Ralstonia solanacearum is a plant pathogenic bacterium. It is a soil-borne disease and colonises the xylem, causing bacterial wilt and brown rot in a very wide range of potential host plants.
Life cycle and appearance of Bacterial wilt, brown rot
Ralstonia solanacearum is a quarantine organism in Europe and considered a bioterrorist organism in the United States. It is a commonly found soil-inhabitant in tropical and subtropical regions and is transferred by the introduction of propagation materials (seed potatoes, cuttings of ornamentals) from warmer regions. Different races are distinguished with a differing, but overlapping host-range. Race 3 is pretty much restricted to potato and tomato. Whereas other races have optimum temperatures of 35-37 °C, the optimum for Race 3 is 27 °C, which makes it more dangerous in temperate regions. It survives well in water and in many different soil types and on alternative hosts like weeds. It invades the plants through wounds or stomata and then spreads into the xylem vessels. In potato, it is tuber-borne. Spread from plant to plant results from bacteria moving from the roots of infected plants to neighbouring plants. The bacteria can also be transported by irrigation water.
Disease incidence is highest when soil moisture is high, such as during rainy periods. When temperature is low, infection may remain latent until conditions are more favourable. This makes it more difficult to recognize infected fields.
In potato, field symptoms of Ralstonia solanacearum are wilting and yellowing of leaves and stunted plants. Inside, vascular browning is common with bacterial slime oozing from the cut. When the bacteria transfers to young plants through infected tubers, the wilting and collapse of plants happens rapidly. Leaves remain green after wilting, until they are completely desiccated. Infected stems show long, dark brown streaks. The vascular tissue in the tubers becomes grey-brown and the tuber eyes also turn grey-brown. This disease can be distinguished from Clavibacter by the bacterial slime produced from wounds, which is not seen in Clavibacter-infected plants.
In tomato, the youngest leaves are affected first. They show signs of wilting during the day which may disappear in the early stages of infection. Under conditions favourable for the bacteria, the whole plant will wilt soon after the first symptoms appear. When conditions are less conducive for disease development, the plants may show signs of stunting and produce many adventitious roots on the stem. The vascular stem tissue is coloured brown and when the stem is cut, bacterial slime oozes from the wound. This is the way to distinguish it from Verticillium or Fusarium wilts.