During 2004, rot outbreaks very occasionally affected the plant cover of an industrial tomato plantation in the south-west of France. All the aerial organs were more or less decomposed. Affected tissues rotted abruptly as a result of wet lesions rapidly invading the leaves. Wet and brown alterations gradually developed on sections of the stem and eventually surrounded them (Figure 1). A dense, whitish felting covered them. The tissues of the stem cortex would eventually be completely rotten and fall apart .
 

A microscopic observation of the mycelium allowed us to see that it was in fact a hymenium on which basidia formed, structures ensuring sexual reproduction in Basidiomycetes.
 

Fruits were also affected. They presented two types of lesions appearing on the parts in contact with the ground:

- more or less circular and chancery alterations (figure 2), sometimes superficially suberized, which revealed more or less brown concentric patterns. Superficial, brown mycelial filaments of Rhizoctonia solani could be observed on lesions and / or on nearby healthy tissue;

- moist and soft rots covered by the same dense and whitish felting already observed on the other aerial parts of the tomato.
 

This pathological context, unprecedented in tomato, was due to the simultaneous manifestation on this crop of Rhizoctonia solani and its teleomorph Thanatephorus cucumeris ( Bottom rot ). It was therefore not surprising to see the same plants, the mycelium brown characterizing R. solani and white hymenium with basidia features of T. cucumeris (Figure 3), especially as R . solani is well known on tomatoes, responsible in particular for damping off, lesions on roots and crowns and on fruits .

The symptoms observed occasionally in the southwest are quite original and seem to be able to be attributed in part to T. cucumeris . Remember that this basidiomycete is known in other plant species to be responsible for damage related to airborne contaminations, initiated by its basidiospores. This is the case, for example, on another nightshade, tobacco, on which it is the source of leaf spots called “ target spots” .

For additional information on this fungus, you can consult the sheet Thanatephorus cucumeris ( Rhizoctonia solani ) fact .