Causal agent(s) and transmission
Potato leaf roll virus is a Luteovirus transmitted by aphids feeding on the plant sap. The transmission of Potato leaf roll virus is called persistent (or circulative) because the aphid only becomes infectious after a latency period required for the transit of the virus in the insect’s intestine and then in its salivary glands. Few aphid species can transmit PLRV: Myzus persicae is the most efficient vector but Aulacorthum solani and Macrosiphum euphorbiae may also transmit PLRV.
The natural host range of PLRV is rather narrow and most known hosts (about 20 species) are in the Solanaceae family. The hosts belong to the cultivated plants (such as potato or tomato) or to the uncultivated species (e.g. Physalis floridana, Datura stramonium which can be a virus reservoir). Non-solanaceous hosts include Amaranthus caudatus, Celosia argentea, Gomphrena globosa and Capsella bursa-pastoris.
PLRV can cause a yield reduction of up to 50- 80% on susceptible cultivars and, in some cases, it may cause tuber net necrosis.
Facteurs de risque
The development of PLRV infection results from the combination of different sources of infection:
- internal to the field (volunteers, seed tubers and weeds);
- external (nearby potato crops, home-gardens and weeds);
- 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 or field and lot inspection);
- the susceptibility of the potato cultivar.
Symptoms on foliage
The symptoms vary with the cultivar, the climatic conditions and according to whether the infection has occurred during the current or the previous year:
- infection during the current year:
Primary infection results in yellowing of the apical leaves and sometimes an erect growth habit. The upper leaflets may be slightly curled at their base (inward rolling) and have a lighter colour (photo 1). Purple pigmentation can sometimes be seen on the edges.
- infection from the previous year:
Secondary infection usually results in more severe symptoms with stunted plants, erect growth habit (photo 2) and typical curled leaves (photos 3 and 4). The in-rolling of leaflets is less pronounced on the young leaves which are usually pale yellow than on the bottom leaves which are tightly curled and hardened (leaves break with a cracking sound when crushed). Sometimes leaflets have purple edges due to the formation of anthocyanin (photos 4 and 5).
Severe yellowing and in-rolling of the upper leaves can, more rarely, be associated with red-purplish discolouration at the edges of the leaflets (photo 6).
Symptoms on tubers
During the growing period, the mother-tubers of infected plants decompose more slowly and the daughter tubers remain small.
Some potato cultivars like Russet Burbank can react to PLRV infection with the formation of many brown necrotic spots, usually localised near the surface of the tuber (photo 7) or around the vascular ring (net necrosis). The necrosis seems to be related to warm climatic conditions in the field and can develop in storage.
For potato crops intended for ware and processing, the use of certified seed potatoes and of resistant cultivars are the main ways of controlling infection due to viral diseases.
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 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);
- 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 groundkeepers, to limit the spread of viruses within the field;
- The treatment with insecticides in order to reduce the number of aphids. Do not treat in very hot weather and check the quality of the application. An aphid warning system, based on field monitoring, can be used to evaluate aphid pressure and the need for insecticide application.
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.