Main Symptoms
 

Thanatephorus cucumeris is responsible for a multitude of symptoms on tobacco. It is a common agent of damping-off in nurseries (figures 1 and 2). It causes lack of germination and water-soaked lesions on the stem at soil line. Afterwards these lesions turn brown and gradually girdle the collar. Simultaneously, or later, the seedlings wilt and die very quickly, giving the impression of having "melted".
 
If the contamination or damage is not detected in the nursery and the infected seedlings are planted, a reddish brown canker (figure 3 and 4) occurs within a few weeks of planting. When the canker expands, the affected plants wilt, suddenly turn yellow and dry up.

Under certain conditions (heavily contaminated soils, favourable climate ...), the root system of mature plants is invaded externally by the asexual form of T. cucumeris,  Rhizoctonia solani. A brown mycelial network is easily distinguished on the surface. Invaded roots turn yellowish to brown, they are superficially corky and many rootlets are decomposed (figure 5).
 
The perfect form or teleomorph of the fungus, T. cucumeris, causes leaf spots on both the young nursery plantlets and on mature plants in the field (figures 6-8). The spots are often wet and circular at the beginning. Then, depending on climatic conditions, they show wide concentric patterns and a chlorotic halo of varying intensity. At the centre of the spots, the most fragile tissues fall out, leaving holes. During wet periods, a hymenium is formed at lesion margins, upon which basidia and basidiospores start developing.
 
 
Some of the information on the worldwide distribution of the disease is obtained from surveys conducted by CORESTA (Cooperation Centre for Scientific Research Relative to Tobacco). The reliability of the information is not confirmed in all countries.