Ralstonia solanacearum is an aerobic non-spore-forming, 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. Bacterial wilts of tomato, pepper, eggplant, and Irish potato caused by R. solanacearum were among the first diseases that Erwin Frink Smith proved to be caused by a bacterial pathogen. Because of its devastating lethality, R. solanacearum is now one of the more intensively studied phytopathogenic bacteria, and bacterial wilt of tomato is a model system for investigating mechanisms of pathogenesis. Ralstonia was until recently classified as Pseudomonas, with similarity in most aspects, except that it does not produce fluorescent pigment like Pseudomonas. The genomes from different strains vary from 5.5 Mb up to 6 Mb, roughly being 3.5 Mb of a chromosome and 2 Mb of a megaplasmid. While the strain GMI1000 was one of the first phytopathogenic bacteria to have its genome completed, the strain UY031 was the first R. solanacearum to have its methylome reported. Within the R. solanacearum species complex, the four major monophyletic clusters of strains are termed phylotypes, that are geographically distinct: phylotypes I-IV are found in Asia, the Americas, Africa, and Oceania, respectively.
Ralstonia solanacearum was once considered as a possible biological control agent for Kahili ginger (Hedychium gardnerianum), a highly invasive species. However, R. solanacearum is no longer used as a biological control for Kahili ginger in Hawaiian forests because of its wide host range. The ginger-parasitizing strain will infect numerous ginger species, including edible ginger (Zingiber officinale), shampoo ginger (Z. zerumbet), and pink and red ginger (Alpinia purpurata).
Hosts and symptoms
Hosts Plant hosts that R. solanacearum infects include:
Crops Potato (Solanum tuberosum); tomato (Lycopersicum esculentum); soybean (Glycine max); eggplant/aubergine (Solanum melongena); banana, (Musa spp); geranium (Pelargonium spp.); ginger (Zingiber officinale); tobacco (Nicotiana tabacum); bell pepper/sweet pepper (Capsicum spp.); olive (Olea europea); rose (Rosa).
Wild hosts Woody nightshade (Solanum dulcamara)
Symptoms Geranium: Wilting begins with lower leaves and petioles and works its way up the plant. Wilted leaves have chlorotic, wedge-shaped areas or chlorotic and/or necrotic leaf margins. No leaf spots are evident. Eventually, the entire plant collapses on the medium. White runny material oozes from cut stems.
Potato: Wilting of the leaves occurs at the end of the day with recovery at night. Plants eventually fail to recover and die. Brown staining of vascular ring happens and pus may exude from the ring when the tuber is squeezed. Pale ooze may exude from eyes and heel end of potato. Soil will adhere to the oozing eyes. Cross-section of a stem placed in water will exude milky white strands. Unlike the fungal wilts, the leaves remains green in bacterial wilt.
Disease cycle Survival Ralstonia solanacearum can overwinter in plant debris or diseased plants, wild hosts, seeds, or vegetative propagative organs (other germplasm) like tubers. The bacteria can survive for a long time in water (up to 40 years at in pure water), and the bacterial population is reduced in extreme conditions (temperature, pH, salts, e.g.). Infected land sometimes cannot be used again for susceptible crops for several years. 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.Denny T., "Plant Pathogenic Ralstonia species" in GNANAMANICKAM, S. S. (2006). Plant-associated bacteria. Dordrecht, Springer. pp 1-62
Dispersal Ralstonia solanacearum causes wilting at high populations (108 – 1010 cfu/g tissue) and disperses in several routes. The large number of R. solanacearum can shed from roots of symptomatic and nonsymptomatic plants. Besides that, bacterial ooze (which is usually used as a sign for detection) on plant surfaces can enter the surrounding soil or water, contaminating farming equipment or may be acquired by insect vectors. In addition, this pathogen can be spread by contaminated flood water, irrigation, contaminated tools, or infected seeds. 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. The race 3 biovar 2 strain can survive in perennial nightshades which act as secondary hosts, and can also cause bacterial wilt of tomato. Some EU states and Middle Eastern countries have not yet been able to eradicate this pathogen.
Infection Ralstonia solanacearum usually enters the plant by a wound. Natural wounds (created by abscission of flowers, genesis of lateral roots) and unnatural ones (by agricultural practices or nematodes and xylem-feeding insects) could become entry sites for R. solanacearum. The bacteria get access to the wounds partially by flagellar-mediated swimming motility and chemotaxic attraction toward root exudates. Unlike many phytopathogenic bacteria, R. solanacearum potentially requires only one entry site to establish a systemic infection that results in bacterial wilt.
After invading a susceptible host, R. solanacearum multiplies and moves systemically within the plant before bacterial wilt symptoms occur. Wilting should be considered as the most visible side effect that usually occurs after extensive colonization of the pathogen. When the pathogen gets into the xylem through natural openings or wounds, tyloses may form to block the axial migration of bacteria within the plant. In susceptible plants, this sometimes happens slowly and infrequently to prevent pathogen migration, and may instead lead to vascular dysfunction by unspecifically obstructing uncolonized vessels.
Wilting occurs at high bacterial populations in the xylem and is partially due to vascular dysfunction in which sufficient water cannot reach the leaves. At this time, extracellular polysaccharide (EPS1) content is about 10 μg/g tissue in the taproot, hypocotyl, and midstem; EPS1 concentration is higher later on at more than 100 μg/g tissue in fully wilted plant. Ralstonia's systemic toxin also causes loss of stomatal control, but no evidence shows excessive transpiration as its consequence. The primary factor contributing to wilting is probably blocking of pit membranes in the petioles and leaves by the high molecular weight EPS1. High bacterial densities are byproducts of plant cell wall degradation; tyloses and gums produced by the plant itself are other contributing factors to wilting.
Natural genetic transformation Most strains of R. solanacearum are competent for genetic transformation. Natural genetic transformation is a sexual process involving DNA transfer from one bacterial cell to another through the intervening medium, and the integration of the donor sequence into the recipient genome by homologous recombination. R. solanacearum is able to exchange large DNA fragments ranging from 30 to 90 thousand bases.
Virulence mechanisms Ralstonia solanacearum possesses genes for all six protein secretion pathways that have been characterized in Gram-negative bacteria. Perhaps the best-studied of these is the Type III secretion system (T3SS or TTSS), which secretes infection-promoting effector proteins (T3Es) into host cells. Around 74 suspected or confirmed T3Es have been identified in R. solanacearum to date, although the functions of very few are currently known. Despite being just one of several protein secretion systems, T3SS is necessary for R. solanacearum to cause disease. No single effector protein has been found to significantly alter pathogenicity of R. solanacearum, but simultaneous disruption of certain subsets of effectors (such as the set of seven GALA effectors in strain GMI1000) strongly affects virulence of the pathogen. For example, GALA 7 is necessary for virulence on Medicago truncatula, hinting that T3E diversity may play a role in determining the broad host range of the R. solanacearum species complex.
The type III secretion system is not unique to R. solanacearum, and is, in fact, very ancient. The evolutionary history of the T3SS is contested; a high degree of similarity to the flagellum has sparked debate over the relationship between these two structures.
About half of T3SS proteins are highly conserved in R. solanacearum and likely constitute a very old and stabilized group of effectors in the core genome of the species complex. Among the other half showing variation among different strains of R. solanacearum, only a third show evidence of lateral gene transfer. The origins of the remaining effectors are unknown, although some researchers hypothesize that gene-for-gene interactions may play a significant role in shaping virulence genes in R. solanacearum. Some of these effector proteins are homologous to Transcription Activator-like effectors (TAL effectors) from Xanthomonas and could possibly have a similar function of activating specific genes in the host plant cells during R. solanacearum pathogenesis.
Environment The environment in which R. solanacearum is commonly found is affected by the particular race (a genetically diverse population within a species), and the particular biovar (a strain that differs physiologically or biochemically from other strains.) Race 1, race 2 biovar 1, and race 3, biovar 2 are three of the most common and important strains. Race 1 strains have a broad host range including tobacco and bananas, and are usually found in tropical and subtropical environments, as they have trouble surviving cooler temperatures, and are endemic to the southeastern United States. Race 2 strains have a more limited host range than race 1, and are mostly restricted to tropical environments. Race 3 strains are more cold tolerant than the other two and are found in tropical highlands and temperate areas. The host range for race 3 biovar 2 includes potatoes, tomatoes, and geraniums. Race 3 biovar 2 is very common throughout the world, but is not generally reported in North America, so is the focus of many sanitation and quarantine management practices to prevent the introduction or spread of the pathogen.
Although it is not there yet, researchers at the University of Guam are concerned about the possible spread of R. solanacearum to Guam.
Management General management Commercial chemicals have generally proven to be ineffective in controlling the pathogen and are not recommended as a means of control. In regions where the pathogen is established, a strategy of integrated disease management is the best method to reduce any impact of the pathogen. Using pathogen-free planting materials is a necessity. Planting resistant cultivars minimizes the ill effects of the pathogen, although no completely immune cultivars are now available. Finally, a good rotation system that follows susceptible crops with resistant or nonhost crops can assist in diminishing the pathogen. The pathogen is listed as a select agent in the United States; if the pathogen is detected by a proper authority, a number of management protocols may be implemented. These can range from surveys to quarantines of infected and potentially infected plant material, which in turn may lead to larger eradication and sanitation programs.
Specific host plant symptoms and management Potato