Symptoms

• 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.