Viruses are minute parasitic entities, too small to be seen by a light microscope. They consist only of a nucleic acid core and a protein coat and are unable to grow outside the living cells of their plant hosts. They cause a bewildering variety of symptoms, some of which are very hard to distinguish from mineral deficiency or herbicide damage. Virus infection can reduce yields drastically, but rarely kills the plant.
Viruses have no means of movement and rely on other organisms (vectors) to transmit them from diseased to healthy plants. These are often sap-sucking insects such as aphids, thrips, whiteflies, or by leaf-feeding beetles, or plant-feeding mites. Soil-inhabiting nematodes (eelworms) and fungal pathogens can also transmit viruses. Some viruses are also very contagious and can be transmitted on pruning knives or gardeners’ hands. Viruses are also transmitted from one plant generation to the next via infected cuttings or graft material and a relatively small number can pass through infected seed.
Once a virus particle enters the host plant cell, it hijacks the cell’s synthetic chemistry to create more virus particles, diverting the cell’s resources from normal activities. The virus nucleic acid takes over the cell’s own nucleic acid, instructing it to synthesise new virus nucleic acid and protein. In the process, the plant’s normal growth activities are disrupted and the visible symptoms are produced.
Viruses then require a fresh vector to transmit them to new host plants. Once acquired by a host, some viruses immediately start to die away and the host only remains infective for a few hours. This is referred to as non-persistent transmission. In other cases the host remains permanently infective (persistent transmission).
Many viruses are unable to survive long outside their host plant or vector and are rapidly killed by exposure to the heat and ultra-violet light in sunlight. However, some are robust enough to survive and be transmitted via pruning tools, and a few can survive composting.
The host ranges of some viruses are very wide, others are restricted to a few closely related plants. Plants may therefore be infected by more than one virus at any one time. The symptoms that a plant shows are determined by which viruses are present, the relative proportions of each, the order in which they infected the plant, the plant’s natural resistance to the strains of the viruses, and the environmental conditions: some symptoms are temperature dependent, for example. Some viruses are symptomless, allowing invisibly infected plants to be reservoirs of infection.
A note on virus names: Viruses are named according to the type of symptom they cause in the first host in which they were discovered, though this is often only the first of many. Thus Bean yellow mosaic virus actually infects a wide range of plants as well as beans; some are in the family Iridaceae, completely unrelated to legumes. To some extent the host range is determined by the host preferences of the vectors.