Biology, epidemiology

- Conservation, sources of phytoplasmas

The phytoplasmas are preserved in the vector leafhoppers and on different hosts , cultivated (carrot, spinach, celery ...) or not, which constitute reservoirs for important phytoplasmas. In the case of " Candidatus Phytoplasma sp. " (Aster yellows phytoplasma), it overwinters in the body of its vectors. This phytoplasma is polyphagous , it is found in particular on several weeds ( Salsola tragus , several species of Sonchus and Plantago , Taraxacum officinale , Lactuca serriola ... It has also been reported on Brussels sprouts, Senecio cruentus , Argyranthenium frutescens , dahlia , Hydrangea macrophylla , Lavandula officinalis , olive tree, Primula sp., Spartium junceum , Violeta odora , tomato . These plants harbor it, multiply it and also serve as sources of inoculum.

The cycle of vector leafhoppers involves eggs which play no role in the winter conservation of the phytoplasma but ensure the survival of the insect from one season to another.


- Transmission, dissemination

As previously suggested, this phytoplasma is transmitted persistently , during feeding bites, by several species of leafhoppers: Macrosteles laevis , Macrosteles quadripunctulatus , Macrosteles severini , Macrosteles quadrilineatus (syn. Macrosteles fascifrons ). The latter species seems to be the most frequently involved in the transmission of this phytoplasmosis. Leafhoppers, stinging and sap-sucking insects, contaminate salads during their migration. Once in contact with the leaf, they bite into the phloem vessels to feed, injecting or removing phytoplasmas as they pass (figure 1). The phytoplasma (s) multiply in these insects, reaching various organs, including the salivary glands, making leafhoppers infectious. Macrosteles quadrilineatus can remain so for at least a hundred days.

These insects are usually occasional on infected plants. They are able to fly long distances or stay put. Five generations can succeed each other during a season on different hosts. The date of onset of symptoms, which generally occurs 30 to 45 days after contamination, depends on the period of migration of the vector (s). Migration is a complex phenomenon that results from the transfer of insect populations from place to place in the form of mass flights. The causes of migration are poorly understood. They seem to be linked to local conditions unfavorable to leafhoppers. Among the factors that influence migration and the nature of flights, we can cite hunger, overcrowding, host deterioration, endocrine deficiency in insects or genetic effects, photoperiod, temperature, wind. .. Insects prefer young plants with succulent tissue. In times of drought, they are more likely to switch from wild plants to irrigated crops. Appetite phenomena are sometimes observed, but they are poorly understood. Cold winters help reduce winter populations.

This aster yellows phytoplasma is not seed transmissible .