Plant viruses share many of the characteristics of those that infect animals, though they do not cross infect (plant viruses only infect plants). Viruses are extremely minute and consist of a protein coat and a core of nucleic acid. They have no means of self-dispersal, but rely on various vectors to transmit them from infected to healthy plants. Once viruses penetrate into the plant cells they take over the cells’ nucleic acid and protein synthesis systems and hijack them to produce more virus. They then require another vector to feed on the infected tissue and carry them to a new host.
The VIDE (Virus Identification Data Exchange) database lists 57 viruses worldwide that are capable of infecting sweet peas, at least 11 of which (and probably more) occur in the UK. A large number were first recorded from related crop legumes such as peas, beans, clover and alfalfa.
Many viruses have aphids as vectors (disease-carriers) but some are transmitted by mechanical means (for example on tools or by handling) and a few are transmitted by soil-dwelling nematodes (eelworms). Some can be transmitted through seed to the next generation but the majority are not.