Pathogenic micro-organisms

Among the parasitic causes, the role of certain fungi is important, such as Peronospora hyoscyami f.sp. tabacina (figure 1) and Colletotrichum capsici f. sp. nicotianae. This specialised form of Colletotrichum capsici is mainly reported in some tobacco production regions of China. It induces elliptical cankers on the stems of tobacco seedlings leading to their death. The lesions turn brown to black, hence the name of the disease "black death". Note that the existence of this specialised form seems to be questionable, because this species is not reported in the international basis of "Fungorum Index".

Figure 1	Figure 2

Various viruses induce lesions sometimes on the stem cortex. The lesions  are often brown, of variable length, in line on one side of the stem ("streaks"). Among these viruses are: PVY (potato virus Y), CMV (cucumber mosaic virus) and TSWV (tomato spotted wilt virus). In France, the symptoms above mentioned are often associated with the presence of PVY (figure 2).