains. Multiple PVY strains are often found in the same field and occasionally within the same plant, but little is known about how PVY strains interact in plants or in aphid stylets. Immuno-staining and confocal microscopy were used to examine the spatial and temporal dynamics of PVY strain mixtures (PVYO and PVYNTN or PVYO and PVYN) in epidermal leaf cells of 'Samsun NN' tobacco and 'Goldrush' potato. Virus binding and localization was also examined in aphid stylets following acquisition. Both strains systemically infected tobacco and co-localized in cells of all leaves examined; however, the relative amounts of each virus changed over time. Early in the tobacco infection, when mosaic symptoms were observed, PVYO dominated the infection although PVYNTN was detected in some cells. As the infection progressed and vein necrosis developed, PVYNTN was prevalent. Co-localization of PVYO and PVYN was also observed in epidermal cells of potato leaves with most cells infected with both viruses. Furthermore, two strains could be detected binding to the distal end of aphid stylets following virus acquisition from a plant infected with a strain mixture. These data are in contrast with the traditional belief of spatial separation of two closely related potyviruses and suggest apparent non-antagonistic interaction between PVY strains that could help explain the multitude of emerging recombinant PVY strains discovered in potato in recent years.

Valuing Disease Prevention in a Vegetatively Propagated Annual Crop: Benefits From the Montana Seed Potato Certification Program

We examine the economic costs of potato virus Y (PVY) and benefits to commercial potato growers from using screened seed. To do so, we use a quantile regression model to explore disease spread. We use this model to predict disease prevalence and corresponding losses in commercial potato operations with and without a screening and certification program in place. Our analysis suggests that this screening is very important; the amount of PVY in seed in the summer test is the strongest predictor of PVY in the winter test of the variables in our model. The amount of PVY in the seed can have major effects on commercial potato grower revenues and profitability. Using data and models from Idaho, a major purchaser of Montana seed, we estimate the annual benefit from Montana's program to Idaho to average $205 per acre or $22 million for the state.[Formula: see text] Copyright © 2020 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Transmission modes affect the population structure of potato virus Y in potato

Transmission is a crucial part of a viral life cycle and transmission mode can have an important impact on virus biology. It was demonstrated that transmission mode can influence the virulence and evolution of a virus; however, few empirical data are available to describe the direct underlying changes in virus population structure dynamics within the host. Potato virus Y (PVY) is an RNA virus and one of the most damaging pathogens of potato. It comprises several genetically variable strains that are transmitted between plants via different transmission modes. To investigate how transmission modes affect the within-plant viral population structure, we have used a deep sequencing approach to examine the changes in the genetic structure of populations (in leaves and tubers) of three PVY strains after successive passages by horizontal (aphid and mechanical) and vertical (via tubers) transmission modes. Nucleotide diversities of viral populations were significantly influenced by transmission modes; lineages transmitted by aphids were the least diverse, whereas lineages transmitted by tubers were the most diverse. Differences in nucleotide diversities of viral populations between leaves and tubers were transmission mode-dependent, with higher diversities in tubers than in leaves for aphid and mechanically transmitted lineages. Furthermore, aphid and tuber transmissions were shown to impose stronger genetic bottlenecks than mechanical transmission. To better understand the structure of virus populations within the host, transmission mode, movement of the virus within the host, and the number of replication cycles after transmission event need to be considered. Collectively, our results suggest a significant impact of virus transmission modes on the within-plant diversity of virus populations and provide quantitative fundamental data for understanding how transmission can shape virus diversity in the natural ecosystems, where different transmission modes are expected to affect virus population structure and consequently its evolution.

Efficient methods for the estimation of the multinomial parameter for the two-trait group testing model

Estimation of a single Bernoulli parameter using pooled sampling is among the oldest problems in the group testing literature. To carry out such estimation, an array of efficient estimators have been introduced covering a wide range of situations routinely encountered in applications. More recently, there has been growing interest in using group testing to simultaneously estimate the joint probabilities of two correlated traits using a multinomial model. Unfortunately, basic estimation results, such as the maximum likelihood estimator (MLE), have not been adequately addressed in the literature for such cases. In this paper, we show that finding the MLE for this problem is equivalent to maximizing a multinomial likelihood with a restricted parameter space. A solution using the EM algorithm is presented which is guaranteed to converge to the global maximizer, even on the boundary of the parameter space. Two additional closed form estimators are presented with the goal of minimizing the bias and/or mean square error. The methods are illustrated by considering an application to the joint estimation of transmission prevalence for two strains of the Potato virus Y by the aphid Myzus persicae.

Vector population growth and condition-dependent movement drive the spread of plant pathogens

Plant viruses, often spread by arthropod vectors, impact natural and agricultural ecosystems worldwide. Intuitively, the movement behavior and life history of vectors influence pathogen spread, but the relative contribution of each factor has not been examined. Recent research has highlighted the influence of host infection status on vector behavior and life history. Here, we developed a model to explore how vector traits influence the spread of vector-borne plant viruses. We allowed vector life history (growth rate, carrying capacity) and movement behavior (departure and settlement rates) parameters to be conditional on whether the plant host is infected or healthy and whether the vector is viruliferous (carrying the virus) or not. We ran simulations under a wide range of parameter combinations and quantified the fraction of hosts infected over time. We also ran case studies of the model for Barley yellow dwarf virus, a persistently transmitted virus, and for Potato virus Y, a non-persistently transmitted virus. We quantified the relative importance of each parameter on pathogen spread using Latin hypercube sampling with the statistical partial rank correlation coefficient technique. We found two general types of mechanisms in our model that increased the rate of pathogen spread. First, increasing factors such as vector intrinsic growth rate, carrying capacity, and departure rate from hosts (independent of whether these factors were condition-dependent) led to more vectors moving between hosts, which increased pathogen spread. Second, changing condition-dependent factors such as a vector's preference for settling on a host with a different infection status than itself, and vector tendency to leave a host of the same infection status, led to increased contact between hosts and vectors with different infection statuses, which also increased pathogen spread. Overall, our findings suggest that vector population growth rates had the greatest influence on rates of virus spread, but rates of vector dispersal from infected hosts and from hosts of the same infection status were also very important. Our model highlights the importance of simultaneously considering vector life history and behavior to better understand pathogen spread. Although developed for plant viruses, our model could readily be utilized with other vector-borne pathogen systems.

Effect of Potato virus Y Presence in Solanum tuberosum (Solanales: Solanaceae) and Chenopodium album on Aphid (Hemiptera: Aphididae) Behavior

This study establishes the effect of Potato Virus Y (PVY; Potyvirus) in potatoes, Solanum tuberosum L. (Solanales: Solanaceae) and in common-lambs' quarter Chenopodium album L. (Amaranthaceae) on Macrosiphum euphorbiae Thomas (Hemiptera: Aphididae) and Myzus persicae Sulzer (Hemiptera: Aphididae) behavior, host preference, transmission, and fitness under field and laboratory studies. In the field, several weeds, besides C. album, were collected, including: Sisymbrium altissimum L. (Brassicaceae), Erodium cicutarium L., Lactuca serriola L., Solanum sarrachoides Sendtner (Solanaceae), and S. dulcamara L. (Solanaceae). All weeds were serologically tested for the presence of PVY. From all weeds collected, 2.3 and 34% of C. album and S. sarrachoides, respectively, were PVY-positive. From those positive samples, 72% of the PVY found were PVYN; the remaining 28% was PVYO. In addition, several aphid species were collected from those weeds: Ovatus crataegarious Walker, Macrosiphum euphorbiae (Hemiptera: Aphididae), Hyalopterus pruni Geoffroy (Hemiptera: Aphididae), Rophalosiphum madis Fitch, and 'others aphid' species were collected. The highest number of aphids were collected in E. cicutarium, followed by S. dulcamara, L. serriola, S. altissimum, and C. album. In laboratory studies, PVY-infected C. album does not induce the production of aphids. Moreover, M. persicae did not appear to have a strong preference for either healthy or PVY-infected potato plant, but they did develop a preference for infected plants after prolonged feeding. M. persicae and M. euphorbiae transmitted PVY from C. album to S. tuberosum, 44 and 37.5 % of the time. Future research should seek to identify not only other sources and prevalence of PVY in the field but vector relationships. In insect-pathogen complex continues to persist in solanaceous field crops around the world.

Sequential acquisition of Potato virus Y strains by Myzus persicae favors the transmission of the emerging recombinant strains

In the past decade recombinant strains of Potato virus Y (PVY) have overtaken the ordinary strain, PVY^O, as the predominant viruses affecting the US seed potato crop. Aphids may be a contributing factor in the emergence of the recombinant strains, but studies indicate that differences in transmission efficiency of individual PVY strains either from single or mixed infections, although variable, are not generally significant. Multiple strains of PVY are present in all potato production areas and common in many potato fields. Therefore, it is likely that individual alate aphids moving through a potato field will sequentially encounter multiple strains as they "taste test" multiple potato plants while looking for a suitable host. This study examined the transmission likelihood and efficiency of three common PVY strains when acquired sequentially by individual aphids. Green peach aphids (Myzus persicae, Sulzer) were allowed a 2-3min acquisition access period (AAP) on potato leaves infected with PVY^O, PVY^N:O or PVY^NTN, followed by another 2-3min AAP on a second potato leaf infected with a different PVY strain before being transferred to healthy potato seedlings for a 24h inoculation access period. All possible combinations of the three strains were tested. Strain-specific infection of the recipient plants was determined by TAS-ELISA and RT-PCR 3-4wk post-inoculation. The recombinant strains, PVY^N:O and PVY^NTN, were transmitted more efficiently than PVY^O when they were sequentially acquired regardless of the order acquired. PVY^N:O and PVY^NTN were transmitted with similar efficiencies when they were sequentially acquired regardless of the order. The recombinant strains appear to preferentially bind to the aphid stylet over PVY^O or they may be preferentially released during inoculation. This may contribute to the increased incidence of the recombinant strains over PVY^O in fields or production regions where multiple PVY strains are detected.

Potato virus Y Transmission Efficiency from Potato Infected with Single or Multiple Virus Strains

There has been a recent shift in the prevalence of Potato virus Y (PVY) strains affecting potato with the ordinary strain PVY^O declining and the recombinant strains PVY^NTN and PVY^N:O emerging in the United States. Multiple PVY strains are commonly found in potato fields and even in individual plants. Factors contributing to the emergence of the recombinant strains are not well defined but differential aphid transmission of strains from single and mixed infections may play a role. We found that the transmission efficiencies by Myzus persicae, the green peach aphid, of PVY^NTN, PVY^N:O, and PVY^O varied depending on the potato cultivar serving as the virus source. Overall transmission efficiency was highest from sources infected with three virus strains, whereas transmission from sources infected with one or two virus strains was not significantly different. Two strains were concomitantly transmitted by individual aphids from many of the mixed-source combinations, especially if PVY^O was present. Triple-strain infections were not transmitted by any single aphid. PVY^O was transmitted most efficiently from mixed-strain infection sources. The data do not support the hypothesis that differential transmission of PVY strains by M. persicae is a major contributing factor in the emergence of recombinant PVY strains in the U.S. potato crop.

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 PVY^O 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 PVY^O was concomitant with the rise of the recombinant PVY^N-Wi strain incidence, from less than 27% of all PVY-positive plants in 2011 to 53% in 2015. The proportion of the PVY^NTN 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 PVY^O. Reduced levels of systemic movement of PVY^O in such cultivars would favor spread of recombinant strains in the field. The negative selection against the nonrecombinant PVY^O strain is likely caused by the presence of the Ny_tbr 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.

Interactions among potato genotypes, growth stages, virus strains, and inoculation methods in the potato virus Y and green peach aphid pathosystem

Potato virus Y (PVY) is an economically important and reemerging potato pathogen in North America. PVY infection reduces yield, and some necrotic and recombinant strains render tubers unmarketable. Although PVY(O) is the most prevalent strain in the United States, the necrotic and recombinant strains PVY(NTN) and PVY(N:O) are becoming more widespread. Infection rates in aphid-inoculated (Myzus persicae (Sulzer)) and mechanically inoculated plants were compared across two potato genotypes ('Yukon Gold' and A98345-1), three PVY strains (PVY(O), PVY(N:O), and PVY(NTN)), and two growth stages at inoculation (pre- and postflowering). Susceptibility of genotypes was measured as infection rate using a double-antibody sandwich-enzyme-linked immunosorbent assay; virus titer and tuber mass also were recorded from the infected plants. Yukon Gold generally was more susceptible than A98345-1 to all three PVY strains, especially following mechanical inoculation. Within genotypes, Yukon Gold was most susceptible to PVY(O) and A98345-1 was most susceptible to PVY(N:O). Plants exhibited age-based resistance, with both genotypes showing higher susceptibility at the pre- than postflowering stage. The overall ranking pattern of virus titer in infected plants was PVY(O) > PVY(NTN) > PVY(N:O); across all three strains, infected Yukon Gold had higher titer than infected A98345-1 plants. Yukon Gold plants had lower tuber mass than A98345-1 when infected, and there were differences between the two inoculation methods in regard to tuber mass for the three stains. The results showed differences in infection response between inoculation methods and as a function of genotype, strain, inoculation stage, and their interactions. These factors should be considered when screening genotypes for resistance.

Virus strains differentially induce plant susceptibility to aphid vectors and chewing herbivores

Plants are frequently attacked by both pathogens and insects, and an attack from one can induce plant responses that affect resistance to the other. However, we currently lack a predictive framework for understanding how pathogens, their vectors, and other herbivores interact. To address this gap, we have investigated the effects of a viral infection in the host plant on both its aphid vector and non-vector herbivores. We tested whether the infection by three different strains of Potato virus Y (PVY(NTN), PVY(NO) and PVY(O)) on tomato plants affected: (1) the induced plant defense pathways; (2) the abundance and fecundity of the aphid vector (Macrosiphum euphorbiae); and (3) the performance of two non-vector species: a caterpillar (Trichoplusia ni) and a beetle (Leptinotarsa decemlineata). While infection by all three strains of PVY induced the salicylate pathway, PVY(NTN) induced a stronger and longer response. Fecundity and density of aphids increased on all PVY-infected plants, suggesting that the aphid response is not negatively associated with salicylate induction. In contrast, the performance of non-vector herbivores positively correlated with the strength of salicylate induction. PVY(NTN) infection decreased plant resistance to both non-vector herbivores, increasing their growth rates. We also demonstrated that the impact of host plant viral infection on the caterpillar results from host plant responses and not the effects of aphid vector feeding. We propose that pathogens chemically mediate insect-plant interactions by activating the salicylate pathway and decreasing plant resistance to chewing insects, which has implications for both disease transmission and insect community structure.

Continuous and emerging challenges of Potato virus Y in potato

Potato virus Y (PVY) is one of the oldest known plant viruses, and yet in the past 20 years it emerged in the United States as a relatively new and very serious problem in potato. The virus exists as a complex of strains that induce a wide variety of foliar and tuber symptoms in potato, leading to yield reduction and loss of tuber quality. PVY has displayed a distinct ability to evolve through accumulation of mutations and more rapidly through recombination between different strains, adapting to new potato cultivars across different environments. Factors behind PVY emergence as a serious potato threat are not clear at the moment, and here an attempt is made to analyze various properties of the virus and its interactions with potato resistance genes and with aphid vectors to explain this recent PVY spread in potato production areas. Recent advances in PVY resistance identification and mapping of corresponding genes are described. An updated classification is proposed for PVY strains that takes into account the most current information on virus molecular genetics, serology, and host reactivity.