Potato spindle tuber viroid (spindle tuber of potato)
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- Risk of Introduction
- Hosts/Species Affected
- Growth Stages
- List of Symptoms/Signs
- Biology and Ecology
- Seedborne Aspects
- Economic Impact
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Potato spindle tuber viroid
Preferred Common Name
- spindle tuber of potato
Other Scientific Names
- potato gothic virus
- potato spindle viroid
- spindle tuber viroid
- tomato bunchy top viroid
International Common Names
- English: bunchy top of tomato
Local Common Names
- Germany: Spindelknollenkrankheit
- PSTVD0 (Potato spindle tuber pospiviroid)
Taxonomic TreeTop of page
- Domain: Virus
- Unknown: Viroids
- Family: Pospiviroidae
- Genus: Pospiviroid
- Species: Potato spindle tuber viroid
Notes on Taxonomy and NomenclatureTop of page
Viroids are small (246-375 nucleotides), single-stranded, covalently closed, circular, unencapsidated RNAs characterized by a highly base-paired, rod-like secondary structure (Diener, 1971, 1979, 1987). They are classified as subviral agents, and the name viroid is derived from the name given to the virus-like agent causing potato spindle tuber disease by Diener (1971) and Diener and Raymer (1971). All viroids contain the -Vd ending in their abbreviation to distinguish them from viruses (Owens et al., 2011). The presence or absence of both a Central Conserved Region (CCR) and hammerhead ribozymes are the main criteria for discriminating the two viroid families i.e., Avsunviroidae and Pospiviroidae. In addition to the type of CCR, the presence of either a Terminal Conserved Hairpin (TCH) or Region (TCR) allocates members of the Pospiviroidae into five genera, one of which is the genus Pospiviroid with Potato spindle tuber viroid (PSTVd) as its type species. An arbitrary level of less than 90% sequence identity and distinct biological properties are the main criteria for separating viroid species within a genus. This classification is supported by phylogenetic analyses (Owens et al., 2011).
DescriptionTop of page
PSTVd is a small, unencapsidated, covalently closed, circular RNA of circa 359 nucleotides. Variants consisting of 356-363 nucleotides have been described (Gross et al., 1978; Puchta et al., 1990; Lakshman and Tavantzis, 1993; Behjatnia et al., 1996; Verhoeven et al., 2010b). Electron micrographs reveal a rod-like conformation of 37+/-6 nm in length of the renatured state. In the denatured state, rod-like molecules as well as completely open circles are found (Riesner et al., 1979).
DistributionTop of page
PSTVd has been reported to occur in potato fields in northern USA, Canada, China, the former USSR and Turkey (Singh et al., 1970, 1991, 1993b; Tien, 1985; He et al., 1987; Güner et al., 2012). However, successful eradication of the viroid in seed potato production has been reported for the USA and Canada (Sun et al., 2004; De Boer et al., 2005). PSTVd was detected in the UK and was eliminated from potato accessions in the Commonwealth Potato Collection (Cammack and Richardson, 1963; Scottish Plant Breeding Station, 1976). It was detected and eradicated in breeding material in South Australia (Schwinghamer et al., 1983; Cartwright, 1984). In South America, PSTVd was reported in Argentina (Fernandez-Valiela, 1965) and in Peru, Venezuela and Brazil (Singh, 1983). It was detected in 1994 on breeding germplasm at Balcarce Experimental Station, INTA, Argentina, and was immediately quarantined and eradicated. Subsequent surveys (1994-1999) failed to detect the viroid and COSAVE and MERCOSUR now recognize Argentina as a country free from PSTVd. For additional information on geographic distribution in potato, see Salazar (1989).
In addition, erratic PSTVd outbreaks have been reported in tomato in e.g. Australia, Belgium, Italy, Japan, New Zealand, The Netherlands, UK, USA (Puchta et al., 1990; Elliott et al., 2001; Mackie et al., 2002; Mumford et al., 2004; Verhoeven et al., 2004, 2007; Matshushita et al., 2008; Navarro et al., 2009; Ling and Sfetcu, 2010). Furthermore, in many European countries PSTVd infections have been found in a substantial number ornamental crops of Brugmansia spp. and Solanum jasminoides (EFSA Panel on Plant Health, 2011). The emergency measures taken by the EU appear to have significantly reduced the PSTVd incidence in both crops but the viroid has not yet been eradicated everywhere (De Hoop et al., 2008; EFSA Panel on Plant Health, 2011). Other natural hosts include pepper in New Zealand (Lebas et al., 2005), various ornamentals in Europe (Verhoeven et al., 2008b; Mertelik et al., 2010; Lemmetty et al., 2011), Physalis peruviana in Germany, Turkey and New Zealand (Verhoeven et al., 2009; Ward et al., 2010), wild Solanum spp. in Australia (Behjatnia et al., 1996) and India (Owens et al., 1992), Solanum muricatum in Czech Republic (Mertelik et al., 2010), avocado in Peru (Querci et al., 1995) and dahlia in Japan (Tsushima et al., 2011).
A record of PSTVd in Chile (Shamloul et al., 1997) published in previous versions of the Compendium has been removed as the isolate (designated PSTVd-P) was a variant of PSTVd recorded on an asymptomatic plant (Solanum muricatum) and differed in five positions from the type strain of PSTVd in potato. There are no records of PSTVd on potato in Chile as the pathogen has not been detected in potato surveys, neither national seed potato certification by Servicio Agrícola y Ganadero (SAG) or by the Institute of Agricultural Research (INIA), Chile. PSTVd is a quarantine pest for Chile (Servicio Agrícola y Ganadero, 2013).
See also CABI/EPPO (1998, No. 324).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Afghanistan||Present||CABI/EPPO, 2014; EPPO, 2018|
|Azerbaijan||Present||CABI/EPPO, 2014; EPPO, 2018|
|Bangladesh||Present||CABI/EPPO, 2014; EPPO, 2018|
|China||Restricted distribution||CABI/EPPO, 2014; EPPO, 2018|
|-Hebei||Present||CABI/EPPO, 2014; EPPO, 2018|
|-Heilongjiang||Present||Singh et al., 1993b; Tien, 1985; CABI/EPPO, 2014; EPPO, 2018|
|-Jiangsu||Present||CABI/EPPO, 2014; EPPO, 2018|
|-Qinghai||Present||Tien, 1985; CABI/EPPO, 2014; EPPO, 2018|
|Georgia (Republic of)||Present||CABI/EPPO, 2014; EPPO, 2018|
|India||Present, few occurrences||He et al., 1987; Owens et al., 1992; CABI/EPPO, 2014; EPPO, 2018|
|-Himachal Pradesh||Present||Owens et al., 1992; CABI/EPPO, 2014; EPPO, 2018|
|-Maharashtra||Present||CABI/EPPO, 2014; EPPO, 2018|
|Iran||Present, few occurrences||Arezou et al., 2008; CABI/EPPO, 2014; EPPO, 2018|
|Israel||Absent, unreliable record||CABI/EPPO, 2014; EPPO, 2018|
|Japan||Present, few occurrences||Takahashi, 1987; CABI/EPPO, 2014; EPPO, 2018|
|-Honshu||Present, few occurrences||EPPO, 2012; EPPO, 2018|
|Turkey||Present, few occurrences||Bostan et al., 2010; CABI/EPPO, 2014; EPPO, 2018|
|Egypt||Present||CABI/EPPO, 2014; EPPO, 2018|
|Kenya||Absent, unreliable record||EPPO, 2018|
|Nigeria||Present||CABI/EPPO, 2014; EPPO, 2018|
|South Africa||Absent, unreliable record||Pietersen, 1985; CABI/EPPO, 2014; EPPO, 2018|
|Canada||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Alberta||Absent, formerly present||CABI/EPPO, 2014; EPPO, 2018|
|-British Columbia||Absent, formerly present||CABI/EPPO, 2014; EPPO, 2018|
|-Manitoba||Absent, formerly present||CABI/EPPO, 2014|
|-New Brunswick||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Newfoundland and Labrador||Absent, formerly present||CABI/EPPO, 2014|
|-Nova Scotia||Absent, formerly present||CABI/EPPO, 2014|
|-Ontario||Absent, unreliable record||CABI/EPPO, 2014; EPPO, 2018|
|-Prince Edward Island||Eradicated||Singh et al., 1988b; CABI/EPPO, 2014; EPPO, 2018|
|-Quebec||Absent, unreliable record||CABI/EPPO, 2014; EPPO, 2018|
|-Saskatchewan||Absent, formerly present||CABI/EPPO, 2014|
|Mexico||Present||CABI/EPPO, 2014; EPPO, 2018|
|USA||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Alaska||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-California||Eradicated||Ling and Sfetcu, 2010; EPPO, 2014; CABI/EPPO, 2018|
|-Colorado||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Idaho||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Kansas||Absent, invalid record||EPPO, 2018|
|-Maine||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Maryland||Absent, invalid record||EPPO, 2018|
|-Michigan||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Minnesota||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Mississippi||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Montana||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Nebraska||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-New Hampshire||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-New York||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-North Carolina||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-North Dakota||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Ohio||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Oregon||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Washington||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Wisconsin||Eradicated||Singh et al., 1993b; CABI/EPPO, 2014; EPPO, 2018|
|-Wyoming||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
Central America and Caribbean
|Costa Rica||Present||CABI/EPPO, 2014; EPPO, 2018|
|Cuba||Absent, unreliable record||CABI/EPPO, 2014; EPPO, 2018|
|Dominican Republic||Present||Ling et al., 2014; EPPO, 2018|
|Argentina||Eradicated||SENASA, personal communication, 2008; Fernandez and Calderoni, 1965; CABI/EPPO, 2014; EPPO, 2018|
|Brazil||Absent, unreliable record||Singh, 1983; CABI/EPPO, 2014; EPPO, 2014|
|-Sao Paulo||Present||Luigi et al., 2016|
|Chile||Absent, invalid record||CABI/EPPO, 2014; EPPO, 2018|
|Peru||Present||Singh, 1983; Querci et al., 1995; CABI/EPPO, 2014; EPPO, 2018|
|Uruguay||Absent, confirmed by survey||EPPO, 2018|
|Venezuela||Present||Singh, 1983; CABI/EPPO, 2014; EPPO, 2018|
|Austria||Present, few occurrences||CABI/EPPO, 2014; EPPO, 2018|
|Belarus||Widespread||CABI/EPPO, 2014; EPPO, 2018|
|Belgium||Present, few occurrences||CABI/EPPO, 2014; EPPO, 2018|
|Bulgaria||Absent, formerly present||CABI/EPPO, 2014; EPPO, 2018|
|Croatia||Present, few occurrences||CABI/EPPO, 2014; EPPO, 2018|
|Czech Republic||Present, few occurrences||Mertelik et al., 2010; Cervená et al., 2011; CABI/EPPO, 2014; EPPO, 2018|
|Denmark||Absent, unreliable record||EPPO, 2018|
|Estonia||Absent, confirmed by survey||EPPO, 2018|
|Finland||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|France||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|Germany||Present, few occurrences||CABI/EPPO, 2014; EPPO, 2018|
|Greece||Transient: actionable, under eradication||CABI/EPPO, 2014; EPPO, 2014; EPPO, 2018|
|-Crete||Transient: actionable, under eradication||CABI/EPPO, 2014; EPPO, 2018|
|Hungary||Transient: actionable, under eradication||CABI/EPPO, 2014; EPPO, 2018|
|Ireland||Absent, formerly present||CABI/EPPO, 2014; EPPO, 2018|
|Italy||Present, few occurrences||EPPO, 2011; CABI/EPPO, 2014; EPPO, 2018|
|-Italy (mainland)||Present, few occurrences||CABI/EPPO, 2014|
|Lithuania||Absent, confirmed by survey||IPPC, 2016; EPPO, 2018|
|Malta||Present, few occurrences||IPPC, 2013; CABI/EPPO, 2014; EPPO, 2018|
|Montenegro||Present||Luigi et al., 2016; EPPO, 2018|
|Netherlands||Present, few occurrences||IPPC, 2014a; IPPC, 2014b; NPPO of the Netherlands, 2013; Verhoeven et al., 2008a; CABI/EPPO, 2014; EPPO, 2016; EPPO, 2018|
|Poland||Present, few occurrences||****||EPPO, 2011; CABI/EPPO, 2014; EPPO, 2016; EPPO, 2018|
|Portugal||Absent, unreliable record||EPPO, 2014; EPPO, 2018|
|Russian Federation||Present||CABI/EPPO, 2014; EPPO, 2018|
|-Central Russia||Present||CABI/EPPO, 2014; EPPO, 2018|
|-Northern Russia||Present||CABI/EPPO, 2014; EPPO, 2018|
|-Russian Far East||Present||CABI/EPPO, 2014; EPPO, 2018|
|-Southern Russia||Present||CABI/EPPO, 2014; EPPO, 2018|
|Slovakia||Absent, intercepted only||EPPO, 2018|
|Slovenia||Present, few occurrences||IPPC, 2007a; Marn and Plesko, 2012; CABI/EPPO, 2014; EPPO, 2018|
|Spain||Present||CABI/EPPO, 2014; EPPO, 2018|
|Switzerland||Absent, formerly present||CABI/EPPO, 2014; EPPO, 2018|
|UK||Transient: actionable, under eradication||EPPO, 2011; IPPC, 2011; CABI/EPPO, 2014; EPPO, 2018|
|-England and Wales||Transient: actionable, under eradication||CABI/EPPO, 2014; EPPO, 2018|
|-Scotland||Absent, intercepted only||EPPO, 2018|
|Ukraine||Present||CABI/EPPO, 2014; EPPO, 2018|
|Australia||Restricted distribution||CABI/EPPO, 2014; EPPO, 2018|
|-Australian Northern Territory||Eradicated||Behjatnia et al., 1996; CABI/EPPO, 2014; EPPO, 2018|
|-New South Wales||Eradicated||2001||Cartwright, 1984; CABI/EPPO, 2014; EPPO, 2018|
|-Queensland||Present, few occurrences||EPPO, 2018|
|-South Australia||Eradicated||Schwinghamer et al., 1983; Cartwright, 1984; CABI/EPPO, 2014; EPPO, 2018|
|-Victoria||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|-Western Australia||Eradicated||CABI/EPPO, 2014; EPPO, 2018|
|New Zealand||Present, few occurrences||Ward et al., 2010; CABI/EPPO, 2014; EPPO, 2018|
Risk of IntroductionTop of page Risk Criteria Category
Economic Importance Moderate
Seedborne Incidence High
Seed Transmitted Yes
Seed Treatment None
Overall Risk Low
Hosts/Species AffectedTop of page
Due to serious symptoms and large scale outbreaks, potato is considered the main host of PSTVd. However, many more hosts are known. The viroid also causes symptoms in tomato and pepper (Capsicum annuum) (Mackie et al., 2002; Lebas et al., 2005). In addition symptomless infections have been reported from avocado (Persea americana), Brugmansia spp., Chrysanthemum sp., Calibrachoa sp., Cestrum spp., Dahlia sp., Datura sp. Lycianthes rantonnei, Petunia sp., Physalis peruviana, Solanum pseudocapsicum, Streptosolen jamesonii, Solanum jasminoides, Solanum muricatum, sweet potato (Ipomoea batatas) and wild Solanum spp. (Salazar, 1989; Owens et al., 1992; Querci et al., 1995; Behjatnia et al., 1996; Di Serio, 2007; Verhoeven et al., 2008a, b, 2009, 2010b; Lemmetty et al., 2011; Luigi et al., 2011; Mertelik et al., 2010; Verhoeven, 2010; Tsushima et al., 2011). The experimental host range of PSTVd includes a wide range of Solanaceous species, as well as species from other families (Singh, 1973; Diener, 1979).
Growth StagesTop of page Flowering stage, Fruiting stage, Vegetative growing stage
SymptomsTop of page
In potato, PSTVd can induce severe growth reduction; however, reduction may also be hardly visible. Vines of infected plants may be smaller, more upright, and produce smaller leaves than their healthy counterparts. Infected tubers may be small, elongated (from which the disease derives its name), misshapen and cracked. Their eyes may be more pronounced than normal and may be borne on knob-like protuberances that may even develop into small tubers. Symptom expression is influenced by the potato cultivar, strain of PSTVd, environmental conditions and method of inoculation (Pfannenstiel and Slack, 1980; Diener, 1987; Owens and Verhoeven, 2009).
The first symptoms of PSTVd infection in tomato are growth reduction and chlorosis in the top of the plant. Subsequently, this growth reduction may develop into stunting, and the chlorosis may become more severe, turning into reddening and/or purpling. In this stage, leaves may become brittle. Generally, this stunting is permanent; occasionally, however, plants may either die or partially recover. As stunting begins, flower and fruit initiation stop. Generally, the disease spreads along the rows (Mackie et al., 2002; Owens and Verhoeven, 2009).
Peppers display only very mild symptoms in response to PSTVd infection. The only visible symptom is a certain 'waviness' or distortion of the leaf margins near the top of infected plants (Lebas et al., 2005).
List of Symptoms/SignsTop of page
|Leaves / abnormal forms|
|Roots / reduced root system|
|Whole plant / dwarfing|
Biology and EcologyTop of page
PSTVd infects all, or most parts, of the susceptible plant (Diener, 1987; Weidemann, 1987). It is very efficiently transmitted by propagation of infected tubers, cuttings, and micro-plants. For example, the widespread occurrence of PSTVd in Russia in the 1980s and 1990s was contributed to the large-scale propagation of infected in vitro plants (Owens et al., 2009). Once established in a vegetatively propagated crop, the PSTVd infection is persistent; therefore, plants from infected lots act as a permanent source of inoculum for other lots and crops. Vegetative propagation has been the major pathway for PSTVd transmission in potato and ornamentals such as Brugmansia spp. and Solanum jasminoides. The absence of symptoms increases the risk that infected plants will be used for propagation (Owens and Verhoeven, 2009).
In addition, PSTVd is readily transmitted by normal cultivation activities. Bonde and Merriam (1951) found that transmission could occur when infectious sap contaminated cutting knives used for tuber propagation, with contamination of young tuber sprouts with infectious sap resulting in the highest percentage of infection. Merriam and Bonde (1954) found 80-100% field transmission of PSTVd to healthy potato plants when tractor wheels became contaminated with infectious sap due to bruising of diseased potato plants. Manzer and Merriam (1961) found nearly 100% transmission with cultivating and hilling equipment, where large plants were present, and very little transmission when plants were smaller and when cultivation was performed earlier in the season. Verhoeven et al. (2010a) showed transmission of PSTVd by sap, contaminated fingers and contaminated knives from either Brugmansia suaveolens or S. jasminoides to both potato and tomato. However, S. jasminoides was a better source of inoculum than B. suaveolens, and tomato was more susceptible than potato. Moreover, transmission to potato failed and was only rarely successful for tomato at a continuous temperature of 15°C.
PSTVd can also be transmitted through botanical seed and pollen of tomato and potato (Benson and Singh, 1964; Hunter et al., 1969; Singh, 1970; EUPHRESCO, 2011). Once present in a germ bank, the viroid can be transmitted to other (wild) potato plants either mechanically or by pollen exchange. Seed is also a potential source of infection for other crops such as tomato and pepper that are propagated by seed.
Finally, PSTVd transmission by aphids has been reported. De Bokx and Piron (1981) demonstrated a low rate of transmission in tomato by the aphid Macrosiphum euphorbiae but not by Myzus persicae or Aulacorthum solani. However, M. persicae transmitted PSTVd in plants doubly infected with PSTVd and Potato leafroll virus (PLRV) (Salazar et al., 1995). It has subsequently been shown that heterologous encapsidation of PSTVd in particles of PLRV occurs, and may explain the observed insect transmission. This has important implications for the epidemiology and spread of PSTVd in potato fields (Querci et al., 1997).
Viroids are low molecular weight infectious nucleic acids. They replicate autonomously in susceptible plant hosts (Diener, 1987). Pospiviroids are predominantly located in the nuclei of infected cells and they replicate via a rolling circle mechanism (Diener, 1987).
The epidemiology of PSTVd is complicated because of the large number of host species and multiple potential transmission routes. For vegetatively propagated crops such as potato and ornamentals, the main mode of spread is propagation by infected seed potatoes and/or cuttings. Transmission by contaminated machinery or aphids carrying both PSTVd and PLRV may result in further spread. The spindle tuber disease was first described by Martin (1922) in potato fields in New Jersey, USA, and was subsequently reported in Maine (Martin, 1922; Schulz and Folsom, 1923). It is speculated that the disease was imported into New Jersey from Maine in infected planting material. Similarly, PSTVd infections in potato in China and South Korea are assumed to have been introduced via infected seed potatoes from North America (Singh et al., 1993b).
Contaminated seed is an important source of infection for crops such as tomato that are grown from seed. In addition, the increased use of true potato seed (TPS) for potato propagation is another potential avenue for spread of the disease if measures are not maintained to certify disease-free status (Singh and Crowley, 1985a; Salazar, 1989). However, other routes of transmission should also be considered. For example, comparison of the respective nucleotide sequences of PSTVd indicated connections between PSTVd infections in several lots of tomato and infections in lots of the ornamental S. jasminoides. As the vegetatively propagated plants of S. jasminoides are kept in the greenhouse year-round, they can serve as a permanent source of inoculum for tomato crops, which are removed from the greenhouse after every production cycle. This indicates that S. jasminoides was the original source of PSTVd infection in various outbreaks in tomato (Navarro et al., 2009; Verhoeven et al., 2010b).
Seedborne AspectsTop of page
PSTVd has been found in true potato seed (TPS) of several potato germplasm collections and breeding lines (Cammack and Harris, 1973; Salazar et al., 1983; Grasmick and Slack, 1986, 1987; Shamloul et al., 1997). It has also been eliminated from these collections (Scottish Plant Breeding Station, 1976; Schwinghamer et al., 1983). PSTVd has been reported at levels of up to 70% in seed lots of TPS from inbred lines in China, and was found in TPS stored for 21 years (Singh et al., 1991).
Singh et al. (1992) pollinated the flowers of healthy Katahdin plants with pollen from PSTVd-infected plants resulting in infection of leaves at the base of inflorescences, apical leaves and tubers. Electrophoretic analysis of fruits indicated sporadic PSTVd infection of sepals, fruit skin and fruit pulp. True potato seed from each fruit was 35-66% infected with PSTVd. Percentage of seed infection varied with individual plants and was not affected by the location of an inflorescence or by the number of fruits produced on the same inflorescence.
Effect on Seed Quality
Benson and Singh (1964) reported that seed obtained from tomato infected with PSTVd was smaller, and rates of germination were reduced by 24-48%. Grasmick and Slack (1986) monitored the effect of PSTVd infection on several aspects of sexual reproduction, seed set and seed germination in cultivated potato. Their results showed that for some cultivars, infected maternal plants increased frequency of fruit development and seed weight compared with the healthy controls, and that true potato seed from viroid-infected crosses germinated at a higher rate than did seed from non-infected parents.
PSTVd is readily transmitted through botanical seed (TPS) and pollen of tomato and potato. Efficiency of transmission varied between 6 and 87% for potato and between 2 and 11% for tomato (Benson and Singh, 1964; Hunter et al., 1969; Singh, 1970; Kryczynski et al., 1992; EUPHRESCO, 2011).
Disinfection of tomato seeds using sodium hypochlorite, hydrochloric acid or pectinase did not prevent seed transmission (EUPHRESCO, 2011).
Seed Health Tests
In general, the methods discussed in the section on Diagnosis can be applied to detect PSTVd in true seeds. However, infection rates in seed lots and viroid concentrations in individual seeds vary and may be very low. Consequently, sensitive methods are required to allow reliable detection in samples of adequate numbers of seeds. For both potato and tomato, Hoshino et al. (2006) reported the detection of PSTVd in a mixture of one infected seed in 2000 non-infected seeds. However, seeds had to be grinded in groups of 400. Using 10 seeds from PSTVd-infected tomato plants in mixtures with either 90 or 990 non-infected seeds, the viroid could clearly be detected by RT-PCR and real-time RT-PCR (EUPHRESCO, 2011). However, mixing a single seed from an infected tomato plant with the same numbers of non-infected seeds produced variable results, probably due to different viroid concentrations per seed. In this case, realtime RT-PCR performed better than conventional RT-PCR. Very crucial in PSTVd testing is seed disruption and homogenization prior to RNA extraction. Ambiguities in detecting PSTVd in seeds were attributed to variable concentrations of PSTVd in/at seeds but also to the processing of seed extracts (EUPHRESCO, 2011).
ImpactTop of page
Several reports attribute losses in potato yield to infection by PSTVd. Le Clerg et al. (1944) determined the effect of different amounts of PSTVd on yield of marketable tubers. In 31 trials with five varieties in several states of the USA, a 4% level of infection resulted in a loss of marketable tubers of 2.6%, which includes effects on the yield and market quality of the crop. Up to 24% reductions in tuber yield have been reported in cultivar Saco infected with mild strains of PSTVd (Singh et al., 1971). However, the severe strain reduced the yield by up to 64%. Estimates of up to 4.6% incidence in cultivar Russet Burbank were observed. On the basis of a number of factors, Singh et al. (1971) estimated an overall loss of 1% of the potato crop. Studies by Pfannenstiel and Slack (1980) showed that reduction of tuber weight depended on the potato cultivar and the length of time they were infected with PSTVd. There was a significant decrease in tuber weight per plant from the first to the third year that the tubers were infected.
Also for tomato, the number of infected plants and the plant age at the time the infection starts are important factors for the total yield loss. Reported infection rates vary from a few plants to 10% (Elliott et al., 2001; Hailstones et al., 2003; Verhoeven et al., 2007). Main symptoms are severe chlorosis and stunting. In general, no more fruits will be produced when stunting starts (Owens and Verhoeven, 2009). In pepper, no obvious effects on plant vigour and fruit production were observed (Lebas et al., 2005). Also in ornamental crops, no yield and quality losses were observed for PSTVd (Verhoeven et al., 2008a, b, 2010b; Luigi et al., 2011).
Economic ImpactTop of page
Soliman (2012) estimated the cost of an unregulated PSTVd infestation in Europe to cost 4.4 million euros for potatoes and 5.7 million euros for tomatoes.
DiagnosisTop of page
PSTVd is readily transmissible by mechanical means to reliable indicator species, including Solanum berthaultii and tomato (Raymer et al., 1964; Fernow et al., 1969; Singh, 1984; Grasmick and Slack, 1987). Mechanically inoculated tomato plants develop the symptoms described above in 2-4 weeks following inoculation of cotyledons, depending upon the strain of PSTVd, viroid concentration, and environmental conditions. High temperature and high light intensity result in more obvious symptoms (Harris and Browning, 1980). However, tomato plants infected with mild strains may not develop symptoms of viroid disease even though the viroid concentration may be quite high. Fernow (1967) developed a cross-protection test to identify the presence of mild viroid strains, but since the advent of nucleic acid-based laboratory technologies, this test is not currently used. The biological indicator test is quite sensitive, but it requires large amounts of greenhouse bench space and time for symptoms to appear, if they appear at all.
Singh (1971) described a local lesion host, Scopolia sinensis, for the detection of viroid infection after inoculation. However, this indicator host requires specific reaction conditions for symptom development and may therefore be a poor indicator host for reliable and reproducible indexing for PSTVd (Kahn, 1989; Salazar, 1989).
Nucleic Acid Based Methods
PAGE techniques have been developed for the detection and discrimination of PSTVd strains from small amounts of tissue (Morris and Wright, 1975; Schumann et al., 1978; Schumacher et al., 1986). Morris and Wright (1975) described the first diagnostic application of PAGE for detection of PSTVd. The PAGE test is based on the migration of circular PSTVd RNA from host cellular RNAs on denaturing gels. Depending upon the resolution and sensitivity required, the viroid RNA is visualized by staining with ethidium bromide or silver nitrate, or with nucleic acid probes following transfer to solid support membranes. Modifications of the so-called bi-directional, two-dimensional, or return gel PAGE techniques have led to increased resolution and sensitivity (Singh and Boucher, 1987; Singh et al., 1988a; Roenhorst et al., 2000).
Singh et al. (1993a) detected both mild and severe strains of PSTVd in potato with return PAGE, but found only mild strains in pollen. TPS were not doubly infected with the two strains (Singh and Boucher, 1987). Schumann et al. (1978) compared the tomato bioassay and gel electrophoresis and found the latter to be more reliable.
Nucleic acid hybridization
With the determination of the nucleotide sequence of PSTVd (Gross et al., 1978), the molecular cloning of cDNA copies was made feasible. Owens and Cress (1980) prepared full-length copies of PSTVd in bacterial plasmids, the DNA of which was then labelled with radioactive tags for detection of PSTVd on a solid support. The first report of a membrane filter hybridization technique for detection of PSTVd utilized radioactively labelled, 32P DNA probes (Owens and Diener, 1981). Using this method, PSTVd detection was shown to be 10 times more sensitive and reliable than PAGE. Since that first report, many modifications of the technique have been reported, including use of non-radioactive, biotinylated and 32P cRNA probes for nucleic acid spot hybridization of plant extracts (Salazar et al., 1983, 1988a; Lakshman et al., 1986; Roy et al., 1989; Candresse et al., 1990; Singh et al., 1994) to non-radioactive, chemiluminscent, digoxigenenin-labelled cRNA probes for nucleic acid spot hybridization and tissue printing (Podleckis et al., 1993; OEPP/EPPO, 2004) and fluorescent-labelled probes using glass slide hybridization in stead of nylon membrane hybridization (Du et al., 2007). Using their own variants of nucleic acid hybridization, Salazar could detect PSTVd in 1 infected potato tuber in 80 healthy tubers, Singh et al. (1994) in dilutions up to 16,384 times, and Khan et al. (2009) RNA preparations down to only 1 µg of potato leaf tissue. However, the probes are generally not specific as they may hybridize with other pospiviroids (Jeffries and James, 2005). The non-specific nature of the assay was used by Torchetti et al. (2012) to develop a polyprobe that detects at least eight pospiviroids including PSTVd.
Singh and Crowley (1985b) and Huttinga et al. (1987) compared the techniques of PAGE and molecular hybridization for PSTVd detection. Singh and Crowley (1985b) found that dot blot hybridization could reliably detect PSTVd at levels that were beyond the sensitivity of PAGE, whereas Huttinga et al. (1987) determined that the two methods were equally sensitive. PAGE allows detection of a range of viroids and depending on the probes used hybridizations assays are not fully specific for PSTVd either (Verhoeven et al., 2004; Jeffries and James, 2005).
Polymerase chain reaction (PCR)
The technique of reverse transcription-polymerase chain reaction (RT-PCR) has been developed for the detection of PSTVd in potato tubers, leaves, pollen and true seeds (Shamloul et al., 1997; OEPP/EPPO, 2004). In addition, the technique also successfully detected PSTVd in various other plant species, i.e., pepino, pepper, tomato and various ornamentals (Shamloul et al., 1997; Mackie et al., 2002; Lebas et al., 2005; Verhoeven et al., 2008a). The method is very sensitive but, concerning PSTVd, generally not specific for identification to the species level because the related viroids Mexican papita viroid (MPVd), Tomato chlorotic dwarf viroid (TCDVd) and Tomato planta macho viroid (TPMVd) may be detected with the primers developed for PSTVd (Singh et al., 1999; Verhoeven et al., 2011). Unequivocal identification, therefore, requires sequencing of complete genome PCR-products followed by BLAST analysis. In addition to the more PSTVd-specific RT-PCRs, generic RT-PCRs for detection of a broad range of pospiviroids were developed (Bostan et al., 2004; Verhoeven et al., 2004). Also after these RT-PCRs identification should be based on sequencing and BLAST analysis. For simultaneous detection and discrimination of PSTVd and TCDVd, Matsushita et al. (2010) developed a duplex RT-PCR. In addition multiplex RT-PCRs have been developed for simultaneous detection and various potato viruses (Nie and Singh, 2001; Peiman and Xie, 2006; Khan et al., 2009). In all cases one should be aware that mismatches due to nucleotide substitutions, deletions or insertions at primer positions may result in false negative results. An internal RNA isolation control targeting plant RNA can be added to monitor false negative reactions due to unsuccessful RNA isolation (Menzel et al., 2002; Seigner et al., 2008; Hataya, 2009).
In addition to conventional RT-PCR, real-time RT-PCR (using TaqMan® chemistry) has been developed for PSTVd detection and validated for large-scale testing of potato leaves (Boonham et al., 2004; Roenhorst et al., 2005). However, this assay will also detect isolates of MPVd, TCDVd and TPMVd and consequently, definite identification requires subsequent conventional RT-PCR, sequencing and BLAST analysis (Boonham et al., 2005; Verhoeven, 2010b). An internal PCR control can be added to check the performance of the PCR reactions (Kox et al., 2005). Similar to conventional RT-PCRs, generic real-time RT-PCRs were developed allowing the detection of PSTVd and five or nine other pospiviroid species, and including an internal PCR control or internal RNA isolation control, respectively (Monger et al., 2010; Botermans et al., 2012).
Reverse transcription loop-mediated isothermal amplification (RT-LAMP) is another variant of PCR that has been applied for the detection of PSTVd in potato and tomato. The assay does not need expensive equipment, is quick and quite sensitive (Tsutsumi et al., 2010). A real-time variant of RT-LAMP proved even quicker and more sensitive and was suitable for application in the field (Lenarcic et al., 2012). Finally, PSTVd can also be detected by macro and microarray platforms (Agindotan and Perry, 2008; Tiberini and Barba, 2012), assays that allow detection of multiple pathogens including viruses and viroids.
The above paragraphs show that there is a large number of assays available for the detection of PSTVd. Each assay has its characteristic advantages and disadvantages e.g. concerning sensitivity, cost-effectiveness, required laboratory facilities, large scale use, etc. However, it should be considered that the assays referred to, allow detection but generally not identification to the species level. Unequivocal identification requires complete genome sequences for BLAST analysis.
A comparison of four detection methods between 13 laboratories showed the following detection limits for most laboratories: 10-20 mg of PSTVd-infective tissue for R-PAGE; 0.25-0.5 mg for hybridization by the DIG-probe; 0.062 mg for RT-PCR; and 0.0155 mg for real-time RT-PCR, but some laboratories managed to equal the detection limit of real-time RT-PCR by the conventional RT-PCR or the DIG-probe (Jeffries and James, 2005). Many infections were not detected by the participating laboratories and a few non-infected samples were falsely found positive. This indicates that successful testing requires much experience for each individual assay.
Detection and InspectionTop of page
In crops like tomato and potato, symptoms may indicate the presence of a pospiviroid. After mechanical inoculation to potato cultivar Nicola all pospiviroids except Iresine viroid 1 (IrVd-1) evoked similar tuber symptoms although the intensity varied with viroid and with isolate. Pospiviroid infections in commercial tomato crops also incite symptoms independent of the viroid species (Verhoeven et al., 2004; EFSA Panel on Plant Health, 2011). However, mild strains may not evoke symptoms, and symptom development is affected by temperature and light (Diener, 1979; Harris and Browning, 1980). In addition, true seeds of potato (TPS) and tomato and plants for planting of ornamental species, the primary means of shipment, may not show symptoms. Therefore, diagnosis on the basis of symptoms alone is not acceptable for quarantine purposes. Laboratory tests are the most reliable method of detection (see Diagnosis). Kahn (1989) and Salazar (1989) give comprehensive reviews of plant protection measures and quarantine implications for viroids in general, and PSTVd in particular, respectively. Furthermore, an comprehensive list of management options for pospiviroids has been evaluated by the EFSA Panel on Plant Health (2011).
Similarities to Other Species/ConditionsTop of page
PSTVd is a member of the genus Pospiviroid. The host range and symptomalogy of PSTVd resembles those of most pospiviroids. However, natural infections in potato have only been reported for PSTVd, so far.
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
The most effective means of control of PSTVd is the prevention of introduction of infected materials into the field, i.e., the use of materials certified as viroid-free as planting material and the maintenance of sanitary cultural practices (Singh and Crowley, 1985a; Harris and James, 1987; Salazar, 1989; Forster and Hadidi, 1998). Other methods of control include elimination of PSTVd from stock cultivars by meristem tip culture and the development of resistant cultivars (see also Maramorosch, 1985).
Resistant Crop Cultivars
No known naturally occurring resistance to PSTVd occurs in cultivars of potato. Solanum acaule OCH 11603 is resistant to mechanical inoculation by PSTVd, but was susceptible following agroinfection with PSTVd-containing cDNAs (Salazar et al., 1988b). It therefore exhibits 'field resistance', but not true immunity. Some sources of resistance have been reported in potatoes (Harris et al., 1979; Pfannenstiel and Slack, 1980) but the viroid still appears to replicate even though no symptoms are evident. These plants are therefore tolerant, not resistant, to PSTVd infection. Singh (1985) also reported that clones of Solanum berthaultii are resistant to PSTVd by sap inoculation, but not by graft inoculation.
Recent attempts to engineer potato plants resistant to PSTVd have shown some success. Yang et al. (1997) expressed a ribozyme to PSTVd in transgenic potato plants, with effective resistance in 23 of 34 lines tested. Sano et al. (1997) expressed a double-stranded RNA specific ribonuclease in potato and these plants conferred some resistance to PSTVd infection as shown by the lack of infection in some plants, and a reduction in viroid concentration and symptoms. For tomato, Schwind et al. (2009) found that transgenic plants expressing a hairpin RNA construct derived from PSTVd sequences exhibit resistance to PSTVd infection. Resistance seemed to be correlated with high-level accumulation of hpRNA-derived short interfering RNAs in the plant.
Decontamination of Tools
Tools can be successfully disinfected by 1-3% sodium hypochlorite (Singh et al., 1989; Roenhorst et al., 2005). The EPPO Standard PM 9/13(1) (OEPP/EPPO, 2011) lists the following commercial products to be used for 'infested' and 'probably infected' objects like equipment, machinery and storage facilities: Menno florades, Menno clean and Virkon S.
No effective chemical control for PSTVd has been reported.
Viroids are able to cross-protect against one another, at least in some combinations (Niblett et al., 1978). Singh et al. (1993a) observed no interference between mild and severe strains of PSTVd in secondarily infected plants obtained by grafting two infected potato cultivars together. However, they suggest that it may occur in the initial infection step made by mechanical inoculation.
Clean Stock by Meristem Culture
Elimination of PSTVd by low temperature treatments (5-8°C) of infected plants and subsequent meristem culture was found to be effective by Lizarraga et al. (1980). Thermotherapy and axillary bud culture was also reported to be effective in eliminating PSTVd (Stace-Smith and Mellor, 1970).
With the development of rapid and sensitive detection methods (see Diagnosis), early indexing of TPS and breeding/planting materials can virtually eliminate the incidence of PSTVd and reduce its impact on potato tuber marketability (Morris and Smith, 1977; Singh and Crowley, 1985a). Even large-scale distributions of PSTVd in potato crops in Canada and USA were eradicated successfully (Sun et al., 2004; De Boer and De Haan, 2005).
Quarantine Enforcement - Phytosanitary Certification
The enforcement of seed regulations and strong seed certification programmes (Morris and Smith, 1977) and establishment of so-called 'elite' seed stocks will reduce or eliminate the introduction of PSTVd onto commercial production farms, as it has in Canada (Singh and Crowley, 1985a).
Many countries consider PSTVd to be a phytosanitary risk and, depending upon the country, certification of disease-free status that is required before import of plant materials varies, in addition to the post entry regulations (Salazar, 1989). In countries where PSTVd was known to occur, for example USA and Canada, seed potatoes must have been grown under a production system shown to be free of PSTVd. These seed potatoes are then subject to testing by the importing country (Salazar, 1989). In addition to potato, various countries also regulate the import of tomato seeds with regard to PSTVd. Furthermore, in the EU emergency measures (Commission Decision 2007/410/EC) were taken for plants of Brugmansia spp. and Solanum jasminoides to prevent the introduction and the spread of PSTVd.
Reducing probability of spread and potential consequences during cultivation
Several measures are listed by the EFSA Panel on Plant Health (2011) for reducing the consequences in case of pospiviroid outbreaks. Among them are hygiene best practice, separation of host plant cultivations, control of aphid vectors, weeds and volunteer plants, and monitoring for symptoms. After outbreaks additional measures are suggested, e.g., tracing the sources of infection, destruction of infected plants, sanitation of the production location, cleaning and disinfecting of machinery and equipment, and temporary banning of the cultivation of host plants.
For potato, EPPO developed a national regulatory control system for PSTVd that provides guidance on preventing its introduction, surveillance for the pathogen and its containment and eradication if found infecting potato plants or tubers (OEPP/EPPO, 2011).
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28/11/12 Review by:
J. Th. J. Verhoeven, National Plant Protection Organization, PO Box 9102, 6700 HC Wageningen, The Netherlands.
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