Ralstonia solanacearum (Bacterial Wilt of Geraniums) - Fact Sheet
Ralstonia solanacearum (Rs) is a bacterial plant pathogen that has a very wide host range. Rs has been classified into various races and biovars. Most races of the bacterium, and their associated diseases, appear to be limited to tropical, sub-tropical and warm temperate climates, and thus pose no long-term threat to agriculture in cool temperate climates. However, one particular race of the bacterium, termed race 3 (biovar 2A), or the "potato race", is not known to occur in the United States or Canada and in recent years has been found in some temperate areas of Europe, mainly on potato, but also on tomato, and a few weed species. This race of the bacterium is now considered as a quarantine pest in Europe, Canada and the United States. Recently it has been found that race 3 also causes a wilting disease in geraniums (Pelargonium species). Audits conducted by countries such as the Netherlands and the United States have shown that geraniums imported from certain countries may be a potential pathway for introduction of this bacterium.
Rs (including all strains and biovars) has an extremely wide host range with well over 200 hosts reported. Race 3 on the other hand has a fairly restricted natural host range, being reported mainly from potatoes, sometimes tomato, and rarely from eggplant and peppers. The bacterium is also known to occur on certain weed species, such as nightshades (e.g. Solanum dulcamara), which may act as an overwintering host for the bacterium in temperate climates. As indicated above, geranium (Pelargonium sp.) has also been shown to be a host for this race of the bacterium.
The bacterium (including all strains and biovars) occurs in almost every region of the warm temperate, sub-tropical and tropical zones of the world. Recently race 3 has also been found in some more temperate regions of Europe, where it is now under quarantine and/or has been eradicated.
Rs enters into a plant through injured roots, wounds on the stem, or through stomata. Once inside the plant, the bacteria multiply and move to the vascular tissues; warmer temperatures (i.e., above 25°C) and excess moisture favour multiplication and disease development. Infection eventually leads to wilting of the plant as the xylem vessels get blocked by the bacterium. The end result is death.
Rs is a soil-borne bacterium. In warmer climates research has shown that the bacterium is able to survive for extended periods in deep soil layers, in sheltered sites such as the roots of alternate hosts (e.g., weeds), in infected plant debris, or in volunteer potato tubers from previous crops. In temperate climates, survival of the bacterium in soil (i.e., without a host) for long periods is not considered very likely. However, studies in temperate Europe have shown that overwintering of the bacterium can occur in potato volunteers, or in the roots of certain weeds, mainly Solanum dulcamara (bittersweet nightshade).
Under greenhouses conditions, the movement of the bacterium on cutting knives, in soil, or in contaminated irrigation water, may lead to rapid spread of the bacterium within a facility. Destruction of all infected and contaminated materials, decontamination, the use of disease-free propagative materials, and strict hygiene, are critical in eradication and disease prevention programs.
Detection & Identification
- Bacterial wilt of geranium caused by Ralstonia is similar in symptomology to bacterial blight, caused by a Xanthomonas species. Early symptoms of infection include wilting of the lower leaves with rolling of the leaf margins; sectorial yellowing and necrosis. Stems may show brown to black discolouration around the soil/air level, both externally and internally when cut. Roots of infected plants are often brown or black.
- Wilting of infected geraniums is systemic, starting with lower leaves and petioles and eventually spreading; in some cases only one part of the stem may show wilting symptoms, but eventually the entire plant wilts, collapses, and dies. Symptoms have been reported on plants kept at 21°C.
- Relative to Pelargonium infections reported in warm countries, infected plants usually showed obvious symptoms and very few plants were found to have latent infections. Spread from plant-to-plant only occurred in poorly-drained greenhouse compartments.
Figure 1 - R. solanacearum infection of Geranium (wilting started at the base of the plant and worked its way up)
Figure 2 - Advanced stages of R. solanacearum infection on Geranium (entire plant collapsed)
Figure 3 - Early stages of infection with Xanthomonas campestris pv. Pelargonii (wilting tends to be more random throughout the plant when compared to infection caused by R. solanacearum)
Figure 4 - Geranium, early stages of infection with Xanthomonas campestris pv. Pelargonii (wilting tends to be more random throughout the plant when compared to infection caused by R. Solanacearum)
Figure 5 - Early stages of R. solanacearum infection on Geranium
Figure 6 - Early stages of R. solanacearum infection on Geranium
* Synonym Pseudomonas solanacearum.
Photos: Fig. 1-4, Dr. S.G.P. Nameth, Dept. of Plant Pathology, Ohio State University, Ohio, USA;
Fig. 5 & 6, JoAnn M. Cruse, State Plant Health Director - Wisconsin, USDA, APHIS, Plant Protection & Quarantine
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