Emerging Viruses in Greenhouse Tomatoes

Three new viral diseases have shown up in greenhouse tomato crops worldwide. Pepino Mosaic Virus (PepMV) has been introduced recently into Canada but the other viruses discussed in this fact sheet have not been found here yet. Information on the viruses is limited.

Early detection of viral outbreaks is critical to reducing crop loss in greenhouses. If a new viral disease or its potential vector is found, it is critical to get it properly identified. Viruses are too small to be seen and identified with light microscopes. Specialized techniques such as enzyme-linked-immunosorbent-assay (ELISA) and reverse transcriptase polymerase chain reaction (RT-PCR) are required in order to confirm the disease. Once you know which virus is present, specific control strategies can be implemented

Pepino Mosaic Virus (Potexvirus)

Current Distribution
PepMV was first reported in 1980 from pepino in Peru. It re-appeared in 1999 in greenhouse-grown tomatoes in the Netherlands and UK (south England) and has subsequently been found in North American greenhouse tomatoes. It was first detected in British Columbia in 2003. Studies have shown that potato can also be infected, although the disease has never been seen in a potato crop.

Symptoms
Infected tomato plants (figures 1 – 4) have yellow spots and bubbly areas on the leaves, mild interveinal chlorosis (yellowing), and leaf distortions such as spindly leaves. The heads of infected plants are often stunted and thin. Reports on the severity of the damage have varied from minor to severe.

Figure 1: Leaf symptoms on tomato plants infected with Pepino Mosaic Virus.	Figure 2: Leaf symptoms on tomato plants infected with Pepino Mosaic Virus.
Figure 3: Fruit symptoms on tomato plant infected with Pepino Mosaic Virus.	Figure 4: Fruit symptoms on tomato plant infected with Pepino Mosaic Virus.

Spread
PepMV appears to be spread mainly by contact. Contact can occur from contaminated tools, hands, and clothing. Direct plant-to-plant contact may also spread the virus. Seed or insect transmission appears to be unlikely at this time. Its ability to spread through recirculating irrigation systems is unknown. Further study is required.

Control
European recommended control strategies for PepMV focus on sanitation. Plant removal, limited access to affected rows, sanitation of clothing and tools are all critical. Submerging hands and tools in skim milk prior to and after working with each plant has been reported to reduce transmission of the virus.

Additional Information

• Management of Pepino Mosaic Virus in Greenhouse Tomatoes - Ontario Ministry of Agriculture, Food and Rural Affairs
   

Tomato Infectious Chlorosis Virus (Closterovirus)

Distribution
The disease was first reported in 1993 from California field tomatoes. Infected petunia and ranunculus in greenhouses were also confirmed. In 1997, the virus was found in greenhouse tomatoes in North Carolina and Italy.

Symptoms
Symptoms can be confused with nutritional disorders, pesticide toxicity, or natural senescence. Tomato leaves have yellowing between veins, leaf rolling, and tissue death (browning) (figure 5). Infected leaves (especially older ones) may also turn red.

Figure 5: Tomato plant infected with Tomato Infectious Chlorosis Virus. Courtesy of G. Wisler, USDA-ARS, California

Spread
Transmitted by the greenhouse whitefly (Trialeurodes vaporariorum). Other whiteflies (Bemisia spp.) are not known to transmit TICV. The virus is not mechanically transmitted.

Tomato Yellow Leaf Curl Virus (Bigeminivirus)

Current Distribution
TYLCV has been a major threat to tomato production in Israel since 1930 and, then, was introduced into the Dominican Republic, Cuba, and Jamaica in the early 1990s’. Since then, TYLCV has been reported from Africa, Asia, Australia, Caribbean, Europe and North and Central America. In North America, TYLCV is known to occur in Florida, Georgia, Louisiana, and has recently been detected in California and Arizona.

Symptoms
Visible symptoms can be seen on tomato plants after 2-3 weeks of initial infection by TYLCV (figure 6). Symptoms can vary slightly depending on the tomato cultivar and amount of virus infection. Generally, infected plants have smaller-than-normal leaves that are cupped upward, thick and rubbery with chlorotic margins (figure 7). Young infected plants become severely stunted. Infected plants drop up to 90% of their flowers resulting in major yield losses.

Figure 6: Leaf symptoms of a tomato plant infected with Tomato Yellow Leaf Curl Virus. Courtesy of T. Schubert, Florida Department of Agriculture.

Figure 7: Severe symptoms of a tomato plant infected with Tomato Yellow Leaf Curl Virus. Courtesy of T. Schubert, Florida Department of Agriculture.

Spread
TYLCV is transmitted by adult silverleaf whiteflies, Bemisia argentifolii, previously known as Bemisia tabaci biotype B, (figure 8). The silverleaf whitefly is a different species than the common greenhouse whitefly (figure 9), which does not transmit TYLCV. Silverleaf whiteflies are rarely seen in British Columbia. They are small, phloem-feeding insects which pick up the virus while feeding on infected host plants. Since TYLCV has a broad host range, it can be spread by silverleaf whiteflies that feed on other infected host plants, including cultivated and wild plants. Not all host plants infected with TYLCV show visible symptoms, however the virus can be acquired by silverleaf whiteflies from symptomless infected plants and transmitted to tomato. The virus can also be transmitted through cuttings taken from infected plants or it can be introduced into a greenhouse with virus-infected transplants. TYLCV can not be transmitted mechanically or by workers and is not transmitted by seeds. Even if TYLCV shows up in British Columbia, it may not become established due to the unlikely survival of silverleaf whiteflies in winter months. On the other hand, there is evidence to suggest that TYLCV can be persistent and can overwinter in infected plants and plant materials.

Figure 8: Adult Silverleaf whiteflies, Bemisia argentifolii, feeding on the underside of a leaf. Wings are usually folded vertically along the body while feeding. Nymphs do not have filaments or fringe around their body. Photo courtesy of Scott Bauer, USDA Agricultural Research Service, www.forestryimages.org	Figure 9: Greenhouse whitefly, Trialeurodes vaporariorum, feeding on the underside of a leaf. Wings tend to lie parallel to the body. Nymphs have filaments around their body. Photo courtesy of Whitney Cranshaw, Colorado State University, www.forestryimages.org

Control Measures
TYLCV is very difficult to control once it becomes established. The possible presence of TYLCV on many host plants (including weeds) in the absence of tomato, and the ability of silverleaf whiteflies to feed and reproduce on tomato make it difficult to control the disease successfully. It is highly recommended that a collective management approach, i.e. a combining of cultural practices and insect-vector control strategies, should be adopted to reduce the impact of TYLCV on tomato.

• Use virus-free transplants that come from certified transplant nurseries
• Use tomato cultivars, if any, that are resistant to TYLCV
• Monitor for silverleaf whitefly population throughout the tomato growing season (spring to fall) and, if present, apply appropriate insecticides that are registered in British Columbia for greenhouse tomato. Protection of young tomato plants from silverleaf whitefly-feeding is essential to reduce potential yield losses.
• If silverleaf whiteflies are detected in British Columbia greenhouses, use reflective or coloured (yellow) mulches that may reduce silverleaf whitefly populations feeding on tomato leaves.
• Use of trap plants, preferably cucurbits, that are preferred by whiteflies can reduce infection rate on tomato.
• Maintain periodic scouting for TYLCV symptoms, particularly on young tomato plants, and remove and deep-bury or incinerate the infected or suspected tomato plants.
• Maintain healthy growth of tomato plants to minimize TYLCV damage.
• Since TYLCV has a broad host range, including many weeds that are symptomless, maintain a rigorous weed control program in and around the greenhouse during the growing season and winter months.
• TYLCV is not yet detected in British Columbia and it is very difficult to eradicate once established. Therefore, preventing the introduction and establishment of the disease is very important. Submit any suspected TYLCV-infected plant samples or suspected silverleaf whiteflies to the BCMAL Plant Diagnostic Laboratory for proper diagnosis and confirmation.

References

• Brown, J.K, Frohlich, D.R., and Rosell, R.C. (1995). The sweetpotato or silverleaf whiteflies: biotypes of Bemisia tabaci or a species complex? Annual Review of Entomology 40: 511-534.
• Brunt, A.A., Crabtree, K., Dallwitz, M.J., Gibbs, A.J., Watson, L. and Zurcher, E.J. (eds.) "Plant Viruses Online: Descriptions and Lists from the VIDE Database. Version: 20th August 1996. URL http://biology.anu.edu.au/Groups/MES/vide/
• Csizinszky, A.A., Schuster, D.J. and Kring, J.B. (1997). Evaluation of color mulches and oil sprays for yield and for the control of silverleaf whitefly, Bemisia argentifolii (Bellows and Perring) on tomatoes. Crop Protection 16: 475-481.
• Duffus, J.E., Liu, H., and G.C. Wisler. (1996). Tomato infectious chlorosis virus - a new clostero-like virus transmitted by Trialeurodes vaporariorum. European Journal of Plant Pathology 102:129-226.
• Polston, J.E., McGovern, R.J., and Brown, L.G. (1999). Introduction of Tomato yellow leaf curl virus in Florida and implications for the spread of this and other geminiviruses of tomato. Plant Disease 83:984-988.
• Wisler, G.C., J.E. Duffus, H. Lie, R. Li, and B.W. Falk. (1997). New whitefly-transmitted closterovirus identified in tomatoes. California Agriculture. 51:24-26.
• Pro-MED mail report Feb. 2, 2000 (From EPPO Report, 2000-01) and Feb. 26, 2007 (Yellow Leaf Curl, Tomato – USA, Arizona: First Report)

British Columbia Ministry of Agriculture & Lands
Abbotsford Agriculture Centre
1767 Angus Campbell Road
Abbotsford, BC V3G 2M3
Phone: (604) 556-3001
Fax: (604) 556-3030

Updated May, 2007

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