Causal agent(s) and transmission
PVA is a Potyvirus transmitted by many aphid species in the non-persistent manner, notably Myzus persicae, Macrosiphum euphorbiae, Aphis frangulae and A. nasturtii.
As in the case of PVY, PVA can be acquired by M. persicae in less than one minute and transmitted to another leaf equally rapidly with no latent period. The virus may be retained for 20 min to 2 hours after its acquisition before it loses its infectivity.
The host range of PVA is narrow and confined to Solanaceae such as tobacco or tomato.
Virus A is widespread in most potato-growing countries but it is considered much less important than PVY. Most cultivars are resistant and yield losses are usually low even if the level of infection can reach up to 40% in some cultivars or in the case of co-infection with other viruses.
Symptoms on foliage
Symptoms caused by PVA vary widely according to the combination of the virus strain, the potato cultivar, the environmental conditions and the type of infection, either primary or secondary infection.
In the case of an infection occurring during the current year, the infection may be symptomless or the symptoms may take the form of slight, transient mosaic symptoms which can be seen mainly in overcast weather. Mosaic patterns are alternating patches of light green and dark green areas without any leaf distortion and PVA discolourations/mottles are often not limited by veins. This symptom pattern can be more visible by placing a sheet of white paper under the leaf concerned.
In the case of an infection that occurred the previous year, the symptoms may be far more pronounced, possibly with wrinkling of the leaves accompanied by symptoms of brightness (photo 1) and more or less severe mosaic symptoms depending on the cultivars concerned (photos 2 to 5).
Potatoes infected with PVA in combination with PVX or PVY may show pronounced symptoms such as crinkling, severe leaf deformation and yield reduction.
Symptoms of PVA are more easily observed in cold and cloudy weather.
Facteurs de risques
The development of PVA 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 signifi cance of fl ights 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 and field and/or lot inspection);
- the susceptibility of the potato cultivar.
Most of the cultivars available in Europe are resistant to PVA. Otherwise, PVA control measures are similar to those for PVY as these viruses have the same mode of transmission.
For the production of ware or processed potato crops, the main measures to control virus infection are the use of certified seed potatoes and of resistant cultivars.
Production of certified seed potatoes is based on a range of measures limiting the infection by viral diseases during the growing period and involves:
- the production in a favourable environment: in isolated areas with low virus/vector pressure and in fields away from household gardens or fields 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;
- the treatment with mineral oils to reduce the transmission of non-persistent viruses such as PVA. 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 by mechanical means) prior to maturity to limit the infection of daughter tubers by late flights of aphids.