Other micro-organisms not observed in France

Other micro-organisms causing leaf spots in a nursery have been described occasionally. However, they have not been observed in France yet.
 
- Thanatephorus cucumeris (causing target spot), is the perfect or teleomorph form of Rhizoctonia solani. This fungus is common in float water production systems in the USA and it is also reported in Zimbabwe and in Bulgaria. It causes small circular, water-soaked lesions that may grow rapidly, they are irregularly-shaped and surrounded by a yellow halo. They sometimes develop a pattern of concentric rings; the plant tissue becomes fragile and brittles when drying and then drops (figure 1) (see the fact sheet of Thanatephorus cucumeris).
 
- The fungus Cercospora nicotianae (frog-eye) induces small circular lesions of about a centimeter in diameter, light beige, with parchment-like appearance (figures 2 and 3). They are sometimes sprinkled with tiny black structures, formed by masses of conidiophores and conidia of the fungus. Cercospora nicotianae is common especially in hot producing areas. It has been reported in some European countries (see also the fact sheet of Cercospora nicotianae).
 
The symptoms caused by Colletotrichum tabacum (formerly Colletotrichum gloeosporioides, anthracnose disease) appear as circular lesions, initially dark and oily, then gray and parchment-like surrounded by a brownish border. Eventually browning of a few lateral veins, and small elongated cankers on petioles and stems that may be covered by pink masses of spores (acervuli) are observed (see the fact sheet on Colletotrichum tabacum). This fungus is kept under control in tobacco nurseries treated with anti-mildew fungicides.
 
- Pseudomonas aeruginosa (Schoetter) Migula causes white, opaque leaf spots of a few millimeters in diameter, surrounded by a yellow halo. This bacterium induces also a wet rot which 'damps off' the young seedlings. Although widespread, it has been reported on tobacco only in the Philippines.
 
- Fusarium affine Fautrey & Lambotte (Scab) is causing irregular olive-colour leaf spots and small, brown canker lesions on petioles and stems. It has been reported mainly in the USA and Zimbabwe during periods of high humidity.

- Slime mould in tobacco greenhouses

This is a very rare problem in tobacco greenhouses and has never been reported to cause economic losses in the USA. Slime mold presence in a tobacco greenhouse is a good indication of very high humidity inside the house (close to 100%). Thus a standard recommendation is good air circulation and ventilation that can bring the humidity levels down.