What are strawberry viruses?

A large number of viruses infect strawberries, either singly or in combination. Often, symptoms are worse with multiple infections.

Virus infection can result in a wide range of symptoms, including mottling, leaf crinkling, yellow spotting, vein banding (lighter areas surrounding the veins), plant distortion and stunting. Runner production and fruit yield may also be affected.

Some of most common strawberry viruses in the UK are Strawberry crinkle virus, Strawberry mottle virus, Strawberry mild yellow edge virus, Strawberry vein banding virus, Strawberry latent ringspot virus, Arabis mosaic virus, Tomato black ring virus, Strawberry necrotic shock virus and Raspberry ringspot virus.

Symptoms

Two of the most frequently encountered virus symptoms are known commonly as 'yellow edge' and 'crinkle', and are caused by the combinations of viruses listed below;

• Plants are stunted with a yellow edge to the younger leaves (Strawberry mild yellow edge virus, Strawberry mottle virus, Strawberry crinkle virus)
• Leaves develop yellowish spots and become crinkled (Strawberry mottle virus, Strawberry crinkle virus, Strawberry vein banding virus)
• Plants are stunted, leaves are crinkled and have yellow spots or blotches which may later turn red (Arabis mosaic virus, Tomato black ring virus, Strawberry necrotic shock virus, Raspberry ringspot virus)

In strawberries the symptoms can be very variable and complex. Multiple viruses infecting strawberries at the same time can produce symptoms which vary according to the virus species involved, the relative proportions of each, the environmental conditions and the response of the particular host variety. However, in practical terms the most important effect is the loss of vigour and yield caused by some viruses and virus combinations, which can render the crop worthless.

Control

Non-chemical control

• Always buy plants which are certified as virus free. It is unwise to accept gifts of plants from old strawberry beds – these will almost certainly be infected with one or more viruses, although of course, if the yield is still good the risk may be worth taking
• Destroy and replace plants as soon as yields start to fall, usually after two or three years. Do not use runners from these plants, which will certainly be infected. Instead, buy new certified, virus-free stock
• If possible, avoid replanting strawberries on the same site. Since it is not practical to determine which virus is present on the basis of symptoms (because these are so variable), it would be prudent to assume that some of the viruses spread by soil nematodes may be involved. The nematodes will persist in the soil and may infect new plants

Resistance: Cultivars vary in the degree of resistance they show to each virus, but none are widely resistant to the extent they can be recommended on this basis. 

Chemical control

None available. The insecticides currently available to gardeners are non-persistent and will not provide sufficient control of the aphid vectors to prevent virus spread.

Biology

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.

Strawberry viruses are vectored to new hosts in a number of ways:

• The vectors of Strawberry mottle virus, Strawberry crinkle virus, Strawberry mild yellow edge virus and Strawberry vein banding virus are aphids
• Arabis mosaic virus, Tomato black ring virus and Raspberry ringspot virus are vectored by soil-inhabiting nematodes (eelworms) and by seed, and these can be easily transmitted between plants on tools and hands
• Strawberry necrotic shock virus is transmitted by thrips, and is also likely to be transmitted by seed, pollen and on tools and hands.

Some strawberry-infecting viruses, for example Arabis mosaic virus, Raspberry ringspot virus and Tomato black ring virus, infect a wider range of plants, including other fruit crops, which can act as sources of infection. Some of the others are restricted to strawberries (including wild strawberries) and close relatives such as raspberry.