Preferred name: Potato latent virus
Taxonomic position: Viruses and viroids: Riboviria: Orthornavirae: Kitrinoviricota: Alsuviricetes: Tymovirales: Betaflexiviridae: Carlavirus
Other scientific names: PotLV, Potato Red La Soda virus, Potato latent carlavirus
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EPPO Categorization: Alert list (formerly)
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EPPO Code: POTLV0

The primary host of potato latent virus (PotLV) is Solanum tuberosum (potato) with most of those potato cultivars tested susceptible to infection by PotLV (Brattey et al., 2002; Nie, 2010). Only Physalis alkekengi (Chinese lantern) has been reported as another natural host. The experimental host range is wider than for the carlaviruses, potato virus M (PVM) and potato virus S (PVS).

Host list: Alkekengi officinarum, Solanum tuberosum

PotLV was first described infecting in vitro potatoes of the potato cultivar Red LaSoda imported into the United Kingdom from the USA in the early 1990s through post-entry quarantine (Brattey et al., 1995; Brattey et al., 2002). It was then discovered infecting 8 out of 270 potato cultivars in the Vancouver Collection of Virus Free Potatoes, British Colombia, Canada and 3 out of 137 cultivars in the USDA National Variety field grown collection of potatoes, maintained at Presque Isle, Maine, USA. It was also reported infecting Red LaSoda in the California winter test supplied from the Vancouver Collection, but which was originally from Nebraska and also from Red LaSoda plants grown in Minnesota (Goth et al., 1999). The California winter test is the annual winter grow-out, in which seed lot samples are planted and tested for potato virus Y and other diseases. However, in the past 20 years it has not been reported from Canada where tests for PotLV are done on potato material submitted for nuclear stock production (CFIA, 2018), pre-elite seed potatoes from sources other than nuclear stock (CFIA, 2012) or in leaf or tuber samples submitted by Canadian Food Inspection Agency potato inspectors (H Xu, CFIA, Canada, personal communication, 2021). Similarly, there are no reports of PotLV infecting potato in the USA in the past several decades, where it is not regulated in the seed potato certification schemes and systematic testing is not done (A Karasev, UIDAHO; K Sather and G Secor, NDSU, all USA, personal communications, 2021). Testing appears to be limited, mostly to testing nuclear stocks in a number of states such as Colorado, Idaho, New York and North Dakota (Anon, 2019a; b; c; d) and it has not been detected (A Houser and C Keller, CSU; J Durrin, UIDAHO; K Perry, Cornell, all USA, personal communications, 2022).

Although PotLV was reported by Suica (2000) and Rosas Díaz (2004) infecting accessions in the native Andean potato cultivar collection, maintained by the International Potato Centre, Peru after testing by ELISA, this latter finding was not confirmed by inoculation to and testing of indicator plants (Rosas Díaz, 2004). Furthermore, it has not been found in Peru, following testing by high throughput sequencing of potato leaf samples taken from throughout the Andean region of Peru (Fuentes et al., 2019) or genebank material (J Kreuze CIP, PE, personal communication, 2022) and it was not mentioned as being present in Peru by Kreuze et al. (2020). 

Apart from potato, PotLV has only been reported infecting field grown Physalis alkekengi in Oregon, USA (Diaz-Lara et al., 2017).  

North America: Canada (British Columbia), United States of America (California, Maine, Minnesota, Nebraska, Oregon)

PotLV infects plants systemically and is efficiently transmitted in susceptible potato cultivars through clonal/vegetative propagation (Nie, 2010). PotLV is transmitted by the aphid Myzus persicae (Brattey et al., 2002) with uncertain efficiency, and probably non-persistently as for other carlaviruses (ICTV, 2011). Although it can be contact or mechanically transmitted experimentally, whether this occurs in the field as for other carlaviruses (ICTV 2011) such as PVM and PVS, and its relative importance compared with aphid transmission is not known.

Symptoms

No symptoms have been reported in naturally infected potato cultivars in the field, or in 58 and 36 infected cultivars tested in the UK and USA respectively, following mechanical inoculation with PotLV (Brattey et al., 2002; Nie, 2010). Although symptoms of stunting, chlorotic mottling, and leaf deformation were observed in the USA in field grown Physalis alkekengi, the plants were also infected with tomato mosaic virus (Diaz-Lara et al., 2017).

Morphology

Virions of PotLV are filamentous and slightly curved and have been reported to be bimodal with modal lengths of 530 and 670 nm (Brattey et al., 2002) or with particles 600 to 720 nm in length with a mean of 690 nm (Goth et al., 1999). 

Viruses in the genus Betaflexiviridae are single stranded positive-sense RNA (ICTV, 2011). The complete PotLV genome sequence contains 7890 nucleotides excluding the poly(A) tail and like other carlaviruses has six open reading frames. The PotLV genome is approximately 500–600 nt shorter than the other potato infecting carlaviruses, PVM, potato virus P and PVS, due to the ORF1 gene being shorter (Nie, 2009). 

Detection and inspection methods

In the absence of symptoms under natural conditions, field inspection is unlikely to detect the virus. However, PotLV virus is reliably detected in in vitro plants of at least 4 weeks old and plants grown from infected tubers using serological and molecular methods. The reliability of testing tubers has not been reported.

Detection may be by ELISA using monoclonal antibodies (Brattey et al., 2002; Goth et al.,1999), which are available commercially. 

Detection may also be using the RT-PCR using the forward primer Car‐F2b (Nie et al., 2008) and reverse primer Not1pdt (Badge et al., 1996; EPPO, 2019) to detect carlaviruses with a PCR product of 900 bp, followed by sequencing for identification, or using the primers 1000F/1000R (Nie, 2010) or PotLVfor1/PotLVrev1 (Diaz-Lara et al., 2017) which produce respectively PotLV-specific fragments of 353 bp or ∼1000 bp. 

Use of indicator plants may be unreliable since not all inoculated plants show symptoms, and these may be transient (Brattey et al., 2002). In Chenopodium murale and Nicotiana bigelovii symptoms are respectively, faint local chlorotic/necrotic spots and systemic vein clearing (Brattey et al., 2002), and in N. debneyi and N. occidentalis-P1 chlorotic mottling and leaf deformation (C Jeffries, SASA, UK, personal communication, 2022).

Plants for planting, such as seed potato tubers and in vitro potato plants, is the main pathway for long distance movement. In countries where PotLV is present, even though aphids and mechanical transmission could theoretically contribute to local spreading, planting infected seed tubers is probably the main way PotLV spreads. PotLV is unlikely to be transmitted by true potato seeds since other carlaviruses are not transmitted in this way.

Even though M. persicae is not regulated and is widespread, given the probable nonpersistent transmission of PotLV, entry with the vector would only be possible if the transfer occurs within a few hours of entry (EFSA, 2019).

Economic impact

Although most of those potato cultivars tested were susceptible to infection by PotLV, no studies have been conducted to determine yield loss, or the effect of PotLV in combination with other viruses. However, a yield loss might be expected since the carlavirus PVS, which also does not produce symptoms, or few symptoms, in many potato cultivars, may cause yield losses up to 20% (reviewed by Lambert, 2005). 

Whether the virus is categorized as a quarantine or regulated non-quarantine pest will also determine the level of indirect economic loss. Indeed, should the virus be detected, phytosanitary measures may be required such as respectively crop destruction or downgrading of seed potato crops depending on quarantine or regulated non-quarantine status. The other reported natural host P. alkekengi is cultivated as an ornamental in the EPPO region where it has naturalized in southern countries. However, any fruit production in Europe for this genus is based on other Physalis spp., such as Physalis peruviana (Cape gooseberry) and it is not known whether this can be infected by PotLV. 

Control

Cutting seed potatoes should be avoided because other carlaviruses are mechanically transmitted (ICTV 2011) and if PotLV is as infective as PVS (Franc & Banttari, 1984), it will be efficiently transmitted by this practice. Insecticide control in the field would be as for the other non-persistently transmitted potyvirus such as PVY.  Although fewer PotLV resistant cultivars are available than for PVM or PVS (Brattey et al., 2002), this resistance suggests that resistance genes to PotLV are available for breeding programmes (Nie, 2010). Both authors found that the cultivar Jemseg was resistant to infection by PotLV. 

In countries where PotLV is present, to ensure that infected asymptomatic stocks are not introduced to the certification system, nuclear stocks should be tested for freedom from PotLV. In Canada PotLV is only regulated at the nuclear stock in vitro initiation phase (CFIA, 2018).

Phytosanitary risk

Potato is the main crop at risk in the EPPO region and for many countries the pathway is closed from North America because import of seed potatoes is prohibited except through post-entry quarantine. However, such import is sometimes authorized under derogation procedures, e.g. with Commission Implementing Decision 2011/778/EU, which allows Portugal to import seed potatoes from the Canadian provinces of New Brunswick and Prince Edward Island.

Although the EPPO PRA (EPPO, 2001) recommended listing PotLV as a quarantine pest, this was not done since it was concluded that PotLV was not sufficiently well characterized and the potential for economic damage was unclear. However, it is recommended for specific testing in post-entry potato quarantine (EPPO, 2019). Infected material should not be released from post-entry quarantine since it may be used for seed potato production or for field trials. For the European Union, although not named as a quarantine pest, PotLV is regulated in Commission Implementing Regulation (EU) 2019/2072 Annex II Part A under ‘Potato viruses, viroids and phytoplasmas, such as ... ’. Recently, however, EFSA (2020) indicated that PotLV did not qualify as potential quarantine pest because it was not expected to have an impact on the EU territory primarily because no symptoms had been reported.

Anon (2021a) Nuclear and-Generation 1 lab testing fees and certification requirements. Colorado Certification Service. Available at: https://potatoes.colostate.edu/wp-content/uploads/2021/05/Nuclear-and-Greenhouse-Lab-Costs-Table.pdf (accessed 4 February 2022)

Anon (2021b) Potato varieties in production. Seed potato germplasm program, University of Idaho. Available at: https://www.uidaho.edu/cals/seed-potato-germplasm-program/varieties-in-production (accessed 4 February 2022)

Anon (2021c) Pathogen Testing. The Uihlein Farm of Cornell University. Available at: https://uihleinfarm.pppmb.cals.cornell.edu/pathogen-testing/ (accessed 4 February 2022)

Anon (2021d) Seed stocks & tissue culture. Available at: https://www.seed.nd.gov/potato/seedstocks-tissue-culture (accessed 4 February 2022)

Badge J, Brunt A, Carson R, Dagless E, Karamagioli M, Phillips S, Seal S, Turner R & Foster GD (1966) A carlavirus-specific PCR primer and partial nucleotide sequence provides further evidence for the recognition of cowpea mild mottle virus as a whitefly-transmitted carlavirus. European Journal of Plant Pathology 102, 305–310.

Brattey C, Badge JL, Burns R, Foster GD, George E, Goodfellow HA, Mulholland V, McDonald J G & Jeffries CJ (2002) Potato latent virus: A proposed new species in the genus Carlavirus. Plant Pathology 51, 495-505.

Brattey C, Jeffries C, Goodfellow S, Rose G, Burns R & George E (1995) Some characteristics of a virus infecting Solanum tuberosum cv. Red La Soda imported from the USA. In Proceedings of the 9^th Virology Section Meeting of the European Association for Potato Research, pp. 65-69. Bled (SI) 18-22 June 1995.

CFIA (2012) D-97-11: Seed potato certification program - Requirements for the production of pre-elite seed potatoes from sources other than nuclear stock. Available at: https://inspection.canada.ca/plant-health/invasive-species/directives/potatoes/d-97-11/eng/1323886418103/1323886500636 (accessed 4 February 2022)

CFIA (2018) D-97-08: Production, maintenance, multiplication and certification of nuclear stock class seed potatoes. Available at https://inspection.canada.ca/plant-health/plant-pests-invasive-species/directives/potatoes/d-97-08/eng/1313337803460/1313337874471 (accessed on 4 February 2022)

Diaz-Lara A, Santamaria L & Martin RR (2017) Identification of Tomato mosaic virus (ToMV) and Potato latent virus (PotLV) as mixed infection in Chinese lantern (Physalis alkekengi) expressing virus-like disease symptoms. Plant Disease 101, 1061.

EFSA (2020) (EFSA Panel on Plant Health) Bragard C, Dehnen‐Schmutz K, Gonthier P, Jacques M‐A, Jaques Miret JA, Justesen AF, MacLeod A, Magnusson CS, Milonas P, Navas‐Cortes JA, Parnell S, Potting R, Reignault PL, Thulke H‐H, van der Werf W, Vicent Civera A, Yuen J, Zappalà L, Candresse T, Lacomme C, Bottex B, Oplaat C, Roenhorst A, Schenk M & Di Serio F (2020) Scientific Opinion on the pest categorisation of non‐EU viruses and viroids of potato. EFSA Journal 18, 5853. https://doi.org/10.2903/j.efsa.2020.5853 (accessed on 4 February 2022)

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Nie X (2010) Preliminary assessment of resistance of potato varieties to potato latent virus. American Journal of Potato Research 87, 78–82.

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Suica P (2000) Study of carlavirus populations (PVS, PVM and PVP) in potato germplasm. Thesis for the title of Agronomist. The National Agrarian University, Lima PE (in Spanish).

This datasheet was prepared in 2022 by C Jeffries (SASA, Edinburgh, UK) with help from G Muller and J Krueze (CIP, Peru) and L Salazar (formerly CIP, Peru) for clarifying the situation in Peru. Their valuable contribution mentioned as personal communications is gratefully acknowledged.

EPPO (2024) Potato latent virus. EPPO datasheets on pests recommended for regulation. https://gd.eppo.int (accessed 2024-04-16)

This datasheet was first published in 2022. It is maintained in an electronic format in the EPPO Global Database. The sections on 'Identity', ‘Hosts’, and 'Geographical distribution' are automatically updated from the database. For other sections, the date of last revision is indicated on the right.