Bacterial wilt is a major restricting factor in tomato production throughout the universe. Furthermore, it can be straight related to economic adversity and hungriness in Africa, Asia and South and Central America ( Champoiseau 2009 ) . Even more distressing, is the improbably broad host scope of Ralstonia solanacearum. Since the bacteriums can be found on so many hosts, it is in bend a job for states importing propagules and other works goods ( EPPO 1997 ) . With the addition in universe trade and the unbelievable diverseness of this pathogen, it is important that we understand it and work to forestall the harm it can do. This research will supply insight into the biological science of the bacterial wilt causal agent, current direction tactics and progresss in research that can potentially decrease the desolation bacterial wilt causes to tomato harvests each twelvemonth.
Economic Impact of Bacterial Wilt
Tomato is one of the most widely produced veggies in the universe. In 2004 4,397,873 hectares of tomatoes were planted worldwide. Merely murphy is produced in more countries than tomato. Many states see utmost losingss due to bacterial wilt. In Taiwan bacterial wilt infects anyplace from 15-55 % of the tomato harvest depending on environmental conditions. Infection rate in India is every bit high as 100 % ( AVRDC 2004 ) . The losingss incurred due to bacterial wilt add up to 1000000s of dollars each twelvemonth ( Champoiseau 2009 ) .
Causal Agent of Bacterial Wilt
Ralstonia solanacearum, the causal agent of bacterial wilt, is good adapted to last in utmost conditions. R. solanacearum is capable of life as a saprophytic organism in the dirt or H2O for many old ages. It can use about anything as a nutrient beginning. The bacterium besides have cistrons which contribute to heavy metal opposition. This is similar to bacteriums that have evolved to last in contaminated dirt. Other cistrons in R. solanacearum addition the likeliness of antibiotic opposition ( Genin 2004 ) . Since the bacteria is so good adapted, the best method for pull offing the disease is to develop immune assortments.
Categorization of Ralstonia solanacearum
R. solanacearum is divided into phylotypes, races, and biovars. Phylotype is determined based on geographic beginning of the R. solanacearum strain. Phylotype I strains originate in Asia, while Phylotype II strains isolated in the Americas. Strains from Africa and the environing islands compose Phylotype III and Phylotype IV strains arise in Indonesia ( Prior 2005 ) . The race of a peculiar strain is determined by the hosts which it infects. The biovar is so determined by analyzing how it uses and or oxidise carbohydrates and hexose intoxicants. Race 3 is typically associated with doing disease in murphy and tomato ( Wenneken 1999 ) . Race 1 nevertheless, has a broader host scope doing it capable of infecting solanaceous workss, land nuts and several other host workss ( Jaunet 1998 ) .
Host Range of Ralstonia Solanacearum
R. solanacearum has an improbably wide host scope which merely adds to the challenge of commanding the pathogen. R. solanacearum can infect many of import nutrient harvests besides tomato. Some of these include murphy, eggplant and baccy. R. solanacearum can besides infect workss outside of the solanaceous household every bit good. Even more distressing, is the fact that R. solanacearum can infect weed hosts every bit good. Latent infection in weed hosts can frequently travel unnoticed until the manufacturers tomato harvest becomes infected ( Wenneken 1999 ) . The tabular array below lists the workss that strains of R. solanacearum can infect.
Ralstonia solanacearum is present throughout many temperate, tropical and semitropical parts of the universe. States in Africa, Asia, North America, South America and Europe have reported Ralstonia solanacearum infection. The maps below show states where Ralstonia solanacearum ( race 1 and race 3 ) is present.
Both Maps from EPPO ( beginning listed individually for each map in plants cited )
From these maps, it is obvious that R. solanacearum is going a global job. This pathogen will merely go on to distribute, due to a few different factors. First, the bacteriums are improbably diverse. Different races and biovars infect different hosts. This wide host scope means most states will manage at least one works that the bacteriums can infect. Furthermore, international trade will increase the airing of the bacteriums. Any state that imports workss, film editings, or dirt harbouring the bacterium hazards conveying in new strains ( Norman 2009 ) . States such as the USA import many points that could increase the happening of new strains within the state. The ability of new strains to last in more temperate climes besides raises the hazard of bacteriums distributing to new parts of the universe ( EPPO 1997 ) .
How Bacteria is Disseminated
R. solanacearum is disseminated a assortment of ways, which increases the trouble of pull offing bacterial wilt. Irrigation H2O and cultivating equipment can both distribute the bacteriums throughout Fieldss. Bacterias can besides be transferred by workers places and vesture, every bit good as sniping knives or shears ( AVRDC 2004 ) . Many factors besides contribute to the rate of infection. Wounds from insects or cultivation leave more gaps for bacteriums to come in the works. Hot, humid and wet environmental conditions are favourable to bacterial wilt development and lead to more terrible symptoms ( AVRDC 2004 ) .
When and How Bacteria Infects Plant
Ralstonia solanacearum can infect a works at any growing phase. Symptoms can look in yearss or hebdomads depending on environmental conditions ( Wang 2000 ) . Ralstonia solanacearum is a dirt borne bacterium. Bacteria infect roots through natural gaps or lesions caused by cultivation, insects, or roundworms. Once inside the works, R. solanacearum replicates and colonizes vascular tissue. As colonisation in vascular tissue additions, H2O flow through the xylem is restricted. This limitation of H2O flow consequences in wilting ( Grimault 1993 ) . Wilting workss besides put emphasis on the bacteriums that infected them. Once the works dies, the bacteriums must accommodate to populating in dirt or H2O once more. Bacterias can besides accommodate to populate in alternate host workss ( Boshou 2005 ) .
Adaptations for Increased Virulence
Ralstonia solanacearum is unluckily good adapted to occupy workss and do lay waste toing consequences one time inside the xylem. The bacterium produce proteins that degrade cell walls leting the bacteriums to derive entree to the xylem. Once inside the xylem, the bacteriums synthesize proteins designed to suppress the works defence response. This allows for rapid colonisation of bacteriums in the early phases of infection ( Genin 2004 ) . The bacteriums so produce polyoses which clog the xylem preventing H2O flow throughout the works ( Wang 2000 ) .
As the common name implies, wilting is the most common symptom of Ralstonia solanacearum infection. In the early phases of infection, the wilting may merely be present on one side of the works or occur merely in the afternoon heat. The works will retrieve over dark and look to be all right the following twenty-four hours ( AVRDC 2004 ) . As bacteriums begin to colonise more of the xylem, the works will wilt irreversibly, ensuing in decease. Roots and pith inside the septic workss will ab initio look H2O soaked, and so turn brown ( Sikora 2004 ) . When conditions are ice chest and drier, Ralstonia solanacearum infection typically consequences in scrawny growing and yellowing of leaf before wilting ( AVRDC 2004 ) . Photos of symptoms are featured below ( exposures from Wang 2005 ) .
Wilting caused by R. solanacearum infection Brown pith in septic works
Death as a consequence of R. solanacearum bacteriums colonising xylem
Many direction patterns can assist forestall the spread of the bacterium. Controling insect plagues and roundworms help forestall lesions which allow the bacteriums to come in workss more easy. Sanitation steps, such as cleansing work boots and pruning tools help forestall distributing bacteriums to healthy workss or clean countries of a field. Avoiding over irrigation besides prevents standing H2O which is contributing to rapid bacterial growing. Application of antibiotics is non effectual. Resistant tomato assortments provide protection from losingss due to bacterial wilt, but tomatoes are frequently merely immune to a specific strain of the bacteriums ( Wang 2005 ) .
Geneticss and Physiology Behind Resistance
Since utilizing immune assortments is one of the most effectual ways to pull off bacterial wilt, it is of import to understand the genetic sciences and physiology of immune workss. Resistance to R. solanacearum is a quantitative trait controlled by many cistrons. Genes that contribute to opposition hold been found on chromosomes 2, 6, 8 and 12 ( Wang 2000 ) . Other markers associated with opposition to bacterial wilt were found on chromosomes 7 and 10 ( Young 1996 ) . Loci on chromosome 6 are connected with opposition to multiple strains of R. solanacearum. A venue found on chromosome 12 is associated with opposition to race 1 biovar 3 Pss4, which originated in Taiwan. The venue on chromosomes 2 and 8 merely showed weak association with opposition to bacterial wilt ( Wang 2000 ) . Below is a map of the venue related to R. solanacearum opposition created utilizing informations from Wang & A ; acirc ; ˆ™s survey in 2000. The map depicts the chromosomes of F2 progeny of a cross between two immune parents, Hawaii7996 and WVa700.
Chromosome map from ( Wang 2000 )
Physiology of immune workss is besides different than that of susceptible workss. Harmonizing to research completed by Grimault and Prior, resistant workss still acquire infected by R. solanacearum. Unlike susceptible workss nevertheless, the resistant tomatoes are capable of arousing a defence response to incorporate the bacterium. This works defence response contains the bacterium, forestalling higher colonisation degrees. Higher colonisation degrees of bacteriums lead to clogged xylem and the restricted H2O flow throughout the works ( Grimault 1994 ) . Biochemical differences besides exist between resistant and susceptible cultivars. Research workers noted higher activity of phenylalanine ammonium hydroxide lyase ( PAL ) and polyphenol oxidase ( PPO ) . Increased PAL is linked to the tract that produces compounds such as phenols which are related to the works defence response. PPO is related to the oxidization of phenols to do quinines which are besides related to the works defence response to pathogens ( Vanitha 2009 ) .
How to Screen
Many techniques can be used to test for opposition to bacterial wilt in tomatoes. Phenotypic observations can be used to test for opposition to R. solanacearum. One of the earlier techniques utilized by Grimault and Prior was to mensurate percent colonisation and bacteriums colony denseness in vascular tissue of septic workss. Immune workss exhibit less percent colonisation and lower bacteriums colony denseness ( Grimault 1993 ) .
Newer methods using molecular markers are now in topographic point to screen for opposition every bit good. Markers associated with opposition to a wide scope of strains have been found on chromosome 6. Other molecular markers have been found on chromosomes 7, 10 and 12. These markers nevertheless, are related to resistance that is strain specific ( Wang 2000 ) . Biochemical markers have besides been discovered as a possible manner to test for opposition to R. solanacearum. Researchers found increased activity of phenylalanine ammonium hydroxide lyase and polyphenol oxidase in immune cultivars in ( Vanitha 2009 ) .
Testing for Resistance
Testing for opposition requires pull stringsing both the workss and the environment. When engendering new cultivars for opposition, crosses must be made between parents likely to lend the cistrons of involvement. These crosses must be carefully recorded. Surveies for opposition to R. solanacearum can be conducted in a field or nursery environment. Testing location determines the methods used to fix the experiment. In nursery experiments turning media is sterilized, and so workss are seeded into pots. Once the workss are established ( around 5 hebdomads after seting in Grimault and Prior experiment ) they are inoculated. The benefit to this method is being able to choose the strain of bacteriums used as an inoculum. Environmental controls are besides easier to keep within a nursery ( Grimault 1993 ) .
In field testing, the dirt is typically tested for the strain of R. solanacearum nowadays every bit good as the denseness of bacteriums. In this method, tomato workss are foremost seeded in the nursery, transplanted into the field, and so observations are made to find opposition to R. solanacearum. This method is better able to stand for how immune assortments will act in the field. With this method nevertheless, the research worker does lose a great trade of control over the environment. Once the vaccination is done, observations should be recorded every twenty-four hours due to the fact that wilting can happen quickly in more susceptible assortments. The graduated table for hiting opposition is listed below.
1= No disease, healthy works
2= merely youngest leaves show marks of wilting
3=only one side of the works is wilted, recovers when temperatures drop
4= whole works shows marks of wilting, shows little recovery when temperatures drop
5=complete wilting of works, brown pith, decease of works
Scale devised utilizing information from Plant Disease Notes ( Sikora 2004 ) .
This graduated table is a proposed ocular graduated table for hiting disease opposition. Using phenotypic disease hiting it is sometimes more hard to keep truth. Phenotypic hiting nevertheless, does give a better image of how the opposition appears in the field, instead than merely happening markers associated with opposition found in a lab.
Troubles in Developing Immune Assortments
Developing tomato assortments resistant to R. solanacearum can be hard since the strain of the bacteriums vary widely between parts. With different strains in assorted parts of the universe, multiple immune assortments need to be developed to assist forestall desolation from R. solanacearum infection. The genetic sciences of bacterial wilt opposition besides make engendering for immune traits hard. Bacterial wilt opposition is a quantitative trait. Quantitative traits that are controlled by multiple cistrons make genteelness for those traits much more hard ( Wang 2000 ) .
Furthermore, developing immune assortments is an improbably long procedure, which takes a figure of old ages. This slow development agencies there is non ever a short term solution for states enduring from harvest losingss due to bacterial wilt. Another job, is that developing immune assortments requires financess for research. Many states that suffer the most desolation from R. solanacearum are developing states with largely subsistence husbandmans. Fortunately, one short term solution is available to manufacturers. Agriculturists can graft susceptible assortments onto immune rhizomes. This method is labour intensive, but affectional. One survey in India found 100 per centum endurance of grafted workss while none of the susceptible workss survived ( Louws 2006 ) .
Using Genetic Engineering to Develop Immune Assortments
Genetic technology has already shown to heighten opposition to bacterial wilt. One successful survey was completed utilizing baccy. The cistron isolated for this undertaking was besides supplied to tomato research workers. The cistron of involvement was isolated from Antheraea pernyi. This cistron inhibits bacterial cell growing by bring forthing peptides that create holes in the bacterium cell walls. Despite the success of familial transmutation, there is still one chief job with this attack. Many states are immune to the thought of turning and devouring tomatoes that contain transgenes ( Boshou 2005 ) .
Ralstonia solanacearum is a pathogen capable of doing lay waste toing disease. Bacterial wilt is responsible for significant losingss in tomato harvests throughout the universe. This is debatable due to the fact tomatoes are an of import beginning of lycopene, vitamin A and antioxidants necessary for proper nutrition. Furthermore, tomatoes are an of import economic harvest for many states throughout the universe. Devastation due to bacterial wilt is straight related to economic adversity and hungriness in developing states in Central and South America, Africa and Asia ( Champoiseau 2009 ) .
Other features of Ralstonia solanacearum biological science contribute to the desolation these bacteriums cause. R. solanacearum has an highly wide host scope which adds to the easiness of airing. The bacteria is besides adapted to last in utmost environments every bit good as cause terrible harm to the workss it infects. These versions make R. solanacearum peculiarly hard to pull off.
Presently, research workers have identified markers associated with Ralstonia solanacearum opposition. Due to the quantitative nature of the trait, engendering for opposition has proved hard. With modern scientific discipline, testing for opposition has become faster and more accurate. This will greatly better the opportunities of developing immune cultivars. Genetic technology has besides proved effectual in adding R. solanacearum opposition to workss, but many states are non willing to bring forth or devour harvests incorporating transgenes. It is of import that engendering for immune cultivars continue to be pursued since other tactics for pull offing bacterial wilt merely provide limited effectivity.