Tomato viruses

What are tomato viruses?

More than 20 viruses affect tomatoes worldwide, causing a wide variety of mosaic patterns and distortions to the leaves, stunted growth and marbling patterns on the fruit, whenever the plants are growing from late winter until early autumn.

The most common tomato viruses are:

• Tobacco mosaic virus (TMV)
• Tomato spotted wilt virus (TSWV)
• Pepino mosaic virus (PepMV)
• Cucumber mosaic virus (CMV)

Pepino mosaic virus, has quarantine status in the UK.

For more on how these viruses are transmitted, see the Biology section below.

Symptoms

You may see the following symptoms:

• Stunted growth is a common symptom, especially when several viruses are present
• Mosaic patterns of lighter green on the leaves (TMV, CMV, PepMV)
• Fern leaf, where the leaf blade is reduced in size to give a fern-like appearance (TMV, CMV, PepMV)
• Brown streaked leaves (TMV)
• Young leaves curl downwards and become bronzed (TSWV)
• The leaf surface appears distorted and bubbly (PepMV)
• Dry set, when fruit fail to set (TMV)
• Fruit develop a bronzed, patchy appearance (TMV)
• Fruit ripens unevenly with pale patches or marbling (TSWV, PepMV)

Stunting, distortion and fern leaf may also be caused by exposure to hormone weedkillers, to which tomatoes are very sensitive. These are volatile and can act from a distance, without direct contact. Providing the source is removed, plants usually recover, but they do not usually recover from virus infection.

Control

Non-chemical control

• Remove plants with symptoms promptly
• After handling infected plants, wash hands and tools in hot soapy water. As an additional precaution, sterilise tools in the disinfectant Virkon S, obtainable from farm suppliers
• Avoid growing other susceptible plants in close proximity
• Varieties with claimed resistance to TMV include ‘Cherry Wonder’, ‘Cumulus’, ‘Dombito’, ‘Dona’, ‘Estrella’, ‘Ida’, ‘Nimbus’, ‘Piranto’, ‘Shirley’ and ‘Sonatine’

Chemical control

There are no chemical controls. The use of insecticides to reduce aphid transmission is not practical.

Biology

Plant viruses share many of the characteristics of those that infect animals, though they do not cross infect (plant viruses only infect plants). Viruses are extremely minute and consist of a protein coat and a core of nucleic acid. They have no means of self-dispersal, but rely on various vectors to transmit them from infected to healthy plants. Once viruses penetrate into the plant cells they take over the cells’ nucleic acid and protein synthesis systems and hijack them to produce more virus. They then require another vector to feed on the infected tissue and carry them to a new host.

Vectors

• CMV is vectored by aphids
• TSWV is vectored by thrips, especially the western flower thrips
• TMV is very easily spread mechanically on tools or fingers
• PepMV is mechanically transmitted, although seed transmission is possible
• TMV is occasionally transmitted through seed

Several of these viruses have other common garden host plants. CMV has a very wide range of hosts, not only among cucurbits. TMV also affects tobacco and potato.  TSWV affects many plants in the tomato family (Solanaceae) and also gloxinias (Sinningia), arum lilies and dahlias.

PepMV was first detected in Europe in 1999 and is a quarantine organism. See the Fera website for more information on symptoms.