There are over 50 viruses worldwide that affect sweet peas, including many that affect peas, beans, clover and alfalfa (lucerne) as well.

In reality only four are of major importance to gardeners in the UK. These are:

• Pea enation mosaic virus (PEMV)
• Bean yellow mosaic virus (BYMB)
• Pea mosaic virus (PMV)
• White clover mosaic virus (WCMV)

Symptoms may be seen when the plants are growing from spring until late summer.

You may see the following symptoms:

• Mottled and yellow (chlorotic) or white-streaked leaves
• Dead spots or patches on leaves
• Colourless veins
• Translucent patches on leaves
• ‘Breaks’ (white streaks) on flower petals
• Stunting and loss of vigour

Note that some of these symptoms are similar to those of nutrient deficiencies, but these are unlikely to occur if a balanced fertiliser is used.

Non-chemical control

• If infection is suspected, it may be better to remove and destroy infected plants to prevent them from being a source of infection for other plants, as there are no treatments for virus infection
• Consider buying good quality seed, rather than plants, as most viruses are not seed-borne
• Remove leguminous weeds such as clover, which may harbour viruses
• Clean pruning tools with garden disinfectant between plants
• Sweet peas, particularly cordon-grown ones, are also subject to virus-like symptoms caused by stress due to adverse conditions, so check that appropriate growing conditions are being met

Chemical control

There are no chemical controls. The use of insecticides to reduce aphid transmission is not practical.

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, of which at least 11 (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).