Biology, epidemiology

- Conservation, sources d'inoculum

Thanatephorus cucumeris ( Rhizoctonia solani ) is frequently found in many soils which have repeatedly grown vegetable crops. It has saprophytic potentials allowing it to be preserved in the soil in the absence of sensitive hosts. It is found in the state of mycelium and pseudo-sclerotia (figures 1 and 2), often in organic matter and plant debris the most diverse that it easily colonizes. It grows easily in the soil, especially if the soil has been disinfected and free of potential antagonistic microorganisms. It is a fungus very polyphagous parasitic which can attack and persist on the most diverse hosts and their debris. It can be present in certain substrates and composts, sometimes in certain peats or on a few purchased plants. It is not uncommon for it to pollute non-disinfected material used in nurseries.


- Penetration, invasion

Contamination takes place through the mycelium present in the soil or from sclerotia. This superficially colonizes the leaves in contact with the ground (figure 3). Subsequently, it enters the limbus directly through the cuticle, via the stomata or via various injuries. Its inter- and intracellular evolution is often very rapid , especially if it encounters favorable climatic conditions. This parasitic process is the cause of damping off and basal rots on lettuce.


- Sporulation, dissemination

From the damaged tissues, the fungus forms mycelium (figure 4) which will travel on the tissues and on the ground and gain other healthy salads. These sclerotia , mixed with soil particles contaminating different materials, also contribute to its dissemination. It also has a sexual form which can ensure its aerial dissemination. This is reported in other plants; it involves basidiospores . These spores are formed on basidia present on the surface of a hymenium that is sometimes found on the ground or on the leaves (figure 5). These spores can be spread by wind and air currents, over fairly large distances. Their role in the epidemiology of Thanatephorus cucumeris on salad has, to our knowledge, never been clarified.


- Conditions favorable to its development

Thanatephorus cucumeris can develop in both damp, heavy soils than in lighter, drier soils, to acidic or basic pH and at temperatures between 5 and 36 ° C . Soils that are too dry or too wet seem to inhibit it. It mainly attacks salads near the harvest because it finds very favorable conditions at this stage of their development. In fact, the plants then completely cover the ground and the microclimate under the plant cover is thus modified. The lack of ventilation at the base of the salads leads to an increase in humidity; we then have a real humid chamber. Free water on the leaves is not essential. Unlike Botrytis cinerea and Sclerotinia spp., Attacks of Thanatephorus cucumeris on lettuce tend to take place when temperatures are mild, of the order of 23-27 ° C, and in the presence of humidity. Some authors report "cold" strains developing at low temperatures. The minimum temperature required for infections is 9 ° C. At this temperature, the incubation period is 11 to 15 days while it lasts only 3 days at 20 ° C. Faced with the diversity of strains present in the field, it is difficult to specify the optimal conditions for the development of this fungus.