Nursery

A particular growing environment

Tobacco plants are produced in different ways:
- Traditionally in a soil disinfected with the use of a fumigant (fumigants are less frequent nowadays in France) (figure 1);
- in pots of varying sizes, made from a peat substrate (figure 2);
- in hydroponic float system production (which is the general tendency today) (figure 3).

Similar to many other cultivated species, the production takes place at a time of the year during which growing conditions, including weather, are not favourable for plant growth and this is why the production is carried out indoors. It also allows maintaining favourable  physiological conditions and environment for the seedlings.

Seedlings  as well as mature plants may be infected by the same diseases. Yet, unlike mature plants the seedlings in their juvenile stage have particularly tender tissues, which make them much more susceptible to diseases than mature plants.

Environmental conditions greatly influence the quality of nursery plants. The results of inappropriate conditions, such as high density planting, unsuitable fertilisation and poor climatic conditions, are either etiolated seedlings, or proliferate growth of succulent tissue, which is even more vulnerable to diseases.

In general, the environment in nursery shelters can be characterised:
- by a lower than normal light levels;
- by very limited gas exchange with the outside environment;
- by higher relative humidity as a resultof evap-transpiration from plants and soil;
- by above optimal temperatures for growth .

It is obvious that these conditions, especially the high humidity of the shelters are quite favourable to disease development, especially those caused by fungi and bacteria. It should be noted that the presence of free water on leaves allows the germination of fungal spores, multiplication of plant pathogenic bacteria, etc... Extreme temperatures affect more visibly the leaves than the roots. For instance leaf burns when too high, deformations and leaf curling after frost. However, it is nonetheless the soil temperature or substrate which determines in part the healthy growth of plants. An excessively wet and cold substrate is often stifling and conducive to the development of damping-off agents.

The nursery is also characterised by a high density of plants. This situation is particularly dangerous. In fact, for example, only a few "viruliferous" aphids (capable of transmitting viruses) are sufficient to rapidly contaminate a large number of plants (generalities on virus diseases). In such a situation, diseases with high epidemic potential are spreading very quickly.
 

Figure 1	Figure 2	Figure 3	Figure 4	Figure 5

  
These diseases are numerous and their identification is often quite difficult because of the seedling size, but most importantly because of the similarity of different symptoms on nursery plants.

 
Among the pathogenic micro-organisms causing diseases in a nursery one can mention:
- various viruses;
- bacteria, such as Pectobacterium spp., Pseudomonas syringae pv. tabacina, but also Pseudomonas aeruginosa, Pseudomonas syringae pv. angulata;
- various fungi and alike organisms such as Botrytis cinerea, Olpidium brassicae, various Chytrids, Peronospora hyoscyami f.sp. tabacina, Pythium spp., Rhizoctonia solani, Sclerotinia sclerotiorum, Thielaviopsis basicola (Chalara elegans), Cercospora nicotianae, Colletotrichum tabacum, Fusarium affine, Thanatephorus cucumeris, and other pathogenic fungi namely the airborne ones.

All types of nematodes such as Pratylenchus spp. and Meloidogyne spp. and parasitic plants like Cuscuta spp. may be present. Also the insects such as wireworms, beetles, moths, crane flies and even rabbits may be found... (see Soil pests).

The system of production in aquatic environments (i.e., float water production) is very conducive to the development of certain fungi, but also of various insects. This is the case for example for the crane flies whose larvae quickly invade the cells of polystyrene trays and feed on tobacco roots (figures 4 and 5).

Several non-parasitic disorders can occur in nurseries and sometimes cause considerable damage such as various chemical injuries, "cold injury" caused by low temperatures, the polyphylla (or "Frenching"), nutritional disorders (i.e. CaCO3, lack of nitrogen. ..) or even  waterlogging, hail injury, etc..

Finally it is worth noting that pathogens are much less harmful when the conditions in the nursery are not favourable to their development and when good quality seedlings are used. Sanitation measures used by producers and the way they manage the nursery, aim to accomplish these conditions (see Protection measures to be adopted in nursery).