Damping-off

  
It is common to observe a group of seedlings in a given location (foci) of tobacco nursery starting to wilt, turn yellow, wither and die sooner or later. This is a classic syndrome called "damping-off" (figure 1). 
 
Several soil-borne pathogens may cause damping-off such as: Olpidium brassicae, several species of Pythium spp. and Thielaviopsis basicola. These micro-organisms preferentially attack the roots and collars of young tobacco plants.
 
Other fungi such as Rhizoctonia solani (figure 2), Botrytis cinerea or Sclerotinia sclerotiorum, and bacteria of the genus Pectobacterium are also pathogens infecting the collar and stem of seedlings (see Collar cankers). In some cases these micro-organisms may also induce damping-off. 
 
It is not easy to differentiate the different pathogens with the naked eye. Besides, all these pathogens may easily attack the very soft and not yet lignified seedling tissues.

Figure 1	Figure 2	Figure 3	Figure 4

 
When damping-off occurs one can distinguish the different symptom that are a result of the different pathogens (figures 3 and 4):
- a root system may be completely brown or when Olpidium brassicae is present . These outbreaks are common in float water production systems;
- a soft, wet and brown rot at the collar, and rather light brown roots are observed in the case of Pythium spp. attacks. Note that these outbreaks are rare in the nursery;
- roots totally or partially black may be related to infections by Thielaviopsis basicola which is more frequent at this stage of tobacco production.