• Conservation, sources d'inoculum
  

Thanatephorus cucumeris (Rhizoctonia solani) is frequently found in many soils which have repeatedly grown vegetable crops. It has in fact saprophytic potentialities allowing it to be preserved in the soil in the absence of sensitive hosts. It is found in the state of mycelium (figure 1) and pseudo-sclerotia (figure 2), often in organic matter and the most diverse plant debris that it easily colonizes. It grows easily in the soil, especially if the soil has been disinfected and cleared of potential antagonistic microorganisms. This parasitic fungus, very polyphagous , can attack and persist on the most diverse hosts and on 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 and invasion

Contamination takes place through the mycelium present in the soil or from sclerotia. This can superficially colonize all the organs of the tomato present in the soil or in contact with it. Rare contaminations can also take place through the basidiospores (figure 3). In this case, these germinate and give rise to mycelial filaments. Subsequently, the mycelium enters the tissues directly through the cuticle or through various injuries. Its inter- and intracellular evolution is often very rapid and destructive because of its enzymatic equipment, but also if it encounters favorable climatic conditions. This parasitic process is the cause of damping-off and visible lesions and rots on the various organs of the tomato.

• Sporulation and dissemination

From the damaged tissues, the fungus forms mycelium which travels on the tissues and on the ground, and reaches other healthy organs. Sclerotia, mixed with soil particles contaminating different materials, also contribute to its spread. Its sexual form can also perform this function, as we have seen previously. These are the basidiospores (figure 3) formed on basidia present on the surface of the hymenium which often provide more aerial dispersion. These spores can be spread by wind and air currents, over fairly large distances.

• Conditions favorable to its development

Thanatephorus cucumeris can thrive in moist, heavy soils as well as lighter, drier soils, at acidic or basic pH and temperatures between 5 and 36 ° C. Soils that are too dry or too wet seem to inhibit it. It can attack the tomato throughout its development cycle. It is particularly damaging in the presence of humidity and when the temperatures are rather mild, of the order of 23-27 ° C or when they are unfavorable to the tomato, especially below 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. It should be noted that its attacks can be serious when it forms a parasitic complex with other root pests, in particular root-knot nematodes. Injuries caused by Meloidogyne spp. would promote penetration and infections linked to T. cucumeris . For example, the development on tomato roots of Meloidogyne incognita would increase those of T. cucumeris and Thielaviopsis basicola .