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Beige to dark brown spots

The large beige to dark brown spots observed on tobacco leaves, can be caused by at least three different parasitic agents:

- A bacterium, Pectobacterium carotovorum subsp. carotovorum (formerly Erwinia carotovora subsp. carotovora) (Erwinia leaf spot) (figure 1);
 - Two fungi, such as Botrytis cinerea (Botrytis leaf spot) and Sclerotinia sclerotiorum (Sclerotinia leaf spot) (figures 2 and 3).

Figure 1 Figure 2 Figure 3 Figure 4

These two fungi and the bacterium have about the same behavior on tobacco. Sometimes they even coexist on the same lesions. They are opportunistic micro-organisms that frequently exploit special conditions to invade plants and cause serious damage. These conditions are: a prolonged rainy period and moist weather, free water on the leaves (particularly on lamina margins), the existence of many pruning  or topping wounds; the presence of senescent tissues; well growing plants with succulent tissues.

Therefore these micro-organisms are considered pathogens of nutrient bases (senescent leaves and floral parts) and injuries. They easily colonise injured and senescent tissues which serve as entries or nutrient bases to facilitate their penetration and development in plants. They invade the floral parts which, when falling on the leaves, serve as sources of inoculum (figure 4). Armed with an impressive enzymatic function, they quickly degrade stem and leaf tissues. All these properties make them also fearful enemies of tobacco during curing (see the section on Curing barn diseases).

Note that lesions caused by these organisms, with very advanced symptoms, can be confused with wilting and drying, so we recommend you also see this fact sheet.

Last change : 02/22/13
  • Author :
  • D Blancard (INRA)