Lesions from wounds and senescent tissues

Various lesions can occur on the stem from wounds provoked by topping,harvesting,,various injuries and senescent tissues. They are mainly induced by three pathogens: pectinolytic bacterium Pectobacterium carotovorum subsp. carotovorum (formerly Erwinia carotovora subsp. carotovora) and two fungi, Botrytis cinerea and Sclerotinia sclerotiorum.
 
These three pests have roughly the same behaviour in tobacco. They may even coexist on the same lesions. They are opportunistic microrganisms that often take advantage of special conditions to settle on plants and cause serious damage:

- prolonged period of wet and humid weather;
- free water on the leaves, particularly on the lamina margins;
- existence of many wounds after topping or harvesting (figure 1);
- presence of senescent tissues (figure 2);
- rapid plant growth with succulent tissues.

Figure 1	Figure 2	Figure 3	Figure 4	Figure 5

They easily colonise wounds and senescent tissues which are entries or nutrient bases to facilitate their penetration and development in plants. They invade the floral parts, which when falling, form sources of inoculum on the stem. Armed with  impressive enzymatic equipment, they quickly degrade the stem in the case of Pectobacterium carotovorum subsp. carotovorum, the rot which is accompanied by a foul odour.
 
They can be easily indentified, especially Botrytis cinerea (figure 3) and Sclerotinia sclerotiorum (figure 4). Indeed, they produce rather rapidly characteristic grey, white mycelium and large sclerotia, respectively, on the  infected tissues. To favour their development, you should put some samples in an airtight container or in a plastic bag containing a paper towel-type support soaked in water. It will provide the required humidity for the formation of these structures. The lesions caused by Pectobacterium carotovorum subsp. carotovorum are often of rather specific dark brown to black colour (figure 5).
 
These pathogens also produce other symptoms on tobacco, especially on the leaves. We recommend, therefore, to refer to data on symptoms of Beige to dark brown spots, and fact sheets on these pests.