Most commonly observed micro-organisms

Apart from Botrytis cinerea, previously described in the fact sheet "Collar cankers" are very frequently observed in nurseries, other micro-organisms like bacteria or fungi are described as causal agents of leaf spot in tobacco nurseries. However, these pathogens are rare.
 
Peronospora hyoscyami f.sp. tabacina, downy mildew (Blue Mould ") may cause yellow spots on nursery plants. The leaves tend to curl having an abundant bluish sporulation on the lower leaf surface formed by sporocystophores and the sporangia. Afterwards leaves start to dry up and seedlings die.
 
The fungus is rather common in float water production in particular in the USA. It seems that it does not exist anymore in tobacco nurseries in France (see mildew of tobacco).
 
- Pseudomonas syringae pv. tabaci ("Wild Fire") and Pseudomonas syringae pv. angulata ("Angular leaf spot) are bacteria that cause the formation of small spots (1-5 mm) of moisty appearance  in the early stages of development. They have variable, sometimes angular shapes and turn brown to black when dry up. They may be surrounded or not by a yellow halo. Note that these spots can be easily confused.  
 
This disease rarely occurs in the nurseries in France (see Pseudomonas spp.).