IDENTITY

Name: Ralstonia (Pseudomonas) solanacearum (E. F. Smith 1896) Yabuuchi et al.1995 race 3 biovar 2
Taxonomic Position:
Monera: Bacteria: Proteobacteria: Beta: Pseudomonadaceae
Common Names: Brown Rot, Bacterial Wilt (if on potatoes)

Significance:
Sporadic greenhouse outbreaks of Ralstonia solanacearum race 3 biovar 2 have been detected on geranium in five U.S. states (New Jersey, New York, Pennsylvania, South Dakota, and Wisconsin). All infested material was destroyed, and no field outbreaks have been reported.

The detection of R. solanacearum on U.S. geraniums prompts concern for its potential carry-over to potato and other Solanaceous crops or weeds.

Issues of Concern: Ralstonia solanacearum is not a new pathogen to the U.S. or to other parts of the world. The majority of R. solanacearum diseases have been attributed to race 1, a rather cosmopolitan race with a wide host range and typical sub-temperate to tropical distribution. However since the 1970's the more temperate regions of Europe have experienced outbreaks of R. solanacearum race 3 biovar 2 on potato, tomato and the common watercourse weed, Solanum dulcamara. Currently, short of the United States and Canada, R. solanacearum race 3 biovar 2 is found in nearly all potato growing regions of the world. Since the 1970's many European countries have been working to eradicate race 3 biovar 2 from potato production and nearby alternate hosts. It is easily spread via contaminated irrigation waters and can survive for multiple years in association with alternate hosts. This issue is further complicated with the occurrence of symptomless latent infections and lack of direct chemical control options.

This race is believed to have originated with potato in the Andean Highlands. The European outbreaks were contributed via the introduction of latently infected seed potatoes and waterways contaminated with waste from potato processing plants. Establishment of the bacterium in watercourse weeds like Solanum dulcamara, and use of contaminated water for field irrigation promoted the build-up and spread of the pathogen. This pathogen can manifest itself slowly in hosts without producing symptoms thereby complicating detection schemes and generating a reservoir of inoculum for spread to future cropping systems (EPPO/CABI Bulletin 1997). The outbreak of this bacterium on geranium in the U.S. prompts concern for a similar carry over to potato or other Solanaceous plant hosts. The infected geranium cuttings that surfaced in Wisconsin and Pennsylvania were confirmed to have originated in Guatemala.

Pathways: The bacterium is water transported. "Ebb and flow" irrigation systems are highly conducive to movement in greenhouses. Infected cuttings introduced into glasshouses where other transplants are being produced may infect those plants that will later be transplanted to the field. In field situations contaminated surface irrigation water provides pathogen movement with potential establishment in susceptible watercourse weeds. Alternate susceptible hosts such as Urtica dioica, and S. dulcamara are both common weeds in mid-west potato production states (PLANTS database).

Hosts: Typical hosts are Solanaceae species: potato (Solanum tuberosum), tomato (Lycopersicon esculentum), eggplant (S. melongena), black nightshade (S. nigrum), bittersweet or climbing nightshade (S. dulcamara), and Datura stramonium. Other non-solanaceous hosts include: Portulaca oleracea, Brassica spp., Tropaeolum majus, Urtica dioica, Chenopodium album, Melampodium perfoliatum, and geranium (Pelargonium hortorum).

Distribution:
Africa; Asia; Australia (eastern); Central America (Costa Rica); Europe (western); South America; In North America there have been sporadic reports from Mexico and now the United States.

Quarantines: Ralstonia solanacearum race 3 is a quarantine pest for NAPPO. In Europe it is an A2 quarantine pest.

Detection Strategies
Due to the ability of this bacterium to survive latently in healthy appearing tissues and its similarities to other races within R. solanacearum, detection strategies for both the geranium and potato industry are not straightforward. The European experience with R. solanacearum utilizes a multi-legged approach for detection. Typically bioassays, dilution plating on semi-selective media, fatty-acid analysis, immunfluorescence (IF) microscopy, enzyme-linked immunosorbent assays (ELISA), or polymerase chain reaction (PCR) work are combined, in some manner, to achieve reliable detections (Elphinstone et al.1997). One method alone would not be sufficient due to the occurrence of false negatives and positives possible particularly with cross reactions and contamination during molecular based work (Elphinstone et al 1997, Seal 1997). Previous studies have found the detection of latent infections is more likely if there is enrichment of sample tissues with a semi-selective medium prior to IF, ELISA or PCR testing (Elphinstone et al. 1997). The reliability of a real-time Taq-PCR detection scheme for latent race 3 infections is currently being tested.

Comments:
synonym: Burkholderia solanacearum

OTHER INFORMATION:
Source: USDA, APHIS, PPQ

References:
Elphinstone, J. G., Stanford, H., Stead, D. E. 1997 Detection of Ralstonia solanacearum in Potato Tubers, Solanum dulcamara, and associated irrigation waters. 2nd International Bacterial Wilt Symposium. Central Science Laboratory, Sand Hutton, York . T8 . internet: http://www.inra.fr/Internet/Departements/PATHOV/2nd_IBWS/T8.html

EPPO/CABI Quarantine Pests for Europe (2nd edn) 1997. Ralstonia solanacearum. Smith, I. M., McNamara, D. G., Scott, P. R. Holderness, M. 11pp.

Janse, J. D. 1996 Potato brown rot in western Europe-history, presence occurrence and some remarks on possible origin, epidemiology and control strategies. Bulletin OEPP/EPPO Bulletin 26: 679-985

Seal, S. E. 1997. Molecular methods for detection and discrimination of Ralstonia solanacearum. 2nd International Bacterial Wilt Symposium. Central Science Laboratory, Sand Hutton, York. T3. internet: http://www.inra.fr/Internet/Departements/ PATHOV/2nd_IBWS/T3.html

Williamson, L., Nakaho, K., Allen, C., Hudelson, B. 2001. Ralstonia solanacearum race 3 biovar 2 isolated from geranium in Wisconsin. Phytopathology 91(6-supplement): S95