Causal agent(s) and transmission
Two types of Potato virus S strains may be distinguished: the ordinary strains (PVSO), distributed worldwide and very common in Europe (Germany, Poland, Central Europe), and the Andean strains (PVSA), classified as quarantine parasites in Europe as they cause much more severe reactions.
PVS is a Carlavirus and it is very easily transmitted both by contact between diseased and healthy plants (e.g. with farm implements) and by aphids in a non-persistent mode. The most frequently mentioned vector aphids are Myzus persicae, Aphis nasturtii and Aphis frangulae.
Host range is narrow. Susceptible species belong mainly to the Solanaceae and Chenopodiaceae.
Yield reduction caused by PVS infection is usually low, at worst 10 to 20%, except in the case of mixed infections with other viruses.
Symptoms on foliage
On the majority of potato cultivars, PVS causes few or no symptoms, especially for PVSO strains which usually are symptomless (latency) or cause mild symptoms, which vary according to the cultivar and the viral strain:
- lightening of the foliage (slight chlorosis);
- deepening of the veins on the upper side of the leaves (rough to the touch, rugosity) (photo 1);
- smaller leaf size;
- undulation of the margin;
- tips of the leaflets inclined downwards (photo 2).
In highly susceptible cultivars, the symptoms are more pronounced, with a bronzed appearance, necrotic spots and smaller plants.
PVSA has been reported to cause more severe symptoms.
The development of PVS infection results from a combination of:
- different sources of infection, either internal (volunteers, seed tubers, weeds) and/or external (nearby potato crops, gardens, weeds);
- the relative significance of flights of aphids during the growing season (related to climatic conditions and nearby crops);
- the level of control measures taken in seed potato production (roguing, crop management such as weed control, crop protection, haulm destruction, field and/or lot inspection and lab testing);
- the susceptibility of the potato cultivar.
PVS has little effect on yield and consequently control measures are only justified for seed potato production.
Production of certified seed potatoes is based on a range of measures limiting the infection by viral diseases such as PVS and involves:
- the use of certified seed tubers resulting from the multiplication of virus-free material in a certification scheme (including specific multiplication procedures such as in-vitro micropropagation, intensive field inspections and large-scale post-harvest testing);
- the limited use of machinery for field production of pre-basic and basic seed material;
- the production in a favourable environment: in isolated areas with low virus/vector pressure and in fields away from household gardens or away from areas used for the production of ware and processed potatoes;
- the early roguing of infected plants and eradication of all sources of inoculum, e.g. weeds and volunteers, to limit the spread of viruses within the field by machinery;
- the treatment with mineral oils to reduce the transmission of non-persistent viruses such as the PVS. Insecticides are only effective against persistent viruses such as PLRV.
Additional control measures in seed potato production include:
- early planting;
- early haulm destruction (by chemicals and/or mechanically) prior to maturity to limit the infection of daughter tubers by late flights of aphids.