Thanatephorus cucumeris (eg Rhizoctonia solani ) is a fungus soil which develops preferentially on organs present in the soil or located nearby.

Like Pythium spp., It is a well-known agent damping-off , affecting both pre- and post-emergence of seedlings. It can cause reddish-brown lesions on all parts of the germinated seed. After emergence, a brownish alteration may surround more or less the part of the stem located at ground level, causing the collapse and death of the seedling.

It also produces cankers located at the foot (the collar) of young plants or more developed plants, grown in some damp and heavily contaminated soils. These canker damage is reddish-brown in color and varies in consistency with soil moisture (Figures 1 and 2). Ultimately, they can completely surround the plants. The roots adjacent to these cankers show a more or less marked browning. In addition, this fungus is sometimes capable of producing more or less extensive reddish-brown root lesions and rather moist; the roots can also become superficially corky and mycelium of the fungus is present on the surface (Figure 3).

Aerial symptoms can also appear on plants. Of lesions Blightbeasts brownish , more or less shrunken, have been reported on tomato stems. They start from damaged leaflets in contact with the soil or come from contaminations generated either by projections of soil particles, or by basidiospores produced by the teleomorph of this fungus, Thanatephorus cucumeris .

The green fruits (figure 4), but especially more or less mature (figures 5 and 6), present more or less circular deteriorations appearing in contact with the ground. Firm at first, these lesions consist of light rings alternating with darker ones, gradually softening. Like many diseases occurring on fruits, the damage can be colonized by secondary invaders which amplify the damage.

Several fungal structures make it possible to confirm the presence of this fungus on or near the damaged tissues, whatever the organ attacked:
- discrete whitish to brown filaments of T. cucumeris running on germinated seeds, roots, along the stem and on the fruits;
- poorly defined, brown masses are sometimes visible on damaged tissues (the sclerotia of this fungus);
- mycelium strongly agglomerated , whitish to cream in color, constituting the hymenium of the sexual form T. cucumeris . It is on the latter that the basidia and basidiospores are formed, structures rarely visible on the organs of the tomato.