Symptoms

The two groups of phytoplasmas induce the following symptoms in the potato:

• Yellows group (stolbur, aster yellow and purple top):

On foliage:

- purplish pigmentation of the tips (photo 1);
- yellow leaf discolouration with curling of the young leaves; plants with upright growth habit and numerous thin stems.

In dry weather, the plants wither and die. The disease develops slowly in cool and wet weather.

On the underground parts:

- presence of aerial tubers or long (aerial) stolons with very numerous small tubers;

- necrotic rot at the root tips;

- tubers lose water and become soft and flaccid, and then rot during storage.

Tubers from diseased plants give threadlike and hairy sprouts and weak shoots (photo 2).

• Witches’ broom group:

- proliferation of numerous spindly, upright growing stems.