Biology and Epidemiology

- Survival and sources of inoculum

Pseudomonas syringae pv. tabaci and Pseudomonas syringae pv. angulata easily survive in the soil on plant debris. These two bacteria are also associated with root systems of weeds and some cultivated plants such as cereals. They can be observed on the leaves of numerous weeds that grow in tobacco fields, such as Capsella bursa-pastoris, Solanum nigrum ... These bacteria are frequently reported on tobacco seeds originating from infested capsules. In some countries, this source of contamination has led to serious epidemics.
 
- Penetration into the plant and invasion
 
Once on the surface of wet leaves, bacteria frequently enter the leaves through stomata, hydatodes, but also through wounds of diverse types.
 
 
- Dissemination of bacteria
 
As previously seen, the bacteria can be spread by the seed, but they may also grow in the nurseries in infected plants. Spread of the bacteria from one plant or from a group of diseased plants is facilitated by water. Indeed, during the rain and irrigation with sprinklers, during windy weather the dissemination of bacteria to other non-infested leaves is facilitated by numerous droplets. Under these conditions it is possible that water (rainfall or irrigation) splashes the bacteria from the soil to the lower leaves of a tobacco plant. During implementation of various cultural practices, especially when the plants are wet, the bacteria may also be spread by workers and cultivation tools.
 
 
- Favourable conditions for the bacteria development 
 
Like for many bacteria, the moisture content and the presence of water on plants enables the development of Pseudomonas. Thus, after a prolonged rainy period or a repeated irrigation with sprinklers several contaminations occur and the spread of the bacteria is highly favoured.