Causal agent(s) and transmission
PVM belongs to the Carlavirus group like the Potato virus S (PVS).
It has a limited range of natural hosts. Most susceptible species belong to the Solanaceae, of which the potato is the most important. But experimentally, PVM has been transmitted to some Chenopodiaceae and Fabaceae.
It is transmitted in a non-persistent manner by the aphid Myzus persicae, less efficiently by Aphis frangulae, A. nasturtii, and Macrosiphum euphorbiae. Some isolates may be transmitted mechanically (e.g. by machinery).
PVM is common throughout the world, especially in Eastern Europe and Russia where some cultivars can be 100% infected.
Yield losses are usually low (10-20%) although some virulent strains may cause more severe leaf deformation and lead to higher (up to 50%) yield loss.
Symptoms on foliage
Depending on the virus strain and the potato cultivar, PVM symptoms range from very mild to severe: mottle, mosaic pattern, crinkling and rolling leaves and possibly associated with stunted shoots.
Typical symptoms of PVM have been called paracrinkle or potato leaf rolling mosaic as they have the appearance of rolling in the top of the plants (photo 1) with mild mosaic and ”spoon-shaped” rolled leaflets. Leaf rolling due to PVM is soft when touched whereas Potato leaf roll virus induces a «cracking» leaf rolling. It occurs in developed plants, usually on the top leaflets.
With some cultivars, the following symptoms may also be observed (photos 2 et 3):
- deformed leaflets;
- slight discolouration of the veins in the top leaves;
- waviness on the edges of the leaves;
- leaf mosaic symptoms (photo 2).
Symptoms are more visible with cloudy and temperate weather.
The development of PVM 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 and field and/or lot inspection);
- the susceptibility of the potato cultivar.
As PVM virus has generally a very limited effect on yield, PVM is controlled in most of the seed potato-producing countries by testing the selection material and the basic seed potatoes to ensure healthy seed potatoes.
General measures used for the production of certified seed potatoes limit the infection by viral diseases such as PVM. They include:
- 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 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 PVM. 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.