Ralstonia solanacearum Race 3 Biovar 2
Ralstonia solanacearum Race 3 biovar 2 is a bacterium that causes the following diseases:
- Southern Wilt of Geranium
- Brown Rot and Bacterial Wilt of Potato
- Bacterial Wilt of Tomato and Other Solanaceous Crops
R. solanacearum Race 3 biovar 2 is considered to be a select agent by the United States under the Agricultural Bioterrorism Protection Act of 2002 because it has the potential to be a severe threat to the potato industry and does not naturally occur in the U.S.
It is important to note that there are other races and biovars of Ralstonia solanacearum. Only R. solanacearum Race 3 biovar 2 is on the select agent list. In total, five races of R. solanacearum have been described with differences in host range. Race 1 can infect hundreds of different plant species in 50 families and Race 1 biovar 1 is endemic in the Southern United States where it is commonly found on tomato, tobacco and other crops. (Harris, 1972; Denny and Hayward, 2001). Races 2, 4, and 5 occur outside the U.S. in warm regions and are not considered a threat due to their poor cold-temperature tolerance. Race 3 is commonly found throughout the world except in the U.S. andCanada. The race/biovar of most concern to the U.S. is Race 3 biovar 2, because of its ability to cause damage to potato, tomato, eggplant and geraniums in cooler climates than Race 1. The potato industries in both the United States and Canada are concerned with potential introduction of Race 3 because potato crops in other temperate countries have experienced serious disease problems due to the cold temperature tolerance of Race 3 (Kim et al 2003 and Daughtrey 2003). In the U.S., especially in the south, it is important to be able to discern through diagnosis Race 1 and Race 3 because they can have a number of the same hosts and symptoms.
World wide losses from bacterial wilt on potato crops are more than $950 million per year (Allen 2003). Outbreaks of R. solanacearum Race 3 biovar 2 have occurred throughout Europe in recent years. These outbreaks are thought to be the result of imports of infected plant material from locations where the disease is already established (Janse 1996).
A pathway for introduction of R. solanacearum Race 3 biovar 2 into the U.S. has been through the introduction of geranium cuttings obtained from production greenhouses located outside the country. Geraniums imported into the U.S. on several occasions in 1999 and 2000 were positive for R. solanacearum Race 3 biovar 2. (Kim et al 2003; Williamson et al 2002; Kim, 2002). During 2001 and 2002 the U.S. had no reported cases of R. solanacearum Race 3 biovar 2. In February 2003, the bacterium was again identified in geraniums in the US but this time on plants grown from cuttings from a facility in Kenya. The outbreak derived from a breach of sanitation in Kenya that led to the contamination of 7 stock plants. The 2003 sampling program conducted by USDA-APHIS in collaboration with the state depts. of agriculture resulted in the organism being detected in 127 individual greenhouses in 27 states. Work was conducted by State Departments of Agriculture and APHIS personnel to contain, destroy, and eradicate any diseased geraniums (Daughtrey 2003). Geraniums with symptoms of bacterial wilt caused by Race 3 biovar 2 were detected again in one greenhouse in December of 2003. This was believed to be a new introduction and not a continued contamination from the spring of 2003. The greenhouse had not received plants from the Kenya cutting station earlier that year and the diseased cuttings were traced back to Guatemala (O'Hern 2004).
Signs and Symptoms on Geraniums
The lower leaves of plants infected by R. solanacearum Race 3 biovar 2 may show yellowing, wilting, and browning. Another common symptom is vascular discoloration of the stem. Roots may also show a brown discoloration.
Bacterial ooze and bacterial streaming both represent signs of the pathogen. Bacterial ooze can be observed in cut stems. If the petioles of infected leaves are removed, cut up, and placed in a water-filled test tube the water will become cloudy as the bacteria stream out of the xylem into the water.
Signs and Symptoms on Potatoes
Foliage of potatoes infected with R. solanacearum Race 3 biovar 2 may show stunting, yellowing, and wilting. Early in the infection wilting leaves may be limited to the top portion of the plant. The plants can appear to recover at night, but soon wilting is irreversible and death of the plant follows. The stems of young plants may collapse and/or have narrow dark streaking present. Vascular discoloration of the stem appears to be grey or brown and bacterial ooze is present. Again, upon placing cut stem material in a test tube with water, bacterial streaming may occur from the vascular tissue (Elphinstone 2004).
Symptomatic potato tubers will show discoloration of the vascular ring. Initially the vascular ring appears yellow to light brown, but as the infection progresses the ring will become browner. Bacterial ooze may also be present; in later stages this ooze may emerge from the eyes and heel (stolon) end to which soil particles will attach.
Signs and Symptoms on Tomatoes
Tomato plants infected with R. solanacearum Race 3 biovar 2 initially show a reduction in the firmness of young leaves. Upon further development the plant will wilt on one or both sides during conditions favorable to the pathogen which include saturated humidity and warm soil temperatures around 25 °C (77 °F). When the temperature is below 21 °C (70 °F) the environment is less favorable for the pathogen and excess roots may develop along the stem. Brown discoloration of the vascular tissue as well as bacterial ooze may be present.
The plant pathogenic bacterium known as Ralstonia solanacearum Race 3 biovar 2 has a smaller host range than Race 1.
The host plants of concern in the United States include geraniums and solanaceous plants: potato, tomato and eggplant. In addition, weeds and semi-aquatic plants could be a potential host for Race 3.
R. solanacearum Race 3 biovar 2 is a soilborne pathogen that primarily infects hosts through the roots. Plants may be infected through wounds caused by handling or nematode feeding or by openings occurring during normal root development. Crops susceptible to the disease may become infected when grown in contaminated soils or when they are irrigated with contaminated water (Janse 1996). In greenhouse operations, subirrigation systems could be a means for disseminating the pathogen.
R. solanacearum Race 3 biovar 2 can also be spread through the use of infected plant material. This was what was witnessed in the U.S. with the geranium industry. The disease was introduced into greenhouses through the use of infected plant cuttings. For potatoes, the disease can be spread by the use of infected potato seed stock. (Lemay et al. 2003)
Detection and Diagnosis
The diagnosis to Race and biovar can be difficult requiring the use of several diagnostic tests to make a final diagnosis. Semi-selective media, ELISA and Immunostrips are only specific to genus and species and not Race and biovar. Therefore they are useful as a first step in the diagnostic process. ELISA can be extremely useful for screening large quantities of materials. Immunostrips which are an ELISA based technology can be used in the field.
Definitive identification of R. solanacearum Race 3 biovar 2 requires at least two tests. Biovar is determined by using an assay that measures acid generation from a panel of sugars and sugar alcohols. Race 3 of R. solanacearum may be identified with a PCR reaction containing reagents specific for Race 3.
Currently there are no chemical controls available. Some methods of control include: eradication of weedy hosts, altering soil pH over the growing season and screening tubers for disease before planting. Woody nightshade (Solanum dulcamara) which commonly grows along riverbanks in the UK can serve as a reservoir of inoculum. These plants are thought to have been the source of contamination for potato crops irrigated with river water in some cases (Plant Health Division 2002 and Elphinstone and Harris 2002). A variety of control measures are being utilized in Europe for potato crops including lowering soil pH in the summer and raising it again in the fall, treating waterways to remove woody nightshade, and using non-susceptible crops for rotations (Lemay et al. 2003).
Control on tomato relies on cultural practices including the use of resistant varieties. Unlike potato, there has been some success with chemical control on tomatoes. A plant derived volatile compound thymol was found to reduce bacterial wilt incidence of tomato when used as a pre-plant soil fumigant (Momol 2006). However, its effectiveness post-infection has not been determined. Plant breeding efforts have resulted in some tomato breeds such as Hawaii 7996 with high resistance but also some undesirable traits like small fruit. There are some large fruit varieties with moderate resistance to bacterial wilt caused by Race 1 (Momol 2006).
As with potatoes, there is no chemical control available for this disease on geraniums. Control relies solely on eradication of infected plant material and proper greenhouse sanitation. Sanitation practices include (Lemay et al 2003):
- Limit greenhouse access and practice good sanitation techniques such as the use of foot baths before entry and disinfesting tools between making cuttings.
- Monitor water for presence of pathogen.
- Clean irrigation system on a regular basis.
- Culture index plant material 1-3 years to ensure it is disease free.
- Practice weed control programs within and around greenhouses.
- Allen, C. 2003. Bacterial Wilt Disease & Ralstonia solanacearum. Talk from 2003 Ralstonia Solanacearum race 3 biovar 2 Outbreak in Geraniums: A Program Review meeting held June 17 and 18, 2003 in Riverdale, MD.
- Daughtrey, M. 2003. New and Re-emerging Diseases in 2003. Department of Plant Pathology, Cornell University, Long Island Horticultural Research & Extension Center.
- Denny, T. P. and Hayward, A.C. 2001. Ralstonia, pages 151-174 in: Schaad, N. W. et al. Laboratory guide for the identification of plant pathogenic bacteria, 3rd ed. APS Press, St. Paul, 373 pp.
- Elphinstone, J. and Harris, R. 2002. Monitoring and control of the potato brown rot bacterium in irrigation water. British Potato Council report 190. http://www.potato.org.uk/upload/pdf/researchReports/report190.pdf. Accessed 2004 January 8.
- Elphinstone, J. January 2004. Personal communication. EC project number: SMT project CT97-2179.
- Harris, D. C. 1972. Intra-specific variation in Pseudomonas solanacearum. Pages 289-292 in: Proc. Int. Conf. Plant Pathog. Bact., 3rd.
- Janse, J. 1996. Potato Brown rot in western Europe – history, present occurrence and some remarks on possible origin, epidemiology and control strategies. Bulletin OEPP/EPPO 26: 679-695.
- Kim, S. H., Olson, R. N. and Schaad, N. 2002. Ralstonia solanacearum Biovar 2, Race 3 in geraniums imported from Guatemala to Pennsylvania in 1999. Plant Disease 92:S42.
- Kim, S. H., T. N. Olson, N. W. Schaad, and G. W. Moorman. 2003. Ralstonia solanacearum Race 3, Biovar 2, the Causal Agent of Brown Rot of Potato, Identified in Geraniums in Pennsylvania, Delaware, and Connecticut. Plant Disease Page 87:4.
- Kluepfel, M., Blake, J., and Keinath, A. Home and Garden Information Center: Irish and Sweet Potato Diseases, HGIC 2214. Clemson Extension. http://hgic.clemson.edu/PDF/HGIC2214.pdf. Accessed 2004 January 8.
- Lemay, A., Redlin, S., Fowler, G., Dirani, M. 2003, February 12. Pest Data Sheet Ralstonia solanacearum race 3 biovar 2. USDA/APHIS/PPQ Center for Plant Health Science and Technology Plant Epidemiology and Risk Analysis Laboratory Raleigh, NC.
- Momol, T. 2006, October 12. Recovery Plan for Ralstonia solanacearum Race 3 biovar 2, Causing Brown Rot of Potato, Bacterial Wilt of Tomato and Southern Wilt of Geranium. National Plant Disease Recovery System. http://www.ars.usda.gov/SP2UserFiles/Place/00000000/opmp/Rs3-2RecoveryPlan-v-Oct112006.pdf. Accessed 2006 October 17.
- O’Hern, C. 2004 January 5. Detection of Ralstonia solanacearum race 3 biovar 2 in New York Greenhouse (document by Richard Dunkle) [Distribution list]. Accessed 2004 January 5.
- Plant Health Division. 2002. Potato Brown Rot (RALSTONIA SOLANACEARUM) Report on monitoring and eradication in England 2002. Defra, Plant Health Division. http://www.defra.gov.uk/planth/phnews/monrad02.pdf. Accessed 2004 January 8.
- Williamson, L., Nakoho, K., Hudelson, B. and Allen, C. 2002. Ralstonia solanacearum race 3, biovar 2 strains isolated from geranium are pathogenic on potato. Plant Dis. 86:987-991
A list of federal, state, university, international, and organization websites about R. solanacearum Race 3 biovar 2 can be found at the USDA, National Agricultural Library, National Invasive Species Information Center http://www.invasivespeciesinfo.gov/microbes/bacterialwilt.shtml
We thank Margery Daughtrey, Cornell University and Tim Momol, University of Florida for their editorial review comments.
These materials may be used as long as the original author is given credit.