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 (figures 1 and 2) and pseudo-sclerotia (figure 3), 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 (figure 2) present in the soil or from the sclerotia (figure 3). This can superficially colonize all the organs of the melon present in the soil or in contact with it. To our knowledge, no contamination involving basidiospores (figure 4) has been reported on melon. Subsequently, the mycelium enters the tissues directly through the cuticle or through various injuries. Its inter and intracellular evolution is often very fast and destructive because of its enzymatic equipment, but also if it encounters favorable climatic conditions. This parasitic process is at the origin of damping-off of seedlings, lesions and rots visible on the various organs of the melon.

• Sporulation and dissemination

From the damaged tissues, the fungus forms mycelium which travels on the tissues (figure 1) and on the ground, and reaches other healthy organs. Sclerotia (Figure 3), mixed with soil particles contaminating different materials, also contribute to its dissemination. Its sexual form can also perform this function, this can be the case in other cultures. In this case, it is the basidiospores (figure 4) formed on basidia present on the surface of the hymenium which ensure aerial dispersion. These spores can be spread by wind and air currents, over fairly large distances. The more or less affected plants contribute to its dissemination.

• Conditions favorable to its development

T. cucumeris thrives in moist, heavy soils as well as in 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 melon 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 melon, especially below 20 ° C.

Note that the severity of its attacks fluctuates on Cucurbitaceae depending on the strains, but also on other telluric pests in complex with it on the roots.