What are strawberry viruses?

Many viruses and phytoplasmas affect strawberries, either singly or in combination. These may lead to strange appearances such as green petals, crinkling and yellow spotting and vein banding of the leaves. Worse still, stunting, poor growth and loss of yield.

Those mentioned here all occur in the UK. However, a number of other strawberry viruses exist elsewhere.

Symptoms

These are very variable. Two of the most frequently encountered are commonly known as 'yellow edge' and 'crinkle' and are caused by the combination 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, Tobacco streak virus, Raspberry ringspot virus)
• Flowers are small, have green petals and do not develop worthwhile fruit (Clover phyllody phytoplasma)

In strawberries the symptoms are very variable and complex. Many viruses are involved and they can act to 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

Viruses are minute parasitic entities consisting only of a nucleic acid core and a protein coat. They cannot reproduce except in the cells of the host plant, where they 'hijack' the cell's synthetic mechanisms to produce more virus particles, causing a variety of symptoms in the process.

Viruses require a vector organism to transmit them to new hosts;

• 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, Tobacco streak virus and Raspberry ringspot virus vectors are soil inhabiting nematodes (eelworms)
• Clover phyllody phytoplasma is a phytoplasma, not a virus, but also requires a vector, in this case a leafhopper; the source of infection is usually infected clover plants nearby

The host ranges of some of these viruses, for example Arabis mosaic virus and Tobacco streak virus, are very wide and many plants can act as sources of infection. Some of the others are restricted to strawberries (including wild strawberries) and close relatives such as raspberry.