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R. solanacearum is a Gram-negative bacterium with rod-shaped cells, 0.5-1.5 µm in length, with a single, polar flagellum. The positive staining reaction for poly-ß-hydroxybutyrate granules with Sudan Black B or Nile Blue distinguishes R. solanacearum from many other (phytopathogenic) Gram-negative bacterial species. Gram-negative rods with a polar tuft of flagella, non-fluorescent but diffusible brown pigment often produced. Polyhydroxybutyrate (PHB) is accumulated as cellular reserve and can be detected by Sudan Black staining on nutrient-rich media or the Nile Blue test, also in smears from infected tissues (Anonymous, 1998;2006) On the general nutrient media, virulent isolates of R. solanacearum develop pearly cream-white, flat, irregular and fluidal colonies often with characteristic whorls in the centre. Avirulent forms of R. solanacearum form small, round, non-fluidal, butyrous colonies which are entirely cream-white. On Kelman’s tetrazolium and SMSA media, the whorls are blood red in colour. Avirulent forms of R. solanacearum form small, round, non-fluidal, butyrous colonies which are entirely deep red.

Recoginition


The bacterium may be obtained from infected tubers or stems for staining purposes if a small portion of tissue is pressed onto a clean glass slide. Potato tubers can be visually checked for internal symptoms by cutting. Suspect tubers should be diagnosed in the laboratory. Appropriate laboratory methods to detect the pathogen have been laid down in a harmonized EU-interim scheme for detection of the brown rot bacterium (Anon., 1997). These methods are based on earlier described indirect immunofluorescence antibody staining (IFAS). Standard samples of 200 tubers per 25 t of potatoes are taken (Janse, 1988;OEPP/ EPPO, 1990a;Anon., 1997, 1998, 2006). Recently a very effective selective medium has been described (Engelbrecht, 1994, and modified by Elphinstone et al., 1996), that can also be applied for detection in environmental samples such as surface water, soil and waste (Janse et al., 1998;Wenneker et al., 1999). ELISA and PCR, based on 16S rRNA targeted primers as well as fluorescent in-situ hybridization (FISH) using 16S and 23S rRNA-targeted probes have also been used.
Ralstonia syzygii, causal agent of Sumatra disease of clove (Syzygium) and the distinct Blood Disease Bacterium, causal agent of blood disease of banana in Indonesia, are closely related to R. solanacearum and cross-react in serological and DNA-based detection methods (Wullings et al., 1998;Thwaites et al., 1999).

Related invasive species

  • Ralstonia solanacearum

Related Farm Practice

  • Ecology
  • Groups
  • Staining
  • Hosts
  • Development
  • Host plants
  • Biology

Related location

  • Nepal
  • Sudan
Impact


The strains in the race 3 group are a select agent under the US Agricultural Bioterrorism Protection Act of 2002 (USDA, 2005). Peculiarly, the organism, if not yet already present in North America in pelargonium (Strider et al., 1981), was introduced with cuttings of this host by American companies producing these cuttings for their markets in countries like Kenya and Guatamala (Norman et al., 1999, 2009;Kim et al., 2002;Williamson et al., 2002;Williamson et al., 2002;O’ Hern, 2004). A similar situation led to introductions of the pathogen from Kenya into some northern European nurseries. Once the source (contaminated surface water) was recognized and proper control measures (use of deep soil water, disinfection of cutting producing premises and replacement of mother stock), the problem was solved and the disease in greenhouses eradicated (Janse et al., 2004);Similarly race 1 has been introduced into greenhouses with ornamental plants (rhizomes, cuttings or fully grown plants) such as Epipremnum, Anthurium, Curcuma spp. and Begonia eliator from tropical areas (Norman and Yuen, 1998, 1999;Janse et al., 2006;Janse, 2012). Introduction can and did occur from Costa Rica and the Caribbean, Indonesia, Thailand and South Africa. However, this idea of placing pathogens on bioterrorist list for unclear and perhaps industry-driven reasons and its effects, is strongly opposed in a recent publication from leading phytobacteriologists. This is because R. solanacearum is an endemic pathogen, causing endemic disease in most parts of its geographic occurrence, moreover normal quarantine regulations are already in place where the disease is not present or only sporadically and are thought to be more efficient and less damaging to trade and research than placing this pathogen on select agent lists and treating it as such (Young et al., 2008). Peculiarly, it has been used in the control of a real invasive species, the weed kahili ginger (Hedychium gardenarium) in tropical forests in Hawaii. This is not without risks because strains occurring on this weed host were thought to be non-virulent, but later appeared to be virulent on many edible and ornamental ginger species as well (Anderson and Gardner, 1999;Paret et al., 2008). The earlier mentioned tropical strains belonging to phylotype II/4 NPB could become an emerging problem not only in the Caribbean, but also to Southern Europe and North Africa where higher yearly temperatures prevail. Another threat for these countries could be strains belonging to race 1, biovar 1 (phylotype I) that have already been reported from field-grown potatoes in Portugal (Cruz et al., 2008).

Has Cabi datasheet ID
45009
Symptons


Potato
Foliage: the first visible symptom is a wilting of the leaves at the ends of the branches during the heat of the day with recovery at night. As the disease develops, a streaky brown discoloration of the stem may be observed on stems 2.5 cm or more above the soil line, and the leaves develop a bronze tint. Epinasty of the petioles may occur. Subsequently, plants fail to recover and die. A white, slimy mass of bacteria exudes from vascular bundles when broken or cut.
Tubers: external symptoms may or may not be visible, depending on the state of development of the disease. Bacterial ooze often emerges from the eyes and stem-end attachment of infected tubers. When this bacterial exudate dries, soil masses adhere to the tubers giving affected tubers a 'smutty' appearance. Cutting the diseased tuber will reveal browning and necrosis of the vascular ring and in adjacent tissues. A creamy fluid exudate usually appears spontaneously on the vascular ring of the cut surface.
Atypical symptoms on potato (necrotic spots on the epidermis), possibly caused after lenticel infection, have been described by Rodrigues-Neto et al. (1984).
Symptoms of brown rot may be readily distinguished with those of ring rot caused by Clavibacter michiganensis subsp. sepedonicus (EPPO/ CABI, 1997). R. solanacearum can be distinguished by the bacterial ooze that often emerges from cut stems and from the eyes and stem-end attachment of infected tubers. If cut tissue is placed in water, threads of ooze are exuded. Because such threads are not formed by other pathogens of potato, this test is of presumptive diagnostic value. For ring rot, tubers must be squeezed to press out yellowish dissolved vascular tissue and bacterial slime.
Tomato
The youngest leaves are the first to be affected and have a flaccid appearance, usually at the warmest time of day. Wilting of the whole plant may follow rapidly if environmental conditions are favourable for the pathogen. Under less favourable conditions, the disease develops slowly, stunting may occur and large numbers of adventitious roots are produced on the stem. The vascular tissues of the stem show a brown discoloration and drops of white or yellowish bacterial ooze may be released if the stem is cut (McCarter, 1991).
Tobacco
One of the distinctive symptoms is partial wilting and premature yellowing of leaves. Leaves on one side of the plant or even a half leaf may show wilting symptoms. This occurs because vascular infection may be restricted to limited sectors of stems and leaf petioles. In severe cases, leaves wilt rapidly without changing colour and stay attached to the stem. As in tomato, the vascular tissues show a brown discoloration when cut. The primary and secondary roots may become brown to black (Echandi, 1991).
Banana
On young and fast-growing plants, the youngest leaves turn pale green or yellow and collapse. Within a week all leaves may collapse. Young suckers may be blackened, stunted or twisted. The pseudostems show brown vascular discoloration (Hayward, 1983). Moko disease, caused by R. solanacearum, is easily confused with the disease caused by Fusarium oxysporum f.sp. cubense. A clear distinction is possible when fruits are affected - a brown and dry rot is only seen in Moko disease.
Teak
Seedling wilt manifests itself as yellowing of the mature lower leaves, which show scorching and browning of the tissue between the veins. The younger leaves and terminal shoot become flaccid and droop. Affected seedlings show either a gradual loss of leaf turgidity or sudden wilting. Seedling wilt becomes evident in the early hours of the day and gradually becomes more pronounced by midday, especially on sunny days. The wilted seedlings may partially recover during the afternoon and evening when temperatures fall, but wilting becomes more pronounced on successive days. The roots of affected seedlings exhibit a brownish-black discoloration. In advanced stages of disease, the tuberous portion of the root becomes discoloured and spongy. In due course, seedlings with pronounced wilt symptoms become completely desiccated.
In container nurseries, R. solanacearum infects the cotyledons of emerging seedlings causing greyish-brown, water-soaked lesions, which spread to the entire cotyledon and become necrotic. The infection spreads to the adjoining stem and root tissues and the affected seedlings rot and die. Collar rot appears in 1- to 4-month-old bare-root seedlings as greyish-brown, water-soaked lesions at the collar region of seedlings, just above the soil level. The lesions spread longitudinally on the stem, both above and below ground level, becoming sunken and necrotic. The younger leaves become flaccid and droop followed by leaf scorching and pronounced vascular wilt. In bare-root nurseries, wilt usually occurs in small patches affecting individuals or groups of seedlings, which expand as more seedlings succumb to the infection.
Infection of mature foliage begins as greyish-brown to greyish-black, irregular lesions that spread to the entire leaf lamina. Infection spreads to the petioles and stems.

Hosts

R. solanacearum as a species has an extremely wide host range, but different pathogenic varieties (races) within the species may show more restricted host ranges. Over 200 species, especially tropical and subtropical crops, are susceptible to one or other of the races of R. solanacearum. Worldwide, the most important are: tomato, tobacco, aubergine, potato, banana, plantain and Heliconia. Within the EPPO region, race 3 (see Biology and Ecology) with a limited host range including potato, tomato and the weed Solanum dulcamara, is considered to have potential for spread.
Other host crops are: Anthurium spp., groundnut, Capsicum annuum, cotton, rubber, sweet potato, cassava, castor bean and ginger.
Many weeds are alternative hosts of the pathogen. Solanum cinereum in Australia (Graham and Lloyd, 1978), Solanum nigrum and, in rare cases, Galinsoga parviflora, G. ciliata, Polygonum capitata, Portulaca oleracea (for example, in Nepal;Pradhanang and Elphinstone, 1996a) and Urtica dioica have been reported as weed hosts for race 3 (Wenneker et al., 1998). S. nigrum and S. dulcamara are primary wild hosts for race 3.
Lists of host records have been recorded (Kelman, 1953;Bradbury, 1986;Persley, 1986;Hayward, 1994a) but the original reports, gathered over many years, vary greatly in reliability. Few reference strains from reported host plants have been deposited in publicly accessible culture collections to support the authenticity of records.

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