Background Information
Tomato chlorotic dwarf viroid (TCDVd) is a Pospiviroid which has a
circular single stranded RNA molecule with 360 nucleotides (nt) and lack of a
protein coat. The absence of protein coat distinguishes viroids from viruses.
Viroids are the smallest “organisms” known to cause plant diseases. TCDVd is
closely related to but different from the well known Potato spindle tuber
viroid (PSTVd), the first viroid to be identified, and it shares only 86-88%
sequence homology with PSTVd. In relation to crop plants, very little is known
about the epidemiology, host specificity and host-pathogen interaction of TCDVd.
However, TCDVd is expected to exhibit pathogenicity properties similar to that
of PSTVd. In general, pospiviroids use the plant cell nucleolus to
self-replicate (i.e. multiply in number) and accumulate. The movement of viroids
within an infected plant is via plasmodesmata, from cell to cell, and via phloem
throughout the plant. Viroids have been known to infect both herbaceous and
woody plants, including many ornamentals; many of them are symptomless carriers.
TCDVd is known to occur in field and greenhouse-grown tomatoes in Europe. In
Canada, it was first detected in greenhouse tomatoes, var. “Trust”, in Manitoba
in 1996. It was successfully eradicated and has not been reported since then. In
2007, TCDVd on tomato has been reported from two greenhouses in Arizona. TCDVd
was also reported from petunia and verbena with no visible symptoms (source:
J.Th.J. Verhoeven, The Netherlands).
Infection & Spread
Generally, all pospiviroids, including TCDVd, can easily be transmitted
mechanically and spread by contact with contaminated pruning tools, farm
equipment, clothing, crop handing, and contact between neighboring plants.
Depending on the type of viroid and host plant, viroids can also be transmitted
by seeds, vegetative propagation, pollen, grafting and insects. Although
transmission of TCDVd by seeds is strongly suspected, TCDVd was not detected in
seeds obtained from TCDVd-infected tomato plants (source: Dr. Rudra Singh,
personal communication). However, in potatoes, PSTVd is known to be transmitted
by seeds. Since TCDVd is known to infect other solanaceous plants and herbaceous
ornamentals the possibility of seed transmission of TCDVd needs further
investigation. Transmission of TCDVd by pollen or insects is not yet known. Most
efficient transmission of TCDVd is expected when young growing terminal parts of
the plant are being handled during pruning and training. Least efficient
transmission is expected during picking of fruits and old leaves (source: J.Th.J.
Verhoeven). TCDVd is active at higher temperatures and thus high rate of viroid-transmission
is expected during warm weather conditions.
Disease Symptoms
The symptoms expressed by infected tomato plants vary depending on the type
of TCDVd strain, tomato variety, age of the plant, plant vigour and climatic
conditions. Initial symptoms appear as reduced growth and yellowing of leaves (chlorosis)
in the young terminal parts of the plant, 3-6 weeks after initial infection.
With time, the infected plants may become stunted, chlorosis may become more
pronounced or turn to bronzing and/ or purpling; leaves may show necrotic
lesions and become brittle and distorted (source: J.Th.J. Verhoeven). The spread
of the viroid can readily be seen within a row. The commonly observed symptoms
are stunting, overall bunchiness, reduced leaves and fruit, leaf chlorosis, leaf
and petiolar necrosis, downward bending of leaves, fruit distortion and even
death of plants, similar to the symptoms expressed by PSTVd when artificially
inoculated to tomato plants (source: Dr. Rudra Singh). A tomato plant can be
infected with both TCDVd and PSTVd simultaneously and, thus, becomes difficult
to distinguish one viroid from the other by the symptoms.
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| Greenhouse tomato plants infected with TCDVd. Photo by Dr. Rudra
Singh |
Greenhouse tomato plants infected with TCDVd. Photo by Dr. Rudra
Singh |
Detection of TCDVd
Viroid-infection in tomato is usually recognized by symptoms. However,
detection of viroid infection by symptoms is challenging since many other
viroids and viruses can produce similar symptoms on tomato plants. Unlike
viruses, ELISA (enzyme-linked-immunosorbent assay) technique cannot be used for
detecting viroids since they lack protein coat. TCDVd can only be detected by a
molecular PCR-based technique called reverse transcription PCR (RT-PCR) using
pospiviroids-specific molecular probes as described by Boonham et al. 2004 or
Shamloul et al., 1997.
Note: BCMAL’s Plant Diagnostic Lab has the RT-PCR diagnostic tool
available for testing suspected tomato samples for Pospiviroids.
Eradication & Management of Viroids
Use exclusion and eradication strategies to prevent the introduction and
spread of TCDVd.
- Use viroid-free seeds and transplants; purchase planting stock from
reliable sources that produce transplants from viroid-free certified seeds.
Facilities that produce tomato transplants must be located in viroid-free
areas.
- Enforce strict biosecurity & phytosanitary requirements as appropriate
for your greenhouse operation.
- Maintain strict hygiene practices (e.g. hand wash, foot dip, avoid bring
green plant materials and food items into a greenhouse facility).
- Carry out periodic and rigorous scouting for virus and viroid-like
symptoms during crop production.
- Submit any suspected plant samples to BCMAL Plant Diagnostic Lab for
viroid/virus identification.
- Healthy-looking herbaceous ornamental plants and weeds can be
asymptomatic carriers of TCDVd as well as other potential tomato viroids
(e.g. PSTVd, Columnea latent viroid, Tomato apical stunt viroid, Tomato
planta macho viroid). Therefore, practice a systematic weed control
program and avoid having ornamental plants in and around greenhouses.
- Avoid using contaminated pruning shears and knives. Sterilize pruning
tools before and after pruning each plant (details on sterilization
techniques is attached). Pruners must be sterilized thoroughly at each use.
Use a couple of pruners alternatively; this will give sufficient time to
disinfect a pruner (immerse minimum of 2 min in a recommended disinfectant)
while pruning with the other.
- Control or minimize movement of people and contact with or handling of
plants.
- Remove plant debris and infected plants, including root system and plant
growth medium; deep bury or incinerate. Since viroids can persist in the
root system of infected plants do not reuse the planting bags or containers
from the previous crop. Since TCDVd has a latent period of 3-6 weeks before
any symptoms can be seen, symptomless plants surrounding the infected plant
must also be destroyed.
- Carry out a thorough greenhouse clean up between crop cycles and at the
year-end.
- Pepino mosaic virus control strategies can be adopted in
controlling TCDVd.
Useful websites
References
- Boonham N, Gonzáles Pérez L, Mendez M.S, Lilia Peralta E, Blockly A,
Walsh K, Barker I and Mumford R.A (2004). Development of a real-time RT-PCR
assay for the detection of Potato spindle tuber viroid. Journal of
Virological Methods 116, 139-146.
- Diener, T (1991). Subviral pathogens of plants: viroids and viroidlike
satellite RNAs. The FASEB Journal 5: 2808-2813.
- Hammond, R.W. and Owens, R.A. (2006). Viroids: New and continuing risks
for horticultural and agricultural crops. APSnet Feature, November 2006.
- Shamloul AM, Hadidi A, Zhu SF, Singh RP and Sagredo B (1997) Sensitive
detection of potato spindle tuber viroid using RT-PCR and identification of
a viroid variant naturally infecting pepino plants. Canadian Journal of
Plant Pathology 19: 89-96.
- Singh R.P. Personal Communication, August 2007
- Singh R.P and Dilworth AD (2006) Detection of Citris exocortis viroid,
Iresine viroid and Tomato chlorotic dwarf viroid in new ornamental host
plants in India. Plant Disease 90: 1457.
- Singh R.P, Xianzhou N and Singh M (1999) Tomato chlorotic dwarf viroid:
an evolutionary link in the origin of pospiviroids. Journal of General
Virology 80: 2823-2828.
- Verhoeven, J.Th.J. Personal Communication, August
2007
- Verhoeven, J.Th.J, Jansen, C.C.C, Werkman, A.W. and Roenhorst, J.W.
(2007) First report of Tomato chlorotic dwarf viroid in Petunia hybrida from
the United States of America. Plant Disease 91, 324.
- Verhoeven, J.Th.J, Jansen, C.C.C, Willemen, T.M, Kox, L.F.F, Owens, R.A
and Roenhorst, J.W. (2004) Natural infections of tomato by Citrus exocortis
viroid, Columnea latent viroid, Potato spindle tuber viroid and Tomato
chlorotic dwarf viroid. European Journal of Plant Pathology 110: 823-831.
Prepared by: Dr. Siva Sabaratnam
Plant Pathologist
Abbotsford Agriculture Centre
British Columbia Ministry of Agriculture & Lands
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