Small, often necrotic spots

 
In biotic or abiotic plant stress conditions one can observe small leaf spots often becoming necrotic at the end of their development. Figures 1-4 help to understand the observation of an altered leaf system.

Figure 1	Figure 2	Figure 3	Figure 4

When these kind of symptoms are observed many hypotheses are possible :

•  parasitic causes

- viruses such as cucumber mosaic virus(CMV), potato virus Y (PVY), tobacco ring spot virus (TRSV ) and other viruses like tobacco etch virus (TEV), or tobacco vein mottling virus(TVMV), or TSV, TNV (see the fact sheet Other parasitic tobacco viruses);
 - bacteria including various species of Pseudomonas (P. syringae pv. angulata and pv. tabaci, figure 1, or P. aeruginosa and P. melleum);
- fungi, in particular Cercospora nicotianae, which is the main fungus causing these types of spot (figure 2).
 

• non-parasitic problems causing wet spots (figure 3), pollution spots ("weather fleck") (figure 4) and various chemical injuries(see this link).

During tobacco growth various small spots which are difficult to identify with certainty may appear. In this fact sheet we have summarised the main diseases causing spots. As you can understand there is a high risk of confusion. In addition, many other diseases causing leaf lesions, also initially causing small spots which spread more or less rapidly. We advise you to thoroughly consult the section on Leaf spots.
 
Some rather rare or misunderstood issues will not be treated or are ignored in this part of the application. 

Two rusts have been reported on tobacco: Anastasia Uredo nicotianae, Sacc. & Splendore and Puccinia substriata Ellis & Barth. These two fungi are very rare and have never been observed in France. They form embossed uredinia or ecidiosores on the leaves, they are coloured and very characteristic and release many spores at maturity (follow this link to Rusts). Note that producers, or even a few technicians, use the term ?rust? to describe all brown spots developing on tobacco leaves. They have an abiotic cause and should in no circumstances require rust fungicide treatments.
 
Remember that some Nutritional disorders also cause necrotic spots which can sometimes be confusing. Please, see the above link.