Biology, epidemiology

• Conservation, sources d'inoculum
  

Ralstonia solanacearum easily maintains itself in many soils up to 30 cm deep for several years , especially from plant debris if these soils have a high water retention capacity. The soil inoculum rate can be maintained or even increased when cultivating other susceptible plants on the same plot. Indeed, this bacterium is capable of infecting many cultivated plants (sunflower, peanut, tobacco, pepper, cassava, potato, banana tree ) or weeds by producing or not (latent infections) the withering syndrome described previously. More than 250 plant species , belonging to at least 50 botanical families, are susceptible to attack , mostly dicotyledons. The severity of the attacks often depends on the size of the soil inoculum and the aggressiveness of the strain present. This bacterium has been isolated from many weeds which may or may not express symptoms: Amaranthus spinosus, Chenopodium album, Cyperus rotundus, Erechtites valerianaefolia, Euphorbia hirta, Hydrocotyle ranunculoides, Malva sp., Physalis minima, Polygonum pensylvanicum, Rumex dentatus, Solanum nigrumvanicum, Rumex dentatus, Solanum nigrumvanicum, Rumex dentatus, S. dulcamara, Vicia sp.

Let us point out that  R. solanacearum  adapts remarkably to planktonic survival, in particular in certain irrigation water coming from canals (Holland), rivers (Scotland), ponds or various water reserves.    

• Penetration and invasion

During the growth of the root system in contaminated soil, it suffers various injuries:
- either natural, in particular at the point of emergence of the lateral secondary roots;
- either due to the effects, intentional or not, of tools, punctures and alterations caused by root-knot nematodes belonging to the genus Meloidogyne , to insect bites

These wounds constitute real entry points for R. solanacearum . They facilitate its penetration into the cortex of the roots and its access to the vessels in which the bacterial cells multiply more quickly. It is found in the vessels of the xylem and at the end of the phloem, but also in the cells of the parenchyma lining the latter.

• Multiplication and dissemination

Many bacterial cells are released from the roots and reach the soil and then the healthy roots of neighboring plants which soon become infected. The spread of R. solanacearum often takes place step by step, through runoff , contaminated plants and tillage tools . Workers during disbudding and stripping would contribute to its dispersal. This bacterium could be transmitted by seeds; but let us point out that the great majority of works invalidate this mode of dissemination.

• Conditions favorable to its development

All strains of R. solanacearum fond of tropical temperatures, between 25 and 35 ° C . The wet, heavy soil , the moderate pH are more favorable to contamination and its extension. It does not tolerate dry soil conditions and temperatures below 10 ° C. The addition of a highly nitrogenous fertilizer would help to sensitize the plants.

The presence of root-knot nematodes ( Meloidogyne incognita ) in the same soils aggravates the damage of bacterial wilt. In Brazil, drip irrigation has been shown to promote disease more than sprinkler. Not all soils show the same receptivity to R. solanacearum . For example, in Guadeloupe, ferralitic or recent (alluvial, volcanic) clay soils with a kaolinite or halloysite type sheet, at pH between 5 and 7, are the most easily contaminated because they are very receptive. The bacteria persist and circulate there indefinitely. Calcium vertisols, on the other hand, are not very receptive and are said to be suppressive. It is the presence of smectite-type clay which literally partitions bacteria into a network of tactoids (honeycomb structure), which prevents R. solanacerum from spreading and surviving in it. provided that Solanaceae are not cultivated there and the soil kept moist at all times.