Stolbur and other Phytoplasmas

Causal agent(s) 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.

Symptoms on foliage

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

• 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.

Witches’ broom group:

• proliferation of numerous spindly, upright growing stems.

Symptoms on tubers

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

• 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).

Risk factors

The development of phytoplasmas results mainly from a combination of sources of infection, with the presence of other host plants in the vicinity, and the presence of its vectors, the leafhoppers (hot and dry summers favour both the development and the migration of leafhoppers).

Controlling weeds within and around a susceptible crop can reduce the potential phytoplasmas reservoirs and also the host plants for the survival and the spread of the vectors.

Control

Various methods are possibly used to detect phytoplasmas: biological indexing (grafting on indicator plants), serology for certain strains, molecular PCR tests (universal phytoplasma primers or primers specific to the stolbur phytoplasma) or electron microscopy.

Stolbur is one of the quarantine pathogens listed by the European Union on the potato and is thus subject to mandatory control measures.

In regions where phytoplasmas are present, preventive measures include:

• destroying vector leafhoppers (insecticides) and weeds;
• avoiding the proximity of host plants;
• using only certified seed potatoes.