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Two types of virus S strain can be distinguished: the PVS° or ordinary strain, very widespread in Europe (Germany, Poland, central Europe) and the PVSA Andean strains, classified as quarantine parasites because they produce much more severe reactions.
The virus S can be very easily transmitted both by contact, by passing machinery, for example, as well as by aphids that depend on a non-persistent mode (such as virus Y). The most frequently mentioned carrier aphids are Aphis nasturtii and Myzus persicae.
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The virus S generally causes weak (latent) and variable symptoms according to the variety and viral strain :
• lightening of the foliage;
• depression of the veins on the upper side of the leaves (roughness to the touch) (photo);
• reduction in leaf size;
• tips of the leaflets inclined downwards;
• on very sensitive varieties : leaves with a bronze appearance, necrotic spots, smaller plants.
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