Symptoms

• Symptoms on foliage

PMTV causes a wide range of symptoms which depend greatly upon the cultivar and the environmental conditions.

The three most common foliage symptoms are:

- yellow blotching or mottling: bright yellow spots, sometimes in the form of arcs or chevrons (photo 1) on leaflets, particularly on the lower leaves;

- the most characteristic symptom is a shortening of the internodes of the upper leaves, inducing stunting of the top shoots (photo 2), hence the name mop-top;

- chevrons or light green bands: chlorotic V-shaped markings or flecks on the upper leaves (photo 3).

These symptoms are favoured by cool conditions (5° -15°C).

• Symptoms on tubers

In the year of infection by the soil, the tubers of susceptible cultivars develop internal brown necrotic arcs (‘spraing’) seen as brown rings on the tuber surface. The arcs develop at the boundary of the virus-infected tissue in response to a specific change of temperature before or after harvest but do not prevent the virus from spreading through the tuber. Temperature variations (for example in cold storage) can induce more or less concentric rings or lines (photo 4), which often extend into the flesh of the tuber (photo 5) almost continuously, unlike TRV symptoms. The tubers of other cultivars may develop superficial raised rings without internal symptoms or may show little evidence of infection.

In the year following infection by the soil, the tubers of some cultivars may be symptomless or cracked and distorted, or they may develop internal brown arcs.