English names: Brown Rot (potato), Southern Bacterial Wilt (tomato), Moko Disease (banana), Granville Wilt (tobacco)
Nordic names: Kartoffelbrunbakteriose (DK), Ruskomätä (FI), Mørk potetringbakteriose(NO), Mörk ringröta(SE)
Estonian name: Kartuli pruun-baktermädanik
Major host plants
Species in Solanaceae (potato, tomato, eggplant, black nightshade, bittersweet nightshade) are the main hosts. However, the bacterium has also been reported on more than 200 other wild and cultivated (including Pelargonium and banana) plant species.
- Potato (race 1 or 3): Foliage: Symptoms include rapid wilting of leaves and stems, usually first visible at the warmest time of day. Eventually, plants fail to recover, become yellow and brown necrotic and die. As the disease develops, a streaky brown discoloration of the stem may be observed on stems above the soil line, and the leaves may have a bronze tint. Epinasty of the petioles may occur. Tubers: External symptoms may be visible depending on the stage of development of the disease. Grayish brown discoloration indicates well-established infection.
- Tomato (race 1 or 3): The youngest leaves are the first to be affected and have a flabby appearance, usually at the warmest time of day. Wilting of the whole plant may follow rapidly if environmental conditions are favourable for the pathogen. Under less favourable conditions, the disease develops less rapidly, stunting may occur and large numbers of adventitious roots are produced on the stem.
- Pelargonium (race 3): First symptoms are wilting and subsequent chlorosis (often sectorial yellowing) of leaves. Stems may blacken and eventually become necrotic. Internal vascular browning is often visible. In a later stage, leaves become brown necrotic and the whole plant desiccates and dies. In final stages, plants collapse totally.
A map can be downloaded from EPPO’s website. See instructions here.
R. solanacearum enters the plant through injured roots, wounds on the stem and other plant parts, or through stomata. Within the plant, the bacteria move in the vascular tissues; higher temperatures favour multiplication and disease development. Infection eventually leads to wilting of the plant as the xylem vessels get blocked by bacteria (above 15°C with optimum around 25°C). Although known primarily as a soil-borne plant pathogen, R. solanacearum also survives in soil, water, and in potato groundkeepers (unharvested potatoes from the previous crop).
The pathogen can be spread by contaminated soil, water or equipment. The risk of spread is high when surface water is used for irrigation. Long distance spreading can occur with transportation of latently infected seed potatoes and other vegetative propagative materials.
Detection and inspection
- Potato: Foliage: A white, slimy mass of bacteria exudes from vascular bundles, when broken or cut. This slime oozes spontaneously from the cut surface of a potato stem in the form of threads, when suspended in water. Such threads are not formed by other bacterial pathogens of potato. This test is presumptive diagnostic value in the field. On tubers: Symptoms may be confused with those of Ring Rot due to Clavibacter michiganensis subsp. sepedonicus. R. solanacearum can be distinguished by the bacterial ooze that often emerges from the eyes and stolon-end attachment of infected tubers. Soil may adhere to the tubers at the eyes. Cutting the diseased tuber will reveal a browning and eventual necrosis of the vascular ring and immediately surrounding tissues. A creamy fluid exudate usually appears spontaneously on the vascular ring of the cut surface a few minutes after cutting. In the case of Ring Rot, the tuber has to be squeezed in order to press out a mass of yellowish macerated vascular tissue and bacterial slime.
- Tomato: The vascular tissues of the stem show a brown discoloration and, if the stem is cut crosswise, drops of white or yellowish bacterial ooze may be visible. Wilt symptoms may be confused with those caused by the bacterium Clavibacter michiganensis spp. michiganensis.
- Pelargonium: Symptoms can be confused with those caused by Xanthomonas hortorum ssp. pelargonii. Discoloration of vascular tissues in stems and roots caused by X. h. pv. pelargonii is not so pronounced as when caused by R. solanacearum.
Pest status and importance
The high economic impact of R. solanacearum results from its wide geographical distribution and its wide host range. The occurrence of different races and strains of the pathogen with varying virulence under difference environmental conditions presents a serious danger to European and Mediterranean potato and tomato production. For the EPPO region, Race 3 (biovar 2) appears to present the most important risk.
Source of information
See further information here:
Author:Eigil de Neergaard
Editor:Elise T. Yamamoto Buch