The discovery of tobacco mosaic virus (TMV), the first virus ever identified, marked the beginning of the field of virology. After a major disease outbreak devastated tobacco crops in the mid-1800s in Europe, plant pathologists endeavored to identify the causal agent. In 1898, Martinus W. Beijerinck discovered that the pathogen was neither a fungus nor a bacterium but a different kind of pathogen altogether that could only replicate within the living cells of host organisms, which he called a “virus.” The discovery of the virus was groundbreaking in the field of plant pathology and the medical sciences and greatly advanced our understanding of plant disease.

In the early 1900s, Tobacco mosaic virus (TMV) and Tomato mosaic virus (ToMV) were major threats to the tobacco and tomato industries. Fortunately, today, resistant cultivars and an understanding of the importance of clean seed have minimized the spread of these viruses. It is still vital, however, to understand how to identify, prevent, and manage these viruses, as these highly infectious and persistent pathogens can lead to a major loss in crop yield and quality for growers. This article will discuss TMV and ToMV together because they are genetically similar, and their attributes, such as hosts, symptoms, and management strategies, vary slightly.

Tomato brown rugose fruit virus (ToBRFV) is a recently emerging tobamovirus similar to TMV and ToMV. It was first discovered in Israel in 2014 and has since spread to Mexico, the US, and several countries in Europe and Asia, however, it is not yet widely distributed in the US but has been introduced to operations via contaminated seed.

All three viruses are members of the genus Tobamovirus. ToMV is a specific strain of TMV that primarily infects tomato plants. Virus particles, called virions, are rod-shaped and measure 18 x 300 nm. Each rod contains a single strand of RNA enclosed by a coat of protein. The viruses infect the plant by entering microscopic wounds in the plant cells. TMV and ToMV can survive in infected plant debris such as leaves and roots and are able to remain dormant for up to two years, depending on the temperature and moisture level of the soil. ToBRFV is closely related and can also survive in soil and plant debris for long periods of time.

TMV commolnly infects tobacco while ToMV commonly infects tomato;, however, both viruses have a wide host range of over 200 species of herbaceous plants. Other common hosts include plants in the Solanaceae (pepper, eggplant, potato) and Cucurbitaceae (squash, pumpkin, cucumber) families, as well as ornamentals like begonia, verbena, geranium, and petunia. Due to the availability of resistant crop varieties, only 1% of tobacco crops and about 20% of tomato crops are affected each year. ToBRFV also has a wide host range of over 40 species in the Amaranthaceae, Apocynaceae, Asteraceae, and Solanaceae families. However, the only two major crops ToBRFV is known to affect are tomatoes and peppers.

Temperatures around 24°C (75°F) favor the development of TMV and ToMV. Symptoms tend to be milder at higher temperatures. Specific environmental conditions that promote the development of ToBRFV are mostly unknown, but all tobamoviruses are known to spread most rapidly in areas where plants are grown at a high density, such as in greenhouses.

TMV, ToMV, and ToBRFV rarely cause the entire host plant to die, but the viruses can greatly reduce fruit quality and yield. Symptoms of these viruses vary depending on stage of infection, host plant, strain of virus, and environmental conditions. The most distinct symptoms are mottled leaves (Figure 1), which have blotchy light and dark spots (mosaic pattern). New growth and host plants infected at a young age may be stunted and chlorotic. Leaves of infected tomato plants tend to develop yellow streaks or spots, and may be malformed (Figure 2). Tomato fruit may ripen unevenly, resulting in deformation with rugose or wrinkled appearance (especially so with ToBRFV), and have brown spots and/or ringspots (Figure 3) causing the fruit to be unmarketable.

Figure 1: Mosaic pattern of ToMV on tomato leaf

Inga Meadows, Extension Vegetable Pathology, NC State University

×

Figure 1: Mosaic pattern of ToMV on tomato leaf

Inga Meadows, Extension Vegetable Pathology, NC State University

Print Image

Figure 2: Symptoms of ToMV on tomato plant

Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org

×

Figure 2: Symptoms of ToMV on tomato plant

Mary Ann Hansen, Virginia Polytechnic Institute and State University, Bugwood.org

Print Image

Figure 3: Symptoms of ToBRFV on tomato fruit

Luria et al. 2017. A New Israeli Tobamovirus Isolate Infects Tomato Plants Harboring Tm-22 Resistance Genes

×

Figure 3: Symptoms of ToBRFV on tomato fruit

Luria et al. 2017. A New Israeli Tobamovirus Isolate Infects Tomato Plants Harboring Tm-22 Resistance Genes

Print Image

All three viruses are highly infectious and can be easily transmitted from contaminated transplants, seeds, tools, grafts, sap, and most commonly, the hands of workers. Though the viruses are not typically transmitted by insects, they could potentially spread through insects that have fed on infected plants. These particularly hardy viruses can remain dormant in plant debris and soil for long periods of time and can infect other host plants well after the initial host plant has died. Environmental conditions dramatically affect the ability of the viruses to remain dormant and infectious. For example, ToMV can survive in leaf debris in dry soil for up to two years, whereas in moist soil the virus remains infectious for only up to one month.

There is no cure for plants infected with viruses and there are currently no chemical treatments. Preventive and cultural management practices are therefore necessary to reduce the spread of TMV, ToBRFV and ToMV. Tomato growers should practice the following control measures.

• Sanitize all tools, equipment, and work surfaces before planting.
• Discard all infected plants, contaminated soil, and dead leaves as TMV can remain dormant and infectious in plant and soil debris.
• Rotate crops susceptible to TMV, ToBRFV and ToMV with crops resistant to the viruses, or non-host plants.
• Use resistant tomato varieties. Tomato cultivars resistant to TMV and ToMV currently available contain the Tm-1, Tm-2, and Tm-2² resistance genes (Table 1). Efforts are underway to identify cultivars resistant to ToBRFV, but there are currently no resistant varieties currently commercially available.

Reduce weeds in and around the field or greenhouse as they may harbor the viruses.

Disease Control for Homeowners

Home gardeners should follow the control measures mentioned above. Another simple method for preventing TMV and ToMV infections at home is to spray transplants with milk and/or dip hands in milk before planting, since milk has been shown to reduce TMV infection rates. Avoid touching the hose or watering can to plants since the viruses spread through direct contact. Remove and discard all weeds as they may harbor one of the viruses.

Disease Control for Commercial Growers

Commercial growers should follow the protocols recommended above. Additional measures include:

• Treat seeds with a 10% solution of trisodium phosphate for 15 minutes and/or heat dry seeds at 70°C (158 °F) for 2-4 days. These methods have been shown to eliminate viruses in the seed without inhibiting germination.

If you are a commercial grower and suspect that your tomatoes are infected with ToBRFV it is imperative that you get an accurate diagnosis through the NCSU Plant Disease and Insect Clinic, your local Extension personnel, or your local state plant health regulatory official.

The 2023 North Carolina Agricultural Chemicals Manual provides information on the

selection, application, and safe use of chemicals.

The NC State University Extension Plant Pathology Portal provides information on crop disease

Management.

The Southeastern US Vegetable Crop Handbook provides information regarding control products used for commercially grown vegetables.

The NC State University Plant Disease and Insect Clinic (PDIC) provides diagnostics and control

recommendations.

USDA Animal and Plant Health Inspection Service has more information on ToBRFV

Agrios, G. N. 1988. Plant Pathology. 3^rd edition. Academic Press Inc.

Jones, J. B., Zitter, T. A., Momol, T. M., Miller, S. A. Compendium of Tomato Diseases and Pests. 2nd Edition. APS press.

Lueloff, S., Hudelson, B. 2021. Tobacco Mosaic. University of Wisconsin-Madison Extension.

Moorman, G.W. 2015. Tobacco Mosaic Virus (TMV). PennState Extension.

Scholthof, K-B.G. 2000. Tobacco mosaic virus. The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-1010-01.

Scholthof, K-B. G. 2008. Tobacco Mosaic Virus: The Beginning of Plant Pathology. Online. APSnet Features. doi: 10.1094/APSnetFeatures-2008-0408.

Tomato brown rugose fruit virus. 2023. International Plant Protection Convention (IPPC). Food and Agriculture Organization of the United Nations.

Tomato Mosaic. Department of Plant Pathology. Ohio State University.

Wollaeger, H. 2014. Common questions and answers about tobacco mosaic virus. Michigan State University Extension.

Zhang, S., Griffiths, J., Marchand, G., Bernards, M., Wang, A. 2022. Tomato brown rugose fruit virus: An emerging and rapidly spreading plant RNA virus that threatens tomato production worldwide. https://doi-org.prox.lib.ncsu.edu/10.1111%2Fmpp.13229