By continuing to browse this site you agree to us using cookies as described in About Cookies
Notice: Due to essential maintenance the subscribe/renew pages will be unavailable on Wednesday 26 October between 02:00- 08:00 BST/ 09:00 – 15:00 SGT/ 21:00- 03:00 EDT. Apologies for the inconvenience.
This study was undertaken to determine the current occurrence in Scottish seed potato crops of Potato mop-top virus (PMTV), which is transmitted by Spongospora subterranea and causes spraing (brown arcs and lines) in the flesh of potato tubers, rendering them unsaleable. In 2004, a stratified survey of four commonly grown cultivars was conducted, while in 2007 and 2008, only samples from powdery scab-affected crops were collected. The incidence of crops in which infection by PMTV was present was 37·5% in the stratified survey in 2004, but was greater in surveys in which tubers with powdery scab were tested (47·2% in 2007 and 44·6% in 2008). Similarly, the frequency of crops with incidences of more than 10% tuber infection was lower (9·4%) in 2004 than in 2007 (25·4%) and in 2008 (26·2%). Significant differences in crop infection were found amongst the four major seed-producing regions and the counties within these regions. The incidence of crop and tuber infection was least for class Pre-basic seed potatoes and greatest for class Super Elite 3 and Elite seed potatoes. The results indicate that the prevalence of PMTV has not increased since surveys in the early 1970s.
Potato mop-top virus (PMTV) typically produces slightly raised lines and rings on the tuber surface and/or brown arcs and lines, commonly described as spraing, in the flesh of tubers of sensitive potato cultivars (Calvert & Harrison, 1966; Harrison & Jones, 1971; Kurppa, 1989). Tubers with spraing are not acceptable for consumption or processing. Secondary infected plants, i.e. those grown from infected tubers, may also produce misshapen or cracked tubers (Calvert, 1968), often with reticulate surface cracking, sometimes known as elephant hide blemishing, on the skin (Tenorio et al., 2006). There is a cultivar effect on symptom severity. Foliar symptoms, which generally only affect a proportion of stems, can range from mild (pale yellow or yellow blotches, rings and chevrons on the leaves) to severe (shortening of the internodes, resulting in a dwarfed appearance –‘mop-top’) (Calvert, 1968). The development of foliar symptoms is affected by environmental conditions such as temperature (Calvert, 1968; Carnegie et al., 2010a) and is rare in some countries, e.g. Scandanavian countries except Norway (Santala et al., 2010). As well as symptomless foliar infection, symptomless infection also affects a high proportion of tubers (Latvala-Kilby et al., 2009).
Jones & Harrison (1969) demonstrated that the plasmodiophorid powdery scab pathogen, Spongospora subterranea, was a vector in the transmission of PMTV, and Arif et al. (1995) demonstrated conclusively that PMTV could be acquired from the roots of infected plants by S. subterranea and then subsequently transmitted to healthy bait plants.
PMTV was first reported on potato in Scotland by Todd (1965) and elsewhere in the UK by Calvert & Harrison (1966) and it has subsequently been shown to be widespread in other cool-climate Northern European countries (Denmark (Nielsen & Molgaard, 1997); Finland (Kurppa, 1989); Sweden (Ryden et al., 1989; Sandgren, 1995); Norway (Bjornstad, 1969)). Although the virus has been reported in other countries (Czechoslovakia (Novak et al., 1981); the Netherlands (van Hoof & Rozendaal, 1969); Germany (EPPO, 2009); Costa Rica (Montero-Astúa et al., 2008)), its occurrence often seems to be restricted in distribution, for example, confined to wetter areas of higher altitude (Novak et al., 1981). In Costa Rica, Montero-Astúa et al. (2008) found that PMTV was most prevalent at altitudes >2500 m a.s.l. and they associated infection with cooler temperatures and higher soil moisture. The global distribution of PMTV appears to be much more limited than that of powdery scab (Merz, 2008), even though infection by PMTV is intrinsically linked to the presence of S. subterranea. Following its initial discovery in Scotland, limited surveys of the prevalence of PMTV have been conducted. Jones & Harrison (1972) reported that in 1967 and 1968, 24 out of 37 seed potato crops of the PMTV-sensitive cultivars Arran Pilot and Red Craig’s Royal included plants with PMTV foliar symptoms. In a survey of 224 seed potato crops in 1971 and 1972, Cooper & Harrison (1973) found 103 crops (46%) including plants with PMTV foliar symptoms and/or tubers with spraing. PMTV-affected crops were also found to be more common in Angus, Fife, Kincardineshire, Perthshire and Kinross-shire than in other counties of Scotland. Since these surveys, the Scottish seed potato production system has changed considerably, with clonal selection being replaced by the use of tissue-cultured pathogen-tested initial planting material, ensuring its freedom from a range of pathogens such as PMTV (Carnegie, 1992). This change was complemented by a flush-through certification scheme in which seed production was limited to a maximum of 10 field generations (in practice usually four to six). In the first stage, class Pre-basic (PB) multiplication is limited to a maximum of 4 years, although seed stocks can move before this to lower quality classes by downgrading, or at the request of the grower. The two classes of Basic category seed potatoes are Super Elite (SE) and Elite, each with a maximum of three allowable multiplications, indicated by the generation number after the class designation. However, the flexibility in class movement means that the number of generations that a seed crop has been grown in the field cannot be deduced from the generation number, except for class PB. Seed potatoes are predominantly grown on the east coast of Scotland north of 55°N (Fig. 1), where temperatures during the growing season are generally below 20°C and photoperiod exceeds 15 h for at least 5 months (Hay et al., 2000).
A series of surveys were therefore conducted between 2004 and 2008 to assess the current prevalence of the virus rather than PMTV symptoms in Scottish seed potato crops because of the known occurrence of symptomless infection. In 2004, the sampling was stratified for four commonly grown cultivars and four regions. In 2007 and 2008, sampling was targeted at the prevalence of PMTV in powdery scab-affected tubers from seed potato crops in Scotland in order to examine more accurately the extent to which crops known to be exposed to S. subterranea become infected with PMTV.
Materials and methods
The occurrence of PMTV in tubers of seed potato crops of four commonly grown potato cultivars: Maris Piper, Hermes, Saturna and Nicola, occupying 20, 10, 2·8 and 2·5% of Scottish seed area, respectively, was undertaken in 2004. Sampling was confined to class SE seed potatoes, which occupied 82% of Scottish seed area and covered 16 counties, representing four main seed potato-producing regions of Scotland (Fig. 1). As far as possible, 10 crops of each of the four cultivars were selected at random for each region from the 2004 Scottish Seed Potato Register, with the exception that no more than one crop was selected from a farm. However, in some cases, it was not possible to obtain 10 crops of a cultivar from a region, so all available crops of the cultivar in that region were sampled. From each selected crop, inspectors from Scottish Government Rural Payments and Inspections Directorate collected 200 seed-sized tubers (c. 35–55 mm) at random into new paper sacks. Tubers were stored at c. 4°C until all samples were received.
2007 and 2008 surveys
Samples of tubers from any seed potato crops affected by powdery scab were assessed for the incidence of tuber infection by PMTV. Samples were drawn primarily from SE crops with smaller numbers from PB and Elite classes. Inspectors were requested to collect 30 seed-sized tubers affected by powdery scab from one crop on each farm used by a grower in 2007 and 2008. On receipt at Science and Advice for Scottish Agriculture (SASA), crop details were recorded and tubers stored in new bags at c. 4°C until testing.
Tuber testing for PMTV
Cores of tuber tissue (25–30 mm long × 5 mm in diameter), were taken from the rose- and stolon-ends of individually numbered, largely dormant tubers using a cork borer (No. 2) sheathed in Nalgene tubing 3 cm from the tip to give a standard depth of tissue. The cork borer was rinsed in water between tubers to prevent cross-contamination. The two cores were placed into a Bioreba homogenization bag (BIOREBA AG) and 5 μL tuber extraction buffer (0·01 m phosphate buffered saline containing 0·05% Tween 20, 2% polyvinylpyrrolidone and 1% albumin from chicken egg white) was added to each Bioreba bag and samples homogenized using a Homex 5 homogenizer (BIOREBA AG).
Enzyme-linked immunosorbent assay (ELISA)
Two hundred microlitres of homogenized sample were added to each well of 96-well, polystyrene microtitre plates (Nunc) pre-coated with a PMTV-specific monoclonal antibody (SASA). All samples were analysed in duplicate and positive and negative tuber control material was added to each plate. Following overnight incubation at 4°C, plates were washed [0·1 m PBS containing 0·5% Tween 20 (aq., v/v)] and a PMTV alkaline phosphatase conjugate (SASA) added and incubated for 2 h at 37°C. The enzyme substrate p-nitrophenyl phosphate (Sigma) was then added at 1 mg mL−1. Absorbance was measured at 405 nm after 1 h of incubation at room temperature using a microplate reader (Dynatech Laboratories). Samples were deemed positive when mean OD values were greater than twice that of the negative controls.
The χ2 (chi-square) test of significance, based on contingency tables, was used to examine the effects of cultivar, region, county and seed class on the frequency of PMTV in crop samples of seed potatoes. When the frequency of virus for a factor was zero or close to zero, the datum was excluded from an analysis.
Data on rainfall in months between March and October was obtained from Met Office UK for five stations covering some of counties from which samples were derived: Nairn, Nairnshire; Kinloss, Moray; Dyce, Aberdeenshire; Montrose, Angus and Leuchars, Fife.
The number of seed potato crops sampled for each region and cultivar is shown in Table 1. Infection by PMTV was detected in 48 out of 128 crops sampled. The incidence of tuber infection by PMTV within each crop was generally low, with 90% of the infected crops having <10% infected tubers (Table 2). High incidences of infection were recorded in only three crops (2·4%): two crops of cv. Nicola with 52 and 82% infection, and one of cv. Maris Piper with 59%.
Table 1. Number of seed potato crops sampled for each cultivar and region in 2004
Northern (Caithness, Sutherland, Ross & Cromarty, Inverness-shire, Moray, Nairnshire)
Borders (Berwickshire, East Lothian, Roxburghshire, Dumfriesshire)
Table 2. Distribution of the incidence of Potato mop-top virus (PMTV) infection in samples of tubers from 128 seed potato crops in 2004, 197 in 2007 and 65 in 2008
Tubers infected by PMTV (%)
Crop samples in which PMTV was detected in tubers (%)
Although PMTV was more prevalent in crops of cv. Nicola (59%) than those of the other cultivars (mean 32%), this difference was not significant ( = 7·1). The incidence of crops infected by PMTV was greatest for the Central region and least for the Borders (Table 3). The number of crops was generally too small to allow statistical analysis to be conducted for the various counties within each region; however, within the North-East region, the incidence of crop infection by PMTV was greater for crops in Kincardineshire than those in Aberdeenshire and Banffshire (Table 3). The incidence of crop infection for Caithness, Sutherland, Ross & Cromarty and Inverness-shire was similar to that for Aberdeenshire. In crops from the Central region, the mean incidence of tuber infection was at least three times greater for crops from Angus than for those from Perthshire and Fife.
Table 3. Incidence of Potato mop-top virus (PMTV) infection in samples of tubers of seed potatoes in relation to region and county of origin in 2004
A total of 203 crop samples (of which six were duplicates) of tubers with powdery scab were tested for PMTV by ELISA. The mean of each duplicate crop sample was used for the statistical analysis. PMTV was detected in 47·2% of crops (Table 2). Of these, seven crops (3·5%) had more than 40% of tubers infected, with the greatest incidence being 80% in a sample of cv. Wilja seed potatoes from Perthshire.
Factors affecting occurrence of PMTV
Cultivar Samples were collected from a total of 53 cultivars, but five or more samples were received for only 12 cultivars. The greatest numbers of samples were received for cv. Maris Piper (32) and for cv. Estima (23) (Table 4). The incidence of crop samples infected by PMTV was greatest for cvs Premiere and Cara (83 and 80%, respectively) and lowest (20%) for cvs Hermes and Kennebec, but the differences amongst the cultivars were not significant. However, the mean incidence of tuber infection was greatest (20%) for cv. Nadine, with 75% of tubers being infected in one crop sample (Table 4).
Table 4. Incidence of Potato mop-top virus (PMTV) in samples of powdery scab-affected tubers of seed potatoes of various cultivars in 2007
No. of samples
Crop samples in which PMTV was detected in tubers (%)
Mean tubers infected by PMTV (%)a
NS, not significant.
aRange of PMTV infection is shown in parentheses.
Class of seed potatoes The greatest number of samples (78) was received for class SE2 (Super Elite generation 2) seed potatoes (Table 5). For the statistical analysis, samples of all generations within class PB were grouped together, as were those of class Elite. The incidence of infected crop samples was least for class PB, with 21·4% being infected, and greatest for class SE3, with 65% of crops containing infected tubers. With class PB seed potatoes, the incidence of infected tubers in a crop was never more than 5% and this is reflected in the low mean incidence of tuber infection compared with those for the other classes. The mean incidence of tuber infection tended to be greater in seed potatoes of classes SE3 and Elite than in those of first and second generation of class SE.
Table 5. Incidence of Potato mop-top virus (PMTV) in samples of powdery scab-affected tubers of various classes of seed potatoes in 2007
No. of samples
Crop samples in which PMTV was detected (%)
Mean tubers infected by PMTV (%)b
aSE1, SE2 and SE3: Super Elite generations 1, 2 and 3, respectively.
bRange of PMTV infection is shown in parentheses.
Region and county For the initial statistical analysis, the data were grouped into the four regions as used for the 2004 survey. Significant differences in the incidence of infected crop samples were found amongst the four regions (Table 6). PMTV was least prevalent in seed crops from the Borders and most prevalent in those from the Central region. However, more detailed analysis revealed significant differences amongst the three counties making up the Central and North-East regions. The numbers of samples for each county in the other regions were too low to permit analysis by county. In the North-East region, the incidence of PMTV in crop samples and in tubers was greater for seed potatoes from Kincardineshire than for those from Aberdeenshire or Banffshire. The occurrence and incidence of tuber infection for seed potatoes from Kincardineshire was similar to that for those from Angus and Fife. In the Central region, PMTV was more prevalent in samples and tubers from Perthshire than those from Angus or Fife.
Table 6. Incidence of Potato mop-top virus (PMTV) infection in samples of powdery scab-affected tubers of seed potatoes in relation to region and county of origin in 2007
PMTV was detected in 44·6% of 65 crop samples received (Table 2). The greatest amount of tuber infection was 78·5% in a crop of cv. Almera. Most crop infection by PMTV occurred within the range of >0 to 10% tuber infection, with around 55% of infected crops being in this band (Table 2).
Factors affecting occurrence of PMTV
The low number of samples limited the extent to which statistical comparisons could be made.
Cultivar The samples covered 37 cultivars, with the greatest number being 11 samples for cv. Estima, followed by eight for cv. Markies and five for cv. King Edward. There were, therefore, an insufficient number of samples for a range of cultivars to permit an analysis to be made. Five crops of cv. Estima, six of cv. Markies and four of cv. King Edward were infected by PMTV. The mean incidence of tuber infection for these three cultivars was 4·8, 11·7 and 13·5%, respectively.
Class of seed potatoes Only one sample of PB seed potatoes was received, which was from a PB4 crop of cv. Markies with 16·7% tuber infection. The incidence of crops infected by PMTV did not differ amongst the SE generations or between SE and Elite classes (Table 7). However, the incidence of tuber infection within crops was at least twice as great for SE3 and Elite crops as for the SE1 and SE2 generations.
Table 7. Incidence of Potato mop-top virus (PMTV) in samples of powdery scab-affected tubers of various classes of seed potatoes in 2008
No. of samples
Crop samples in which PMTV was detected in tubers (%)
Mean tubers infected by PMTV (%)
NS, not significant.
aSE1, SE2 and SE3: Super Elite generations 1, 2 and 3, respectively.
Region and county The incidence of crops infected by PMTV did not differ significantly amongst the regions, although, as in 2007, it was lower for the Northern and North-East regions than for the Central region (Table 8). The low numbers received in 2008 did not allow meaningful comparisons to be made between counties. The mean incidence of tuber infection for samples from the most westerly Borders county of Dumfriesshire was 19·7%, which was amongst the greatest for any county. No samples were received from this county in 2007.
Table 8. Incidence of Potato mop-top virus (PMTV) infection in samples of powdery scab-affected tubers of seed potatoes in relation to region and county of origin in 2008
The greatest amount of rainfall between March and October occurred in 2004, when a mean of 564 mm fell at the five stations. However, the pattern of rainfall for this period over the 5 years did not correlate with the rainfall occurring between June and August, probably the key period for PMTV infection. The greatest amount of summer rain was 287 mm in 2007 and the least was 146 mm in 2006.
The prevalence of PMTV in Scottish seed potato crops was less in 2004 than in 2007 and 2008. The differences amongst the years may have been attributable to the targeted sampling of crops affected by powdery scab in 2007 and 2008. In these crops, the presence of powdery scab indicated that there was an opportunity for infection by PMTV, if present. In 2004, crops and tubers without powdery scab may have been selected and thus may not have been at risk of PMTV infection. Environmental conditions may also have affected the differences in crop infection by PMTV amongst the years. Summer rainfall, which is a key determinant for infection by S. subterranea (Merz, 2008) and hence PMTV, was high in 2004 and 2007, but in 2008 it was only c. 62% of the mean of those 2 years. This may explain the lower number of crop samples submitted in 2008 but not the difference in PMTV prevalence amongst the years.
The mean incidence of crop infection over the 3 years was 45%, which is similar to the 46% recorded by Cooper & Harrison (1973) on the basis of plants with symptoms of PMTV infection. This suggests that the prevalence of PMTV in seed potato crops has not changed markedly since the early 1970s. By contrast, Santala et al. (2010) reported that in Sweden, where PMTV is widespread, the virus had spread northwards since 1991 over a 15- to 20-year period, although it had not been found in the specialist seed potato region in the far north around Umea. However, PMTV was already found to be fairly common in most potato areas of Scotland in early 1970s (Cooper & Harrison, 1973). The virus probably continues to be introduced into clean soils by the planting of seed tubers carrying viruliferous soil or sporeballs (Davey et al., 2008), but the surveys in the present study were not suitable for identifying such introductions. Brierley et al. (2008) detected S. subterranea in 62% of soils sampled from fields intended for commercial potato production and PMTV in only 20·5% of these soils, which indicates that PMTV was present in only 35% of soils infested by S. subterranea. This is broadly similar to the findings in the 2007 and 2008 surveys, in which PMTV was detected in only 46% of powdery scab-affected samples, and would indicate that the proportion of field populations of S. subterranea that carry PMTV is relatively low, despite the virus having been widespread for many years. This is somewhat surprising given the close linkage between infection by S. subterranea and PMTV.
The occurrence of crop infection also appeared to be affected by class and by chronological age within the SE seed class. Infection was least frequent on class PB seed potatoes. This may be related to the requirement in Scotland that PB seed potatoes are grown on land with a rotational interval of at least 7 years since the previous potato crop, with virgin land often being used for the first two generations of PB production. By contrast, the minimum interval for the Basic classes SE and Elite is 5 years. Another factor may be the greater proportion of seed potato crops entered for PB certification in the Northern and North-East regions, where PMTV appeared to be less prevalent. However, these explanations do not appear to account for all the differences, because the incidence of crop infection on Basic crops was lower on younger SE1 and SE2 crops than on older SE3 and Elite crops, despite the minimum rotational interval being the same for all. In the absence of new infection from soil inoculum, the proportion of plants with foliar symptoms and of infected tubers in a crop will decline rapidly with each cycle of vegetative propagation but the virus may not be fully eliminated from a stock (Carnegie et al., 2010b). This residual PMTV may thus be more prevalent in older Basic stocks and contribute to increased virus development in these crops under favourable conditions.
Davey et al. (2008) recorded high occurrences of tuber infection of PMTV in only 2% of 143 seed potato crops of cv. Cara tested between 2004 and 2006 and associated these with infection from soilborne inoculum and not with planting of PMTV-infected seed tubers. A similarly low proportion of high incidences of tuber infection were recorded in the surveys in the present study. This indicates that although PMTV is widespread in Scotland, potentially economic damaging outbreaks of PMTV infection may be relatively uncommon, although the risk may differ with region of production. Cooper & Harrison (1973) reported that crops with PMTV symptoms were most prevalent in seed potatoes from the region covering the counties of Angus, Fife, Kincardineshire, Kinross-shire and Perthshire. Regional differences were confirmed in the present surveys. In addition, differences amongst individual counties within regions were demonstrated, with the highest incidences of infected crops in 2007 being recorded for Perthshire and Dumfriesshire. These higher incidences of infected crops were accompanied by higher incidences of tuber infection within the crops.
Cooper & Harrison (1973) indicated that PMTV was two and a half times more prevalent in crops grown in fields in which the water deficit was <25 mm than in those in which it was more than 25 mm. Furthermore, they also reported that crops infected by PMTV increased as annual rainfall increased from 760 to 1140 mm. The use of annual rainfall as a measure of whether yearly growing conditions are likely to be favourable for PMTV infection seems somewhat crude, as infection by the virus is likely to be affected primarily by rainfall in the period of tuber initiation, rather than at other times of the year (Sandgren, 1995). The amount of rainfall occurring between June and August was reported by Brierley et al. (2008) to be a good indicator of the risk of powdery scab development. In their study, powdery scab was more prevalent and severe on crops in 2007 than in 2005 and 2006, and summer rainfall was 78% greater in 2007 than in the preceding 2 years. However, this relationship seems less clear with PMTV because Davey et al. (2008) found the incidence of tuber infection exceeding 4% in 9·7% of 31 crops of cv. Cara in 2004, in 19% of 84 crops in 2005 and in 25% of 28 crops in 2006, despite the mean amount of summer rainfall at the five meteorological stations used in the present study being 269 mm in 2004, compared with 153 and 146 mm in 2005 and 2006, respectively.
Carnegie et al. (2009) demonstrated that cultivars differed less in their susceptibility to primary infection by PMTV than in their sensitivity to spraing development. In the 2004 survey, cv. Nicola appeared to be more susceptible to PMTV infection than cvs Maris Piper and Hermes, which was in agreement with the results of a comparative experiment at a site in Yorkshire, where soilborne inoculum pressure was low (Carnegie et al., 2009). In the 2007 survey, cvs Maris Piper and Hermes again appeared to be relatively resistant to PMTV infection in crops, but the variable and often small number of samples received for each cultivar did not allow statistical differences to be detected. Overall, this suggests that infection by PMTV in seed potato crops is unlikely to be influenced by cultivar to any great extent.
We thank Mr R. Holmes of the Virology and Zoology Section, SASA for conducting the ELISA tests; Dr Rosalind McHugh for extracting the meterological data; the Potato Council Ltd for financial support for Triona Davey; and the inspectors of the Rural Payments and Inspections Directorate, who collected the samples.