Plant viruses are extremely minute infectious particles consisting of a protein coat and a core of nucleic acid. They have no means of self-dispersal, but instead rely on various vectors (including humans) 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.
Viruses are frequently transmitted through propagated material but, depending on the virus, can also be transmitted via insect or mite vectors, pollen, mechanical transfer via contaminated hands and tools, and nematode vectors in the soil. Some viruses can be transmitted via seed, but generally these are a minority and therefore seed propagation is often a useful way to ensure virus-free plant material.
The VIDE (Virus Identification Data Exchange) database lists over 50 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 legume crops such as peas, beans, clover and alfalfa.
Many of the sweet pea viruses have aphids as vectors, but some are transmitted by mechanical means (for example on tools or by handling) and a few are transmitted by soil-dwelling nematodes (eelworms).