Infections by one or more of Canna yellow mottle virus (CaYMV), Bean yellow mosaic virus (BYMV) and Canna yellow streak virus (CaYSV) cause a range of symptoms in cannas including stunting, distortion and streaking in the leaves and colour ‘breaks’ in the flowers.

CaYMV infects only cannas, but BYMV infects a wide range of plants, including beans, peas, freesias and gladioli. CaYSV is a relatively new virus, found for the first time in the UK in 2007. In infected plants, symptoms on newly-infected leaves can be expected whenever the plants are growing from mid spring until late autumn.

For more on how these viruses are transmitted, see the Biology section below.

You may see the following symptoms:

• CaYMV causes streaks of pale tissue between the leaf veins. These later die, resulting in streaks of dead brown tissue
• BYMV causes a yellow mottling of the leaves
• CaYSV causes leaf-streaking similar to CaYMV
• Infections can result in stunted plants with distorted leaves
• ‘Breaks’ (white streaks) may occur in the flowers

When more than one virus is present in the same plant the symptoms may be more severe.

Non-chemical control

• Take care to buy healthy plants, avoiding any which are stunted or have suspicious colour patterns on leaves (though note that some cultivars have highly variegated leaves and it is very difficult to tell if these are infected without specialist tests)
• Do not propagate from any plant suspected of having virus infection, but destroy it before it can act as a source of infection for others

Chemical control

There are no chemical controls for virus infections. It is not practical to protect plants from the aphid vectors by using insecticides.

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 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, or 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 vector of CaYMV is not known, but it is thought it may be transmitted mechanically, for example on propagating tools. BYMV and CaYSV are vectored by aphids, which become contaminated with virus-containing sap when feeding on infected tissues, then fly to healthy plants and transfer the virus infection during feeding. BYMV may also be transmitted through infected seed.