One can observe characteristic sporangia and chlamydospores of Olpidium brassicae in roots and rootlets (particularly at the epidermis and cortex) of the plants produced in the float water system. The presence of an obligate parasitic fungus in these organs seems to lead to two rather opposite situations: 

 

- The roots seem healthy or slightly altered, which is surprising given the number of sporangia and chlamydospores   sometimes present in the cells (figure 1);
- The roots are completely brown and the plants become yellow, wilt and die (figures 2 and 3).
These two situations are also found in other systems of tobacco plant production.  
 

We do not really understand why this aquatic fungus, which is also a vector of several viruses (see its Main Symptoms), can change from a simple coloniser of root cells to a destructive pest. Environmental conditions including temperature and soil oxygenation, substrate or nutrient solution, and especially the root condition, must certainly influence its behavior. Note that this fungus is well adapted to aquatic life. Like Pythium spp. it can produce mobile zoospores allowing its  spread easily in water.
 

 For further information on this fungus, see Olpidium brassicae fact sheet.

 
 

Other aquatic organisms related to fungi and oomycetes belonging to Chytridiomycetes demonstrate similar behavior to this by Olpidium brassicae, but only in the float water  systetm (figures 4 and 5). Their identification is rather difficult. Some of their structures are similar to those that indicate the presence of Rhizophydium or other aquatic organisms. Several of these pathogens  have already been observed in roots of some Nicotiana species, especially N. tabacum: Lagenocystis radicicola (Vanterp. and Ledingham) HF Copel. (1956) (formerly Lagena radicicola) and Rhizophydium graminis Ledingham (1936).
 
 

Last change : 02/08/13

• Author :
• D Blancard (INRAe)