Biology and Epidemiology

- Survival and sources of inoculum

The Pectobacterium spp. are ubiquitous and present in many soils, especially on plant debris, where they easily survive for several years. They are also found on the leaf surface of tobacco, a fact that reenforces the idea that these bacteria are relatively common in the environment of tobacco.  Seed is a significant source of inoculum (figure 1). These bacteria are also relatively polyphagous and are likely to survive on a fairly large number of hosts.


- Penetration into the plant and invasion

The bacteria will usually penetrate the plant through injuries or senescent tissues. For example, they can infect young seedlings through lower leaves in contact with soil as they become yellow and  wet. Similarly, the bacteria invade the stem through topping or stripping wounds (figure 2), or through the surface of senescent or damaged leaves (figure 3). Once in place, the cellulolytic and pectinolytic enzymes the bacteria produce actively contribute to the development of a wet and sometimes nauseating rot.


- Dissemination

Like many bacteria, they are easily spread by water during runoff and splashing. If infections take place in the nursery, it is clear that those plants are carriers of the bacteria and will contaminate one or more fields. The bacteria  can contaminate the seeds subsequently ensuring bacterial transmission to seedlings. Bacteria  are not usually isolated from seed after 8 months of storage


- Favourable conditions for the bacteria development

Bacteria of the genus Pectobacterium are mainly favoured by wet weather conditions. Cloudy and rainy periods increase the risk of their spread and development (figure 4). These bacteria more easily infect plants that have succulent tissues, numerous injuries and some senescent leaves. Temperature does not seem to play a key role in the infection cycle.
They seem to be able to grow at temperatures between 5 and 37 ° C, the optimal temperature varying from one species to another. In dry soils, with moisture below 40%, the damage by Pectobacterium carotovorum subsp. carotovorum seems to decrease, and in some situations even vanish.