The quality of irrigation water is as important as the quality  of seeds and substrate. One should be very careful because the water from a river or storage reservoir can be contaminated with Olpidium brassicae, Phytophthora nicotianae, Pythium spp., Thielaviopsis basicola, or polluted by abandoned herbicide containers. When weed control is carried out near the nursery, one should be careful of any spray or potential runoff.

 
 

The chemical quality of water should not be neglected especially in the floating seedbed systems. It should have a pH between 5 and 7 and low chlorine content. It must also be adequately provided with calcium, magnesium and sulfur. Tobacco tolerates sodium well enough but an excess of accumulated salinity is sometimes observed close to the roots. This phenomenon may occur in nurseries irrigated with sprinkling. To avoid such a situation, the clods should be kept sufficiently moist. Very high bicarbonate content is harmful to tobacco; it induces seedling stunting, leaf yellowing and curving down (figure 1). The bicarbonate content of water can be corrected by adding, for example sulfuric acid. Boron excesses or deficiencies have also been observed in the USA. Regardless of the situation, you should talk to your technician and get your water tested if necessary.

 

We would like to draw your attention to the risks present in  tobacco nurseries especially when pesticides are used in the nutrient solution of floating seedbed systems. Soon after the adoption of this system, technicians and producers had hastily thought they could make their pesticide treatments through the nutrient solution by using systemic pesticides. This measure, while reasonable and feasible in some cases, should not be generalized because it has some constraints (such as use an effective pesticide, with a spectrum of adequate properties) and carries hazards:
- Induction of root necrosis and various phytotoxicities to tobacco (figure 2);
- Inefficiency of some contact fungicides applied at root level against airborne pathogens;
- Questioning certain strategies that involve a contact fungicide with a systemic fungicide already concerned by the resistance phenomenon.

 

One may think , this is just an easy solution that  may eventually cost more than it will benefit, especially considering that  small nursery surfaces will  be treated. Choice of pesticide should be carefully considered and take place after thorough experimentation and verification of pesticide registration for this type of use.
 

Last change : 02/04/13

• Author :
• D Blancard (INRAe)