Main symptoms

Thanatephorus cucumeris ( Rhizoctonia solani ) is a soil fungus which develops preferentially on organs present in the soil or located nearby. Very polyphagous, practically present in all market garden soils, it can be found in nurseries on young seedlings as well as on adult plants. Too often underestimated, it is capable of producing significant damage to the roots of adult plants.

Like Pythium spp. above all, but also other soil fungi, it is a well-known agent of damping-off on melon, raging both pre- and post-emergence of seedlings. In particular, it produces yellowish to more or less dark brown lesions on the roots and hypocotyl. Ultimately, a canker may surround more or less the part of the stem located at the level of the soil or the substrate (figure 1), causing the collapse and death of the seedling.

R. solani can also be responsible for damage roots to melon after planting (Figures 2 and 3):
- yellowing, browning and disappearance of small nourishing roots;
- yellowing, browning of the cortex which may become soft or superficially suberise. The lower part of the stem in the soil may also be affected, with the cortical tissues turning a gray to brown green tint and gradually rotting.

The other organs of the melon are sometimes affected: certain portions of the stems, but especially the part of the fruit in contact with the ground. On it develops a more or less superficial and circular lesion, tawny to brownish in color, showing small bursts (figures 4 and 5).

On or near damaged tissues, the characteristic can be observed brown mycelium of this fungus in the form of discrete isolated filaments (figure 6) or sometimes rather dense cords (figure 3).