Risk factors

Ralstonia solanacearum is a soil borne bacterial phytopathogen which can survive in a free state in a soil, although its ability to survive in a bare soil is still debated. However, its survival depends on the type of soil, the temperature and the moisture content. The bacterium prefers a poorly drained, moist soil. It can survive in the roots of host plants, in crop residues (groundkeepers) or in the rhizosphere of non-host plants.

Temperature is also a key factor for the survival of this bacterium. It may be present at temperatures between 0 to 10°C, but the disease will not develop. The optimal temperature lies between 30 and 35°C in the case of tropical strains and around 23°C for European strains (phylotype II).

Crop residues, host plants and potato tubers, even though they show no symptoms, can be considered as healthy reservoirs as they can carry latent infections and spread the bacterium over long distances. Moreover, numerous weeds such as the bittersweet nightshade (Solanum dulcamara) and the black nightshade (Solanum nigrum) are tolerant host plants and are also potential inoculum sources.

Another very important factor in the dissemination of the bacterium is water: not only irrigation water (surface water), but also water streaming inside the field after rainfalls.

Man is also responsible for the transmission of the disease as a result of cultivation operations (cutting off seed tubers, pruning or pinching out tomatoes) and of the use of infected agricultural equipment (at planting, harvesting or storage).