Several parasitic or non-parasitic diseases cause yellow, beige and greyish spots like those presented on these pictures..
 
The most common parasitic disease is caused by Peronospora hyoscyami f.sp. tabacina (once considered as a fungus, now belonging to stramenopila), causal agent of tobacco mildew or "blue mould" (figure 1). Note that tobacco is very susceptible to this pathogen at all stages of its culture and its presence is characterised by a blue velvety surface developing on spots.
 
Another leaf fungus, Phoma exigua var. exigua (formerly Ascochyta nicotianae), can cause beige-brown spots (figure 2), but it has never been observed in France.

Figure 1	Figure 2	Figure 3	Figure 4	Figure 5	Figure 6

 
Among the biotic causes of leaf spots  there are also some viruses, mainly the cucumber mosaic virus (CMV) (figure 3), but also other viruses (figure. 4). For more information, please see the corresponding fact sheets (Viruses).
 
As for abiotic causes of leaf spots, the first to blame are the chemical injuries (figures 5 and 6). Note that they can occur at all stages of plant development and that no particular structure will be visible on the spots (unlike for fungi), except occasional spots of the product(s) used.