Viral diseases: Tobacco mosaic virus (TMV) is a widespread plant pathogen affecting many species of plants including tomatoes. It gives a distinctive ‘mosaic’ pattern of discolouration on the leaves which affects the growth of the plant due to lack of photosynthesis.

The idea

Tobacco mosaic virus (TMV) is a widespread plant pathogen that infects many species, including tomatoes. It causes a distinctive mosaic pattern of discolouration on leaves: patchy light and dark areas rather than uniform green. The discoloured areas contain less chlorophyll, so the leaf absorbs less light for photosynthesis. Less photosynthesis means less glucose is made for growth, so the plant grows poorly. TMV spreads between plants by direct contact, including contaminated hands, tools and clothing, and via infected sap; it is not spread by insect vectors such as aphids. There is no cure, so control relies on removing infected plants, disinfecting tools and washing hands to reduce spread. A useful comparison is a healthy leaf, which is uniformly green, with an infected leaf showing patchy discolouration.

What earns the marks

• TMV is a widespread plant pathogen affecting many species of plants, including tomatoes.
• The discoloured areas contain less chlorophyll, so less light is absorbed for photosynthesis.
• Reduced photosynthesis lowers glucose production, which affects the growth of the plant.
• TMV spreads by contact between plants, including contaminated tools, hands and sap; it is not spread by aphids or other insect vectors.
• Control relies on removing infected plants and reducing contact spread, for example by disinfecting tools, because there is no cure.

Common mistakes and how to put them right

• Saying TMV kills the plant directly.

  Put it right: It reduces growth because photosynthesis is reduced, and the plant may become weakened.

• Confusing the mosaic pattern with a mineral deficiency.

  Put it right: TMV is caused by a virus and shows a patchy mosaic of discolouration, not a uniform yellowing.

• Stating that TMV spreads by aphids or other insect vectors.

  Put it right: TMV spreads by contact, including contaminated tools, hands and sap; aphids are not vectors for TMV.

• Stating that TMV affects only tobacco.

  Put it right: It is a widespread pathogen affecting many species, including tomatoes.

Exam tips

• Link the mosaic pattern to chlorophyll loss, then to reduced photosynthesis and reduced growth in a clear chain.
• Name at least one other plant affected, such as tomatoes, to show the pathogen is widespread.
• When describing control, mention removing infected plants and reducing contact spread, such as disinfecting tools, rather than suggesting a cure or blaming aphids.