Plant 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.
The names of many raspberry virus diseases are based on the symptoms seen in affected plants, rather than the name of the causal virus. For example raspberry mosaic, probably the most common and damaging virus disease, can be caused by at least five different viruses (the more important being Rubus yellow net virus and Raspberry leaf spot virus).
One disease of raspberries, called Rubus stunt, is caused by a phytoplasma (these share some characteristics of both viruses and bacteria).
Viruses and phytoplasmas require a vector to transfer them between plants. The vectors of raspberry viruses include:
- Aphids: All of the viruses causing raspberry mosaic are aphid-transmitted
- Soil-borne nematodes (eelworms): The most common is Arabis mosaic virus
- Leafhoppers: These are common vectors of phytoplasmas, including Rubus stunt
Some viruses can also invade the seed of raspberries and one, Apple chlorotic leaf spot virus (which causes a disease called raspberry bushy dwarf) can spread from plant to plant in infected pollen grains.
Some of the viruses affecting raspberries (e.g. Arabis mosaic virus, Cucumber mosaic virus) have very wide host ranges, and can affect a large number of garden plants and even weeds.