The behaviour of potato mop-top virus in soil, and evidence for its transmission by Spongospora subterranea (Wallr.) Lagerh.

Potato mop-top virus (PMTV) was best detected in field soils by air-drying them for more than a week before remoistening and growing seedlings of Nicotiana tabacum or N. debneyi for a 6–10 week period. Infection of N. tabacum was assessed by inoculating sap from roots and shoots to Chenopodium amaranticolor. Similar inoculations from N. debneyi were far less convenient for detecting PMTV than recording leaf symptoms, but slightly more efficient. Air-dry soil retained PMTV infectivity for 9 months, when passed through a 50 μ sieve or when diluted with 10³ but not 10⁴ parts of steamed soil. Tobacco seedlings were not infected when their roots were steeped in PMTV-containing tobacco sap. Infective soils contained Spongospora subterranea, spore balls of which resisted air-drying for more than a year and passed a 50 μ sieve. Roots of susceptible seedlings were infected with PMTV when exposed to spore balls of S. subterranea taken from powdery scabs on PMTV-infected potato tubers, or to suspensions obtained by steeping, in nutrient solution, roots infected with virus-carrying cultures of S. subterranea. Plants in several families were hosts of S. subterranea, but probabilities of infection when exposed to spore balls differed greatly between families and only species of Solanaceae were good hosts. The ten species infected with PMTV when grown in infective soil or when exposed to spore balls of S. subterranea taken from PMTV-infected potato tubers are all members of this family. PMTV seems to be carried internally in S. subterranea spore balls and survived in them for at least a year. PMTV was transmitted by S. subterranea to Arran Pilot potato, causing yellow blotches in some leaves and spraing in many tubers. However, when newly infected with PMTV in the field, not all Arran Pilot tubers developed spraing. Also, although many spraing-affected or symptomless but PMTV-infected tubers carried PMTV-containing spore balls of S. subterranea, powdery scabs were rarely found near the centres of the rings of primary spraing. PMTV became established in virus-free soil when PMTV-infected tubers carrying S. subterranea were planted as ‘seed’ but not when virus-free tubers bearing powdery scabs were used. 5. subterranea seems the main, and possibly the only, vector of PMTV in the soils examined. S. subterranea did not transmit potato aucuba mosaic virus from potato to N. debneyi or Capsicum annuum.     

Field and glasshouse experiments on the control of potato mop-top virus

Field observations during 3 yr on a stock of potato cv. Red Craigs Royal partially infected with potato mop-top virus (PMTV) confirmed that the virus was passed by an infected mother plant to only a proportion of its progeny tubers, and showed that in this cultivar symptomless plants gave rise only to symptomless progeny. The elimination of PMTV from stocks can therefore be greatly accelerated by removing symptom-bearing plants. Infected potato tubers were not freed from PMTV by treating them at 37 °C for up to 8 wk. Treating ‘seed’ tubers bearing powdery scabs that contain PMTV-carrying resting spores of Spongospora subterranea with formaldehyde or organo-mercurial fungicide greatly decreased PMTV establishment when the tubers were planted in previously uninfective soil, but fumigation with 2-aminobutane was ineffective. Decreasing the pH of infective soil to 5-0 by applying sulphur greatly decreased the infection of potato cv. Arran Pilot with PMTV and S. subterranea in field experiments, but this treatment did not eliminate either; when the pH of treated soil was raised the transmission of PMTV resumed. Treating infective soil with a range of fungicides greatly decreased the infection of Nicotiana debneyi bait seedlings in glasshouse experiments but only calomel at 75 kg/ha controlled spread of PMTV and 5. subterranea to potato in field experiments. In other field experiments, applying zinc frit, zinc sulphate or zinc oxide to infective soil greatly decreased the spread of both to potato. The amount of zinc required increased with increase in clay content of the soil. However, treatment with zinc compounds did not eliminate PMTV-carrying vectors from soil, and when treated soil was diluted with autoclaved soil many of the bait seedlings planted in the mixture became infected. The zinc frit was phytotoxic because of its boron content but zinc sulphate and zinc oxide caused little or no decrease in tuber yield. The zinc content of potato tubers was increased but not doubled in zinc-treated plots, and during the first year after treatment the zinc content of topsoil decreased greatly. The zinc content of ryegrass grown after potatoes was greater than of potato tubers but did not reach a level considered dangerous to livestock. Treatment of soil with sulphur, zinc oxide or calomel may be useful for small plots used in the early stages of propagation of virus-tested potato clones where there is risk of infection with PMTV.     

Ecological studies on potato mop-top virus in Scotland

Plants with symptoms of potato mop-top virus (PMTV) occurred in many commercial seed stocks of Arran Pilot and Red Craig's Royal potato in Scotland, but their incidence rarely exceeded 5%. In nuclear stocks of seed potatoes, most varieties examined in 1967 and 1968 were infected at one or more locality, but infected plants did not occur in all clones or at all stages of propagation of any one variety. infection of nuclear stocks resulted both from propagation on virus-infested land and from unwitting selection of infected plants to start new clones. PMTV was detected in farm soils ranging from light sands to heavy loams, in five Scottish counties. Soil was infested throughout the ploughed layer but the severity of infestation varied greatly within any one field; some sites of former potato clamps were heavily infested. PMTV was detected in field soil 12 years after potatoes were grown. In glasshouse tests many British crop and wild plants were colonized by Spongospora subterranea. Within some families all species tested were moderate to good hosts. (Solanaceae, Chenopodiaceae and Cruciferae), in others, species differed greatly in susceptibility (Compositae and Umbelliferae), and in a few, species were poor hosts or were not infected (Caryophyllaceae and Gramineae). Of the British crop and weed species that were moderate to good zoosporangial hosts of S. subterranea, only Solanum nigrum, potato, spinach and sugar beet were hosts of vector-borne PMTV. Potato probably survives between potato crops mainly in the resting spores of S. subterranea. PMTV was probably first brought to Europe with potatoes from South or Central America.     

Acquisition and transmission of potato mop-to furovirus by a culture of Spongospora subterranea f.sp. subterranea derived from a single cystosorus

Potato mop-top virus (PMTV) was detected by ELISA in primary zoospores from four out of six isolates of Spongospora subterranea f.sp. subterranea. One virus-free isolate (N) of S. subterranea was used to acquire PMTV from potato roots and to transmit the virus to healthy plants. A mono-fungal culture of S. subterranea (isolate N) was derived by infecting tomato plant roots with a single cystosorus. The culture was used successfully to acquire PMTV from the roots of infected Nicotiana debneyi plants that had been manually inoculated with virus isolates, and subsequently to transmit the virus to healthy bait plants. These experiments confirm that S. subterranea is a vector of PMTV. Two PMTV isolates that had been maintained by manual inoculation for 19 and 21 passages were also acquired and transmitted by the fungus culture.     

Production of Polyclonal Antibodies to a Recombinant Coat Protein of Potato mop-top virus

The coat protein (CP) coding regions of two Czech Potato mop‐top virus (PMTV) isolates were sequenced and shown to be identical. One, the Korneta isolate CP gene, was cloned in several expression vectors. The recombinant PMTV‐CP was expressed in Escherichia coli and the purified recombinant protein was used to produce PMTV‐specific polyclonal antibodies. The antiserum had a titre of 1 : 2000 in an indirect enzyme‐linked immunosorbent assay (ELISA) and reacted specifically in immunoblotting and IPTA‐ ELISA (indirect plate‐trapped antigen (PTA)‐ELISA).     

Molecular and pathobiological characterization of 61 Potato mop‐top virus full‐length cDNAs reveals great variability of the virus in the centre of potato domestication,novel genotypes and evidence for recombination

The evolutionary divergence of Potato mop‐top virus (PMTV), a tri‐partite, single‐stranded RNA virus, is exceptionally low, based on the analysis of sequences obtained from isolates from Europe, Asia and North America. In general, RNA viruses exist as dynamic populations of closely related and recombinant genomes that are subjected to continuous genetic variation. The reason behind the low genetic variation of PMTV remains unclear. The question remains as to whether the low variability is a shared property of all PMTV isolates or is a result of the limited number of isolates characterized so far. We hypothesized that higher divergence of the virus might exist in the Andean regions of South America, the centre of potato domestication. Here, we report high variability of PMTV isolates collected from 12 fields in three locations in the Andean region of Peru. To evaluate PMTV genetic variation in Peru, we generated full‐length cDNA clones, which allowed reliable comparative molecular and pathobiological characterization of individual isolates. We found significant divergence of the CP‐RT and 8K sequences. The 8K cistron, which encodes a viral suppressor of RNA silencing, was found to be under diversifying selection. Phylogenetic analysis determined that, based on the CP‐RT sequence, all PMTV isolates could be categorized into three separate lineages (clades). Moreover, we found evidence for recombination between two clades. Using infectious cDNA clones of the representatives of these two clades, as well as reassortants for the RNA‐CP genomic component, we determined the pathobiological differences between the lineages, which we coined as S (for severe) and M (for mild) types. Interestingly, all isolates characterized previously (from Europe, Asia and North America) fall into the S‐type clade, whereas most of the Peruvian isolates belong to the M‐type. Taken together, our results support the notion of the single introduction of PMTV from the centre of potato origin to Europe, and subsequent spread of the S‐type into Asia and USA. This is also supported by the suggested novel classification of isolates based on genetic constellations.     

Host range and some properties of potato mop-top virus

Potato mop-top virus (PMTV) was transmitted by inoculation of sap to twenty-six species in the Solanaceae or Chenopodiaceae and to Tetragonia expansa; species in eleven other plant families were not infected. The virus was cultured in inoculated leaves of Nicotiana tabacum cv. Xanthi-nc or in N. debneyi. Diagnostic local lesions were produced in Chenopodium amaranticolor. In winter, ten solanaceous species were slowly invaded systemically but the first leaves infected were those immediately above inoculated leaves. When transmitted to Arran Pilot potato by the vector Spongospora subterranea, PMTV induced all the main types of shoot and tuber symptoms found in naturally infected plants. Isolates of PMTV from different sources differed considerably in virulence. PMTV-containing tobacco sap lost infectivity when heated for 10 min at 80 °C, diluted to 10^-4, or stored at 20 °C for 14 weeks. Infectivity was partially stabilized by 0·02% sodium azide. When sap was centrifuged for 10 min at 8000 g, infectivity was mainly in the sediment. Infective sap contained straight rod-shaped particles about 20 nm wide, with lengths up to 900 nm and crossbands at intervals of 2·5 nm. Many of the particles were aggregated side-to-side, and the ends of most seemed damaged. The slight infectivity of phenol-treated leaf extracts was abolished by pancreatic ribonuclease. The present cryptogram of PMTV is R/*:*/*:E/E:S/Fu.     

Evidence that soilborne wheat mosaic virus moves long distance through the xylem in wheat

Summary Soilborne wheat mosaic virus (SBWMV) is a member of the genusFurovirus of plant viruses. SBWMV is transmitted to wheat roots by the plasmodiophorid vectorPolymyxa graminis. Experiments were conducted to determine the path for SBWMV transport from roots to leaves. The results of immunogold labeling suggest that SBWMV enters and moves long distance through the xylem. SBWMV may enter primary xylem elements before cell death occurs and then move upward in the plant after the xylem has matured into hollow vessels. There is also evidence for lateral movement between adjacent xylem vessels.Abbreviations SBWMV Soilborne wheat mosaic virus - TMV Tobacco mosaic virus - BMV Brome mosaic virus - PMTV Potato mop-top virus - BNYVV Beet necrotic yellow vein virus - WSSMV Wheat spindle streak mosaic virus - WSMV Wheat streak mosaic virus     

Information‐theory‐based model selection for determining the main vector and period of transmission of Potato virus Y

Potato virus Y (PVY, genus Potyvirus, family Potyviridae) is transmitted non‐persistently by aphids. It causes major losses in potato production (Solanum tuberosum), especially following seed tuber‐borne infection of plants. To limit the risk of PVY infection, seed potato production is located preferably in regions where vector pressure is low. The northern‐most high‐grade seed potato production area (HG zone) of Europe is in Finland. The aim of this study was to determine the incidence of aphid species with documented ability to transmit PVY and to use a modelling approach to determine their relative importance as vectors of PVY in the HG zone of Finland. Winged aphids were caught from six to seven potato fields in each of three growing seasons (2007–09) using yellow pan traps that were examined twice a week. Identification of more than 30 000 individuals indicated that 37% of the aphids belonged to nine species reported to transmit PVY. Incidence of PVY in seed lots was low (0–5.6%) and the seasonal increase of PVY incidence was also low in the potato crops. No potato‐colonising aphids were found on the plants in any of the years. The seasonal increase in PVY incidence was modelled using aphid counts in traps, the relative vector efficiencies of the aphids, virus resistance of cultivars, and the initial infection rate of the seed tubers as explanatory variables in generalised linear mixed modelling. Akaike's information criterion was employed to find the best set of explanatory variables for PVY in harvested tubers. Results of this modelling approach showed that the incidence of seed‐borne PVY infection and the early‐season vector flights are the most important factors contributing to the incidence of PVY in the yield. Compared to models with data from all potential vector species, models containing data from Aphis fabae only showed a better model fit with regard to the incidence of PVY in the harvested tubers. The explanatory power of the models was lost when A. fabae was omitted from the vector data, suggesting that other species play a negligible role as vectors of PVY in the HG zone of Finland. Results can be used to devise appropriate strategies for enhanced control of PVY.     

Characterization of Viruses Infecting Potato Plants from a Single Location in Shetland,an Isolated Scottish Archipelago

Sequence data were obtained from 29 isolates of Potato virus A (PVA), Potato virus S (PVS), Potato virus V (PVV) and Potato virus X (PVX) infecting nine tubers from Shetland, one of the most remote inhabited islands in the United Kingdom. These isolates were sequenced in the coat protein region, as were 29 Scottish mainland isolates of the same four potato virus species, and these 58 isolates were compared to previously published sequence data. This has allowed the characterization of viruses from a relatively isolated location, where there is little production of ware potatoes and no seed potato production. Phylogenetic homogeneity of the Shetland isolates of PVS and PVV was apparent. PVX was more heterogeneous, and Shetland isolates cluster with the Scottish isolates in a group which includes Asian and European isolates. For PVA, the majority of the Shetland and Scottish mainland isolates formed a predominantly Scottish grouping, with the remaining Shetland and Scottish mainland isolates clustering with a previously characterized Scottish isolate. There were three main groups of PVA, of which the Scottish grouping was the only one which did not have a fully characterized representative. To extend the characterization of PVA, the nucleotide sequence of the full polyprotein region encoding all the gene products of an isolate from Shetland was determined.     

The 18S rDNA sequence of Synchytrium endobioticum and its utility in microarrays for the simultaneous detection of fungal and viral pathogens of potato

Resting spores extracted from wart (Synchytrium endobioticum)-infected potato tubers were used for DNA extraction and amplification of 18S rDNA. Analysis of the cloned, sequenced fragment revealed high similarity to members of the Chytridiomycota. Using this information, specific oligonucleotide probes were designed and arrayed onto glass slides for detection of the pathogen. Viral sequence information available in the databank was retrieved, or new viral sequences were generated, and used to design probes for specific detection of important quarantine viruses of potato. To determine the sensitivity and specificity of the oligonucleotide probes, total RNA from infected plants was reverse transcribed, labelled with Cyanine 5, and hybridised with the microarray. A significant number of the oligonucleotide probes exhibited high specificity to S. endobioticum, Andean potato latent virus, Andean potato mottle virus, Potato black ringspot virus, and Potato spindle tuber viroid. Hybridisation signals of sub-arrays within slides were reproducible (r=0.79) with a high correlation coefficient of hybridisation repetitions (0.73). Our results demonstrate the potential of microarray-based hybridisation for identification of multiple pathogen targets, which will find application in quarantine laboratories, where parallel testing for diverse pathogens is essential.     

The effect of soil- and tuber-borne inoculum on the incidence of potato gangrene

Under optimum growing conditions neither tuber- nor soil-borne Phoma exigua var. foveata inoculum appreciably affected stand or yield of the subsequent potato crop. Seed tubers with gangrene rots caused high levels of stem and tuber symptoms when planted in var. foveata contaminated or uncontaminated land; contaminated seed tubers with no rots also produced progeny with a high gangrene potential. Sufficient soil-borne inoculum was carried over in land that produced a gangrene affected crop in the previous year to override the effect of tuber disinfection. Effective gangrene control was achieved by a combination of tuber disinfection shortly after harvest over successive years with a 1 in 5 yr potato crop rotation. Gangrene rots usually developed through injuries to the tuber periderm, rots in other tubers being associated with pustules of powdery scab (Spon-gospora subterranea).     

Nicotiana velutina mosaic virus: purification, properties and affinities with other rod-shaped viruses

Nicotiana velutina mosaic virus (NVMV), found in Australia, was transmitted by inoculation of sap to twenty species in the Solanaceae and Chenopodiaceae, and to Gomphrena globosa; its host range closely resembles that of potato mop-top virus (PMTV). Infectivity was abolished when sap was kept at room temperature between 1 and 4 days, or when heated for 10 min between 60 and 70 °C. NVMV was frequently transmitted through the seed of four Nicotiana spp. NVMV and PMTV were purified by a method that involved redissolving virus particles sedimented by low speed centrifugation of leaf extracts, followed by sedimentation through sucrose cushions. NVMV preparations contain rod-shaped particles about 18 nm wide and with a large range of lengths, the commonest being 125–150 nm. The particles have a helical structure with a pitch of 2–9 nm, break easily, and contain a single protein of apparent mol. wt. 21|400, slightly larger than that of PMTV (19 800). In serological tests assessed by electron microscopy, no relationship was detected between NVMV and PMTV, or barley stripe mosaic, beet necrotic yellow vein, soil-borne wheat mosaic, tobacco mosaic or tobacco rattle viruses. However, antiserum to soil-borne wheat mosaic virus reacted quite strongly with PMTV and weakly with tobacco mosaic virus. NVMV is considered to be a distinct member of the tobamovirus group; its frequent transmission through seed may be an adaptation to the arid environment where it was found. Its cryptogram is */*:*/*:E/E:S/*.     

Epidemiological analysis of risk factors for the spread of potato viruses in Switzerland

Virus diseases represent important economic threats to the production of seed potatoes worldwide, yet little quantitative information is available on the relative merits of different measures of virus control applied singly or in combination. In this study, we compiled data from the national seed certification programme on the incidence of Potato Virus Y (PVY) and Potato Leaf‐roll Virus (PLRV) in potato tubers in Switzerland for the years 1990–2009 and used generalised linear models to investigate the influence of key epidemiological factors on infection risk. Results showed that post‐harvest virus incidence increased with initial inoculum levels, with the largest change in infection risk occurring between fields with no inoculum and those with low levels of inoculum. Virus incidence decreased with increasing altitude of fields. Surprisingly, infection risk was considerably lower for imported seed lots even though the model controlled for the effect of inoculum level, potato variety and other confounders. Overall, variety was the most important factor influencing virus risk. The results of the present analysis are useful to fine‐tune decision‐support systems that predict disease risk under different epidemiological scenarios.     

Potato virus Y reduction by straw mulch in organic potatoes

Potato virus Y (PVY) is transmitted non-persistently by winged morphs of many aphid species and is a major problem in seed potato production. In order to evaluate the potential of straw mulch applications (4–5 t ha⁻¹) and presprouting on PVY reduction, small scale organically managed field experiments were carried out in Northern Hessen, Germany, over 3 yr. In all years mulching significantly reduced aphid infestation on leaves as well as PVY incidence in tubers. For the effect of presprouting the temporal coincidence of two factors was crucial – crop emergence and aphid flight activity. Presprouting decreased PVY incidence when in the phase of early crop emergence aphid spring flight activity was low, but increased it, although not significantly, when prominent aphid flight peaks occurred in this critical period. Straw mulch was most effective when vector pressure was concentrated early in the year acting as a PVY protectant for young plants. In later growth stages its effect declined gradually with increasing ground coverage of the crop. Combined mulching and presprouting had a synergistic, complementary effect on reduction of PVY incidence. In an on-farm experiment in 2001 scaling up the area mulched stepwise from 100 m² to 900 m² consistently kept aphid infestation at reduced levels.