Viruses are simple organisms which can form devastating diseases in plants. Viruses attack plants at a cellular level and rapidly spread throughout the plant, not just the area showing symptoms, making any vegetative propagation from that plant impossible without risk of passing the virus to the next generation. They can also exist within plants latently, that is to say that they are present but induce no symptoms of their attack until they are transferred to another plant (eg by grafting) at which point the virus becomes apparent. Most plants can be infected by viruses of one type or another, although to date no viruses affecting conifers have been identified.

Viruses can be transmitted by sap, for example via secateurs which aren’t cleaned between plants, through grafting or by sap sucking insects who can carry the disease (insects are known as ‘vectors‘ in this context). Weeds can also host viruses and pass them to other plants.

Many symptoms of viral infection are very similar to symptoms of other diseases, so they can be difficult to identify.

While viruses can’t be ‘cured’ as such, they can be removed from plant material by heat treatment, which is used to produce certified virus free stock (eg of raspberries). The best option for most gardeners is to adhere to good hygiene practices, reduce weeds and sap sucking insects (which can act as vectors) and purchase certified virus free plants where possible.

The naming of viruses has not been as controlled, or Latinised, as for other organisms. Mostly the virus is named after the original plant it was found on, though often the same virus is found on other types of plants, making the name slightly disingenuous at times. For example, the cucumber mosaic virus can also affect dahlias, lilies and narcissi (daffodils).