In order to get the best tobacco plants one must use good quality seed and a healthy substrate. It is common to find producers who, faced with a sanitation problem in the nursery, hastily accuse either the quality of the seed or the soil they use. This behaviour is common no matter what the production is, but is it justified in tobacco production?

 

In France, quality of tobacco seed is relatively good. In general varietal purity and germination are checked before marketing. Furthermore, there is not any serious seed-transmitted tobacco disease known to occur in France. Usually producers use seed that germinate well enough and free of pathogens.

 

 It is difficult to certify that a substrate is perfectly healthy. Cases of contamination by Pythium spp., Olpidium brassicae and T. basicola have been reported in other countries.
 

Analyses carried out in France, of peat of various origins, did not allow us to isolate T. basicola from them. Yet some substrates seemed to be contaminated. In fact, they were originated from farms at the end of plant production cycle and were collected from bags already opened or stored in bad conditions. Under these storage conditions, one may suspect that contamination could have taken place, particularly from dust or soil particles harboring the fungus. It is a good example of the role the environment may play in seedling infection or contamination of the substrate.

 

We should keep in mind that compost or sand mixed to a peat substrate, can sometimes be contaminated as well. For example, Pythium spp. have been isolated from washed river sand. In this case, the source of contamination is not the peat, but the other substrate (sand) mixed.

 
 

It is sometimes reported in the literature that the substrates can be disinfected. Technicians frequently used methyl bromide fumigant (which is now banned in France and in many countries) or steam. With regard to the peat-based substrates, their excessive drying may cause the formation of micro-air pockets that affect the proper distribution of fumigants in them. In case of the steam treatment, it is recommended to provide water retention capacity at 1/3 and to maintain a temperature of 100 ° C for 30 min. Metam sodium is not recommended for organic substrates; however it can be used for the treatment of mineral substrates.

You should note that disinfection can be effective, but it also causes equipment problems and involves a few disadvantages such as:
- Destruction of natural micro-organisms, antagonistic to pathogens;
- Increase of susceptibility of disinfected soils to pests;
- Development of toxic phenomena (excess of exchangeable manganese, excess of ammonia that results in an almost complete blockage of nitrification ...).

Last change : 02/13/13

• Author :
• D Blancard (INRAe)