Ralstonia solanacearum (Smith) Yabuuchi et al. (formerly called Pseudomonas solanacearum), is a soilborne bacterial pathogen that is a major limiting factor in the production of many crop plants around the world. This organism is the causal agent of brown rot of potato, bacterial wilt or southern wilt of tomato, tobacco, eggplant, and some ornamentals, and Moko disease of banana (1, 8).
Ralstonia solanacearum is a widely distributed pathogen found in tropical, subtropical, and some temperate regions of the world (3). The species as a whole has a very broad host range and infects hundreds of species in many plant families. The majority of hosts are dicots with the major exception being bananas and plantains. Most economically important host plants are found in the Solanaceae or nightshade family (8).
Specific host range and
distribution of R. solanacearum
depends on the race and to some degree the biovar of the pathogen (2). These host ranges and distributions have been
changing in recent years. Race 1 is
endemic to the southern
tetrazolium chloride (TZC) agar or 2% sucrose peptone agar (SPA)
used for isolation when R. solanacearum
is suspected. After two days on TZC
wildtype colonies are large, elevated, fluidal, and either entirely
with a pale red center; avirulent mutant colonies are butyrous,
with a bluish border. On SPA, R.
solanacearum colonies are white and
fluidal with characteristic whorls (4).
of virulent Ralstonia
on TZC agar.
(Photo courtsey of
T. Momol and S.M. Olson, University of Florida)
Direct isolation of Ralstonia solanacearum can be obtained from plant ooze and exudates. The infected stems and/or petioles are cut using a sterile sharp knife or razor blade. If bacterial ooze does not actively appear, the plant material is squeezed between two fingers. A suspension of the ooze is prepared in sterile distilled water and then streaked onto either TZC or SPA plates. Pure cultures are usually easily isolated (4). R. solanacearum can also be isolated from water and soil using a modified Kelman's TZC medium (4).
R. solanacearum rapidly lose virulence
when maintained on laboratory media; however, the organism can easily
maintained for years in sterile distilled water or on agar slants
sterile mineral oil and stored at room temperature (7).
Ralstonia solanacearum is a gram-negative motile rod. The organism grows aerobically and does not form endospores. Cells are 0.5-0.7 X 1.5-2.0 µm and are non-encapsulated. R. solanacearum is catalase positive, oxidase positive, and reduces nitrates. The pathogen does not hydrolyze starch and does not readily degrade gelatin. In broth culture, the organism is inhibited by concentrations of NaCl greater than 2% (8).
a quick field diagnostic identification of R.
solanacearum and to distinguish bacterial wilt from vascular wilts
by fungal pathogens, bacterial streaming from infected plant material
used. A stem section is cut from a plant
with vascular discoloration using a sharp knife or razor blade. The stem section is placed against the inside
wall of a water-filled clear beaker or test tube so that the end of the
just touches the water surface. Milky
white strands containing bacteria and extracellular polysaccharide will
from the cut ends of the xylem (7).
Quick serological tests are also now available for detection of
from a cut infected
(Photo courtsey of
T. Momol and S.M. Olson,
University of Florida)
Races and Biovars:
pathogen species is subdivided into races based on host range. Currently, polymerase chain reaction (PCR) is
the primary means of
definitive identification of pathogen race.
Before PCR, a tobacco hypersensitivity test, developed by Lozano
Sequeira, was used to distinguish between Races 1, 2, and 3, the most
economically important races. The
species is also subdivided into biovars based on the utilization of the
disaccharides cellobiose, lactose, and maltose and oxidation of the
alcohols dulcitol, mannitol, and sorbitol (4).
illustrates the classification into biovars based on
Recently, it has been
suggested that Ralstonia solanacearum
should be considered a “species complex.”
Also, some consider the current race and biovar system
it is based on phenotypic measures and propose switching to a
phylogenetically-based hierarchical system.
This scheme is based upon phylogenetic analysis of sequence
using RFLP typing, 16S rDNA phylogeny, and hrpB PCR-RFLP data.
The system divides the species complex into
phylotypes (phylogenetic grouping of strains), sequevars (group of
endoglucanase or mutS gene sequenes diverging by <1%), and clones
strains exhibiting the same genomic fingerprint) (3).
High temperatures and high soil moisture generally favors Ralstonia solanacearum, the exception being certain Race 3 strains that are pathogenic on potato and are able to grow well at lower temperatures (8). The pathogen is found in many different soil types and over a wide range of soil pH. The organism survives in infected plant material, vegetative propagative organs, wild host plants, and soil. Alternate hosts, especially latently infected weed species, are thought to play a major role in the overwintering ability of the organism in temperate regions (5, 8). Bittersweet nightshade (Solanum dulcamara), a common aquatic weed, has been implicated as the source of several brown rot outbreaks (8).
Sources of inoculum for
agricultural fields and methods of spread include irrigation and
aquatic weeds, infested soil and field weeds, contaminated planting
latently infected vegetative propagative material, and contaminated
and equipment. Once established in a
field, plant-to-plant spread may occur when bacteria move from roots of
plants to roots of healthy plants (5, 8).
|Contaminated irrigation pond and
|Latently infected weeds in and around
||Contaminated farm equipment and/or
solanacearum enters the plant
through wounds in the roots from
cultivating equipment, nematodes, insects, and through cracks where
roots emerge (1). The bacteria reach the
large xylem elements and are spread into the plant, where they multiply. Once established in the xylem vessels, the
bacteria are able to enter the intercellular spaces of the parenchyma
the cortex and pith in various areas of the plant.
solanacearum is able to dissolve the cell walls and create slimy
bacteria and cell debris. Production of
highly polymerized polysaccharides increases the viscosity of the
results in plugging (7).
Agrios, G.N. 1997.
Edition. Academic Press,
2. Daughtrey, M. 2003. Southern bacterial wilt, caused by Ralstonia solanacearum. Society of American Florists’ 19th Annual Conference on Insect and Disease Management on Ornamentals.
Fegan, M. and P. Prior. 2004.
Recent developments in the phylogeny and classification of Ralstonia solanacearum. Presentation
at the 1st
International Tomato Symposium.
4. French, E.B., L. Gutarra, P. Aley, and J. Elphinstone. 1995. Culture media for Ralstonia solanacearum isolation, identification, and maintenance. Fitopatologia 30(3):126-130.
Ji, P., T. Momol, S.M. Olson, J. Hong, P.
Pradhanang, A. Narayanan, and J.B. Jones.
2004. New Tactics for Bacterial Wilt Management on
Tomatoes in the
Jones, J.B, J.P. Jones, R.E. Stall, and
T.A. Zitter, Eds. 1991.
Compendium of Tomato Diseases. APS
Shew, H.D. and G.B. Lucas, Eds. 1991. Compendium
of Tobacco Diseases. APS Press,
Stevenson, W.R., R. Loria, G.D. Franc,
and D.P. Weingartner, Eds. 2001. Compendium of Potato Diseases, 2nd
Ed. APS Press,
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