Causal agent(s) and transmission
The causal agent responsible for bacterial wilt is the bacterium Ralstonia solanacearum (synonym Pseudomonas or Burkholderia solanacearum).
It is a phytopathogenic bacterium, distributed worldwide, which has been discovered over the five continents. Typical of the subtropical and warm temperate regions, it has been found more recently in cooler climates, notably in Europe and in the highlands of Southern America and Africa.
The bacterium penetrates the plant either through natural wounds on the roots (axil or point of emergence of the secondary roots) or through artificial wounds caused by man (trans- planting, pruning, pinching off, harvesting equipment) or caused by nematodes (Meloidogyne spp.). The bacterium will then be able to colonise the vascular system, where it will actively multiply and rapidly spread throughout the plant.
Economic and geographical significance
Ralstonia solanacearum has a very wide host range; it affects more than 53 botanical families comprising over 200 plant species. Economically important crop hosts include banana, tobacco, groundnut, potato, tomato, cassava, etc. Among the Solanaceae, other crops or ornamentals can also be affected: aubergine, pepper, petunia, etc.
This bacterium can also be hosted by tolerant plants, i.e. ones in which there is no visible manifestation of the disease; they include weeds such as bittersweet nightshade (Solanum dulcamara) and black nightshade (Solanum nigrum).
Symptoms on foliage
The intensity of the symptoms and the speed of their appearance depend on the host (age, species, cultivar), on the inoculum (quality and quantity) and on the environmental conditions (temperature, moisture, type of soil, etc.). As a result, the symptoms in growing crops are not always obvious and usually start with the crinkling of a few leaflets (photo 1).
In the first stage of the disease wilting is visible only at the warmest time of day and may affect only one or two stems (unilateral wilt), then it finishes by spreading to the entire plant (photo 2).
With a severe attack, a bacterial exudate oozing can be observed at the leaf axils or from the vascular tissues of cut-off stems (water glass test: photo 3)
Symptoms on tubers
Symptoms on tubers are characterised by the browning of the vascular ring, which is visible once the tubers have been cut off. A beige, then brown, rot (which explains the name of the disease) develops at the vascular ring (photo 4) and the tuber finishes by decomposing. Cutting (and pressing on) the tuber reveals a creamy fluid exsudate oozing from the vascular tissues (photo 5).
The exudate may also ooze out of the eyes or from the stolon-end attachment of infected tubers; the emergence of the bacterial ooze is visible externally by the formation of crusts with soil particles adhering (photos 6 and 7).
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).
Control
Ralstonia solanacearum is classified as a quarantine parasite (for which no tolerance is allowed) in most places in the European Union and is therefore subject to mandatory control measures to limit its spread and to eradicate the disease when and where an outbreak is notified.
Integrated pest management and prophylactic measures are the most effective way to reduce the consequences linked to the presence of the disease and to prevent its introduction into pest free areas:
- the use of certified seed potatoes inspected in the field and tested in the laboratory is still the best way of preventing the dissemination of the disease;
- cultivation in a healthy environment: a bacteria-free soil, the effective elimination of weeds and groundkeepers, and soil drainage;
- the use of clean and disinfected agricultural equipment for both cultivation and storage;
- irrigation with non-contaminated water (avoid surface water);
- the introduction of non-host plants, e.g. some cereals (spring barley), some Umbelliferae species (carrots) or flax.