Viruses are minute parasitic entities consisting only of a nucleic acid core and a protein coat. They cannot reproduce except in the cells of the host plant, where they 'hijack' the cell's synthetic mechanisms to produce more virus particles, causing a variety of symptoms in the process.
Viruses require a vector organism to transmit them to new hosts;
- The vectors of Strawberry mottle virus, Strawberry crinkle virus, Strawberry mild yellow edge virus and Strawberry vein banding virus are aphids
- Arabis mosaic virus, Tomato black ring virus, Tobacco streak virus and Raspberry ringspot virus vectors are soil inhabiting nematodes (eelworms)
- Clover phyllody phytoplasma is a phytoplasma, not a virus, but also requires a vector, in this case a leafhopper; the source of infection is usually infected clover plants nearby
The host ranges of some of these viruses, for example Arabis mosaic virus and Tobacco streak virus, are very wide and many plants can act as sources of infection. Some of the others are restricted to strawberries (including wild strawberries) and close relatives such as raspberry.