Biology, epidemiology

- Conservation, sources d'inoculum

Pseudomonas cichorii is very easily maintained in soil and on plant debris. This bacterium (Figure 1) was isolated directly from soil in Japan. It colonizes the rhizosphere of a good number of hosts , cultivated or not. It is found in the pathogenic state on several vegetables, such as cabbage, endive, celery, tomato, eggplant, several legumes, tobacco, flowers (gerbera, chrysanthemum, pelargonium ...) and many weeds: Sonchus oleracea , Veronica sp., Solanum nigrum , Portulaca oleracea , Poa annua , Setaria sp., Senecio vulgaris , Capsella bursa-pasteuris ...

It can be stored on seeds for several months and would survive in certain contaminated water reserves. Certain tools and harvest boxes are also sources of contamination.


- Penetration

Contamination can take place at the end of the rosette stage, thanks to rains or irrigation. Pseudomonas cichorii does not seem to have considerable parasitic potential on lettuce. It mainly penetrates the salad leaves through natural openings, such as the stomata, or wounds (growth bursts, wounds due to the wind, insect damage or related to cultivation operations, etc.). Many bacteria can be noted in the guard cells and in the intercellular spaces of the epidermis.

They subsequently gain the mesophyll. Direct contaminations through the cuticle are rare and occur when the leaves are covered and saturated with water. In fact, this bacterium is present on the surface of the leaves throughout the growing cycle of salads. Population density fluctuates during the salad cycle depending on climatic conditions and perhaps salad types. The outer leaves constitute an important source of inoculum allowing the contamination of the leaves of the heart subsequently. As the harvest approaches, the level of the bacterial population increases and the inoculum pressure intensifies. The first spots appear quickly after contamination (about 30 hours). The browning of the tissue progresses from the epidermis to the mesophyll. In heavily infested soils, contamination via the roots or the taproot could occur. The bacteria, behaving like a vascular microorganism, would be responsible for browning the main veins.


- Dissemination

It certainly takes place through splashing water, runoff and normally through seeds. This is particularly the case on some host plants attacked by this bacterium. It can be acquired and transmitted by the leafminer Liriomyza trifolii on chrysanthemum. Contaminated plants in nurseries also ensure its dissemination.


- Conditions favorable to its development

Pseudomonas cichorii thrives at temperatures between 5 and 35 ° C , its optimum being around 20-25 ° C. Above 53 ° C, the bacteria are killed. She particularly likes environments humid . It is for this reason that it is rife mainly during prolonged rainy periods, during which the water deposited on the leaves is favorable to contamination and its dissemination.