Candidatus Phytoplasma spp.

Stolbur and other phytoplasmas 

(Stolbur, Aster yellow, and purple top phytoplasmas)

• Causal agents and transmission

The diversity of the phytoplasmas and their hosts makes classification very difficult. They are now grouped according to their genetic proximity rather than according to their symptoms.

Usually, two main groups of potato phytoplasmas are identified:

• phytoplasmas causing yellowing diseases (stolbur, aster yellow and purple top), which are identified according to the symptoms, host range or RNA polymorphism;

• phytoplasmas causing witches’ broom diseases.

The main vectors of stolbur MLO are insects of the leafhopper family (Macrosteles sp., Empoaca sp. and Hyalestes sp.) which can naturally disperse it over large distances but phytoplasmas can also be transmitted by a parasitic plant, the dodder (Cuscuta spp.).

The stolbur phytoplasma needs reservoir plants (e.g. dodder, tomato, aubergine, henbane, belladonna, datura, field bindweed) which are an inoculum source for further dispersal, as it appears that Stolbur is not transmitted by the botanical seed of any of its host plants.

The probability of spread in potato tubers is extremely low, because the risk of transmission of the stolbur phytoplasma to the daughter-tubers is rare.

Phytoplasmas, such as Stolbur, are mycoplasma-like organisms (MLOs) that are closely related to bacteria but which have no cell walls and cannot be grown outside a living organism.

• Significance

Outbreaks in potatoes are sporadic and related to the spread of vectors from other infected crops.

The significance of Stolbur and other MLO is usually more important on other crops, such as tomatoes or aubergines, and their incidence on the potato is not usually important, except in rare cases where infected plants may have quite a low yield and produce necrotic or rotted tubers.

Phytoplasma diseases are found especially in Central Europe as well as in Southern Europe, the Middle East, the United States, Australia and some other countries.