Potato Spindle Tuber Viroid

California Plant Pest Rating for
Potato Spindle Tuber Viroid
Pest Rating: A

Initiating Event:

On December 16, 2013, CDFA was notified by USDA APHIS of the detection of potato spindle tuber viroid (PSTVd) in three Cestrum samples that were shipped under federal permit from California to Illinois. The detection was made by Ball Horticultural Company Plant Pathologist, and communicated to USDA APHIS as per federal permit requirement that APHIS is notified within 10 days on receipt of permitted materials if the permitee detects a pathogen that is not widely prevalent in the State from which the infected material was obtained.” The Cestrum plant samples came from plants raised in a Nursery greenhouse in San Diego County, CA. Subsequently, there is a need to reevaluate the current status and pest rating of PSTVd as a necessary step toward dealing with the current and future detections of this pathogen in California.

History & Status:

Background:  Potato spindle tuber viroid is the first recognized viroid that was discovered in 1971 as the causal agent of the potato spindle tuber disease. The disease itself was first discovered in potato fields in New Jersey, USA (Martin, 1922). The 1971 study led to discovery of viroids which are small naked single-stranded, covalently closed, circular molecule of infectious RNAs that multiply autonomously in plant cells and lack a coat protein as they are too small to coat for a single protein. They infect plant cells and are replicated within the plant nucleus. PSTVd consists of 359 nucleotides and is the type species of the genus Pospiviroid.

Hosts: Potato is considered the main host of PSTVd large due to the presence of serious symptoms and large scale outbreaks. Other symptomatic hosts include tomato and pepper. Symptomless infections have been reported on Persea americana (avocado) and mainly solanaceous ornamentals: Brugmansia spp., Chrysanthemum sp., Calibrachoa sp., Cestrum spp., Dahlia sp., Datura sp., Lycianthes rantonnei, Petunia sp., Physalis peruviana, Solanum pserdocapsicum, Streptosolen jamesonii, Solanum jasminoides, Solanum muricatum, Ipomoea batatas (sweet potato) and wild Solanum spp.

Symptoms: In potato, symptom expression is influenced by potato cultivar, viroid strain, environmental conditions and method of inoculation (Pfannenstiel and Slack, 1980; Diener, 1987; Owens and Verhoeven, 2009). Symptoms include severe or barely distinct growth reduction, smaller vines, plants appear more erect and with smaller leaves than healthy, non-infected ones. Also, leaflets are darker green, sometimes with rolling and twisting. Tubers may be small, elongated (spindle-shaped), misshapen and cracked. Tuber eyes are pronounced and sometime borne on knob-like protuberances.
Infected tomatoes show growth reduction which may develop into permanent stunting with occasional death or recovery. Top grow is chlorotic sometimes turning to reddening and/or purpling causing leaves to become brittle. Flowers and fruit fail to develop as stunting begins. Very mild symptoms are produced on infected peppers in the form of slight distortion or ‘waviness’ of the leaf margins near the top of infected plants (Lebas et al., 2005). Infected solanaceous ornamentals do not exhibit any symptoms.

Damage Potential: PSTVd attacks all potato varieties causing severe losses. Up to 24% reduction in tuber yields are reported with mild strains of PSTVd and 64% with a severe strain (Singh et al., 1971). According to Pfannenstiel and Slack (1980) reduction of tuber weight depended on the potato cultivar and length of time they were infected with PSTVd. Plant age at the time of infection and the number of infected plants are factors that determine the amount of yield loss in tomatoes, reportedly from a few to 10% of plants (CABI International, 2013).

Transmission: PSTVd can infect all or most parts of susceptible plants. The viroid is mechanically transmitted and spread mainly through knives used for cutting infected and healthy tubers. Other means by which infected sap of diseased plants is transmitted to healthy plants include normal cultivation equipment and practices. Also, it is readily transmitted by infected plant material, namely cuttings, tubers, and micro-plants. Symptomless ornamental plants may serve as a source of inoculum. It is also transmitted by pollen, seed – especially for crops propagated by botanical seed, and by contaminated mouth parts of several insect vectors including aphids (Agrios, 2005, CABI International, 2013).

Worldwide Distribution:  PSTVd is present in several countries in Europe, although in few occurrences for certain countries. Erratic outbreaks have been reported in tomato and substantial infections of ornamental plants have been reported. Effective measures have reduced the incidence of PSTVd in those crops but it has not been eradicated. However, the viroid was eradicated from Finland and France (CABI International, 2013). It has also been reported in Asia: China, Turkey, India, Iran, Israel, Japan, Afghanistan, Azerbaijan, Bangladesh, and the Republic of Georgia; Africa: Egypt, Nigeria, (absent from Kenya & South Africa – unreliable record according to CABI International 2013); Costa Rica; South America: Peru, Venezuela, Brazil, and Uruguay (it was eradicated from Argentina); Oceania: New Zealand, (it was eradicated from Australia); North America: Mexico, USA. It was successfully eradicated in seed potato production in the USA and Canada.

Official Control: Countries that require phytosanitary certification of PSTVd-free plant commodities imported from the USA include: Israel (tomato and petunia seeds), Austria (solanaceous seeds), Bosnia and Herzegovina (potato tubers), Chile (sweet potato and potato seeds and in vitro plantlets), Columbia (potato plantlets and seeds), India (tomato seeds), Macedonia (potato tubers), Mexico (tomato seedlings and potato in vitro plantlets), Morocco (potato tubers for propagation and consumption), New Caledonia (potato tubers for propagation) and Yemen (pepper and tomato seeds). At least 78 countries worldwide consider PSTVd an actionable pathogen but do not require phytosanitary certification. It is quite probable that these countries may require official certification in the event that PSTVd becomes further established within the USA.

California Distribution:  There are no reports of PSTVd being established in field or natural environments within California – that would indicate an established distribution. However, PSTVd was reported for the first time in 2010 in tomatoes grown in commercial greenhouses in Ventura County (Ling, et al., 2010). The 2013 detection of PSTVd was made from infected Cestrum spp. nursery stock (C. fasciculatum Newellii, C. elegans Smithii, and C. elegans) also grown in greenhouses. It is likely that the 2010 PSTVd infected plants were destroyed and appropriate sanitary measures led to the eradication of the pathogen in the affected greenhouses. Similar eradicative measures will most likely be taken against the recent 2013 detection.

California Interceptions:  There are no state reports of PSTVd detections in plant materials intercepted within or at points of entry in California.

The risk Potato spindle tuber viroid would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction:  Evaluate if the pest would have suitable hosts and climate to establish in California. Score:

Low (1) Not likely to establish in California; or likely to establish in very limited areas.
Medium (2) may be able to establish in a larger but limited part of California.
High (3) likely to establish a widespread distribution in California.

Risk is high (3) as shown by the 2010 and 2013 detections of PSTVd in California greenhouses. Suitable climates for potato, tomato and other host plants would also favor the establishment of PSTVd.

2) Known Pest Host Range:  Evaluate the host range of the pest. Score:

Low (1) has a very limited host range.
Medium (2) has a moderate host range.
High (3) has a wide host range.

Risk is medium (2). Potato and tomato are grown in significant acreages throughout the State. Ornamental plants appreciably increase the host range of this pathogen.

3) Pest Dispersal Potential:  Evaluate the natural and artificial dispersal potential of the pest. Score:

Low (1) does not have high reproductive or dispersal potential.
Medium (2) has either high reproductive or dispersal potential.
High (3) has both high reproduction and dispersal potential.

Risk is High (3): PSTVd multiply autonomously in infected plant material and are easily transmitted through various means.

4) Economic Impact:  Evaluate the economic impact of the pest to California using the criteria below. Score:

A.   The pest could lower crop yield.
B.   The pest could lower crop value (includes increasing crop production costs).
C.   The pest could trigger the loss of markets (includes quarantines).
D.   The pest could negatively change normal cultural practices.
E.   The pest can vector, or is vectored, by another pestiferous organism.
F.   The organism is injurious or poisonous to agriculturally important animals.
G.   The organism can interfere with the delivery or supply of water for agricultural uses.

Low (1) causes 0 or 1 of these impacts.
Medium (2) causes 2 of these impacts.
High (3) causes 3 or more of these impacts.

Risk is High (3): PSTVd infected potato tubers could lower crop yield, crop value, trigger loss of market through the imposition of quarantine regulations by other countries and states, can influence normal cultural practices and is vectored by insects.

5) Environmental Impact:  Evaluate the environmental impact of the pest on California using the criteria below.

A.   The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.
B.   The pest could directly affect threatened or endangered species.
C.   The pest could impact threatened or endangered species by disrupting critical habitats.
D.   The pest could trigger additional official or private treatment programs.
E.   The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score the pest for Environmental Impact. Score:

Low (1) causes none of the above to occur.
Medium (2) causes one of the above to occur.
High (3) causes two or more of the above to occur.

Risk is High (3): Incidence of PSTVd could require additional official and private treatments, impact cultural and horticultural practices, at the very least.

Consequences of Introduction to California for potato spindle tuber viroid:

Add up the total score and include it here. (Score)

–  Low = 5-8 points
–  Medium = 9-12 points
–  High = 13-15 points

Total points obtained on evaluation of consequences of introduction to California = 14 (High).

6) Post Entry Distribution and Survey Information:  Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. (Score)

–  Not established (0) Pest never detected in California, or known only from incursions.
–  Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area (region).
–  Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.
–  High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Evaluation: PSTVd is not established in California (0). PSTVd has only been detected in nursery plants contained in greenhouses. Those detection would have led to eradicative and greenhouse sanitary measures. They do not indicate the establishment of the pathogen in California.

Final Score:

7) The final score is the consequences of introduction score minus the post entry distribution and survey information score: (Score)

Final Score: Score of Consequences of Introduction – Score of Post Entry Distribution and Survey Information = 14 (High).


Information of the source of the pathogen is not known. The pathogen has not been reported from environments outside greenhouses.

Conclusion and Rating Justification:

Based on the evidence provided above the proposed rating for Potato spindle tuber viroid is A.


Agrios, G. N. 2005. Plant Pathology. Fifth edition. Elsevier Academic Press. 922 p.
CABI International. 2013. http://www.cabi.org/cpc/?compid=1&dsid=43659&loadmodule=datasheet&page=868&site=161

Diener TO, ed., 1987. The Viroids. New York, USA: Plenum Press.

Martin WH, 1922. “Spindle tuber”, a new potato trouble. Hints to Potato Growers, New Jersey State Potato Association, 3:8.

Lebas BSM, Clover GRG, Ochoa-Corona FM, Elliott DR, Tang Z, Alexander BJR, 2005. Distribution of potato spindle tuber viroid in New Zealand glasshouse crops of Capsicum and tomato. Australasian Plant Pathology, 34(2):129-133.

Ling, K. S. and D. Sfetcu. 2010. First report of natural infection of greenhouse tomatoes by Potato spindle tuber viroid in the United States. Plant Disease, 94:1376.

Owens RA, Cress DE, 1980. Molecular cloning and characterization of potato spindle tuber viroid cDNA sequences. Proceedings of the National Academy of Sciences USA, 77:5302-5306.

Owens RA, Verhoeven JTJ, 2009. Potato spindle tuber. External factsheets. Minnesota, USA: APSnet, unpaginated. http://dx.doi.org/10.1094/PHI-I-2009-0804-01.

Pfannenstiel MA, Slack SA, 1980. Response of potato cultivars to infection by the potato spindle tuber viroid. Phytopathology, 70(9):922-926.

Singh RP, Finnie RE, Bagnall RH, 1971. Losses due to the potato spindle tuber virus. American Potato Journal, 48:262-267.

Responsible Party:

Dr. John J. Chitambar, Primary Plant Pathologist/Nematologist, California Department of Food and Agriculture, 3294 Meadowview Road, Sacramento, CA 95832. Phone: 916-262-1110, plant.health[@]cdfa.ca.gov.

Pest Rating:   A

Posted by ls