Ralstonia solanacearum

Ralstonia solanacearum
Symptoms caused by Ralstonia solanacearum on tomato
Scientific classification
Kingdom:	Bacteria
Phylum:	Proteobacteria
Class:	Beta Proteobacteria
Order:	Burkholderiales
Family:	Ralstoniaceae
Genus:	Ralstonia
Binomial name
Ralstonia solanacearum (Smith 1896) Yabuuchi et al. 1996
Type strain
ATCC 11696 CCUG 14272 CFBP 2047 DSM 9544 ICMP 5712 JCM 10489 LMG 2299 NCAIM B.01459 NCPPB 325 NRRL B-3212
Synonyms
Burkholderia solanacearum (Smith 1896) Yabuuchi et al. 1993 Bacillus solanacearum Smith 1896 Pseudomonas solanacearum (Smith 1896) Smith 1914 Pseudomonas batatae Cheng and Faan 1962 Pseudomonas ricini (Archibald) Robbs 1954

Ralstonia solanacearum is an aerobic non-sporing, Gram-negative plant pathogenic bacterium. R. solanacearum is soil-borne and motile with a polar flagellar tuft. It colonises the xylem, causing bacterial wilt in a very wide range of potential host plants. It is known as Granville wilt when it occurs in tobacco.

Ralstonia was recently classified as Pseudomonas with similarity in most aspect, except that it does not produce fluorescent pigment like Pseudomonas. The genome of R. solanacearum Strain GMI1000 has been sequenced.

Disease Cycle

Survival

Ralstonia solanacearum can overwinter in plant debris or diseased plants, wild hosts, seeds or vegetative propagative organs like tubers. The bacteria can survive for a long time in water (up to 40 years at 20-25oC in pure water) and can be reduced in extreme condition (temperature, pH, salts, e.g.). R. solanacearum can also survive in cool weather and enter a state of being viable but not culturable. In most cases, this stage is not an agricultural threat because the bacteria usually become avirulent after recovering.

Dispersal

R. solanacearum causes wilting at high population (10^8 – 10^10 cfu/g tissue). The large number of pathogens shed from roots of symptomatic and non-symptomatic plants and bacterial ooze on plant surfaces enter the surrounding soil or water, contaminating farming equipment or may be acquired by insect vectors.

Invasion

R. solanacearum usually enter the plant via a wound. Natural wounds (created by excision of flowers, genesis of lateral roots) as well as unnatural ones (by agricultural practices or nematodes attack) would become entry sites for Ralstonia solanacearum. The bacteria get access to the wounds partially by flagellar-mediated swimming motility and chemotaxis toward root exudates. When the pathogen gets into the xylems, tyloses may form to block the axial migration of bacteria within the plant. In susceptible plants, this sometimes happens slowly and may lead to vascular dysfunction because tyloses can unspecifically obstruct uncolonized vessels. Wilting occurs at high level of bacterial population in the xylem and is due to vascular dysfunction in which water cannot reach the leaves sufficiently. The pathogen’s EPS (exopolysaccharide) blocking petiole pit membrane; by product of cell wall degradation; tyloses and gums produced by the plant itself are other contributing factors to wilting.

Importance

Bacterial wilt caused by Ralstonia solanacearum is of economic importance because it infects plants in over 50 families. It causes a wilt disease in several important agricultural crops such as potato, tomato, tobacco, banana, pepper and eggplant. The disease is known as Southern wilt, bacterial wilt, and brown rot of potato. Many more dicots suffer from the disease than do monocots. Among the monocot host, the order Zingiberales dominates with 5 over 9 families being infected by this bacterium. The reason why some families are more susceptible to bacterial wilt is still unknown.

Plant hosts that R. solanacearum infects includes: Crops: Potatoes (Solanum tuberosum); Tomatoes (Lycopersicum esculentum); Aubergine (egg plant) (Solanum melongena); Banana, (Musa spp); Geranium (common name) (Pelargonium); Ginger (Zingiber officinale); Tobacco (Nicotiana tabacum); Sweet peppers (Capsicum spp); Olive (Olea europea) Wild hosts: Woody nightshade (Solanum dulcamara)

Methods of transmission

Contaminated tools, Machinery etc. Irrigation water, Contaminated flood water, Wash water, Soil, Infected seed.

Ralstonia solanacearum is a high profile alien plant pathogen of A2 Quarantine status affecting a very wide range of crops. This means that it is present in parts of Europe but is under statutory control. Worldwide, the most important crops affected are: Potatoes Tomatoes, Tobacco, Banana and Pelargonium. In the UK and the rest of the EU the most important crops affected are Potato and Tomato. It would cause serious economic damage were it to become more established than it currently is. Losses are due to actual yield reduction and also due to statutary measures taken to eliminate the disease. Control within the EU is governed by an EC Control. The bacterium is found worldwide.

Spread is by contaminated soil, surfaces, infected seed, irrigation and/or wash waters. Ralstonia solanacearum can survive in the soil. Infected land sometimes cannot be used again for susceptible crops for several years. In northern Europe, the pathogen has become established in solanaceous weeds which grow in slow moving rivers. When such contaminated water is used to irrigate potatoes, the pathogen enters the potato production system. Some EU states and Middle Eastern countries have not yet been able to eradicate this pathogen.

Used as a biological control against Kahli Ginger (Hedychium gardnerianum), which is a member of '100 of the World's Worst Invasive Alien Species' in 2004

Licences are required to hold/work with this pathogen and in the EU, export licences are required to export it out of the EU. Import licences are also required to import it into any EU state.

References

• Confirmation of export licence required
• List of Plant Pathogens subject to Export Control
• Agrios, G. N. (2008). Plant pathology. Amsterdam [u.a.], Elsevier Academic Press, pp 647-649
• Denny T., "Plant Pathogenic Ralstonia species" in GNANAMANICKAM, S. S. (2006). Plant-associated bacteria. Dordrecht, Springer. pp 1-62

External links

• http://www.invasivespeciesinfo.gov/microbes/bacterialwilt.shtml
• http://www.bacterio.cict.fr/qr/ralstonia.html