L'antracnosi dels cítrics o "xancre dels cítrics" és una malaltia que afecta diferents espècies de cítrics. És causada pel bacteri Xanthomonas axonopodis. La infecció causa lesions a les fulles, les tiges i els fruits dels arbres del gènere Citrus com el taronger el llimoner, etc. Encara que no són perillosos per l'home, el xancre afecta de manera significativa la vitalitat de les plantes, cosa que provoca la caiguda prematura de fulles i fruits; els fruits infectats pel xancre són comestibles però per morius estètics no es poden comercialitzar. La malaltia, que hauria tingut origen al sud-est d'Àsia, és extremadament persistent quan s'estableix a una zona. Vergers de cítrics sencers s'han eliminat a Austràlia per intentar erradicar aquesta malaltia. Brasil i Estats Units en són focus actius en l'actualitat.
L'antracnosi dels cítrics o "xancre dels cítrics" és una malaltia que afecta diferents espècies de cítrics. És causada pel bacteri Xanthomonas axonopodis. La infecció causa lesions a les fulles, les tiges i els fruits dels arbres del gènere Citrus com el taronger el llimoner, etc. Encara que no són perillosos per l'home, el xancre afecta de manera significativa la vitalitat de les plantes, cosa que provoca la caiguda prematura de fulles i fruits; els fruits infectats pel xancre són comestibles però per morius estètics no es poden comercialitzar. La malaltia, que hauria tingut origen al sud-est d'Àsia, és extremadament persistent quan s'estableix a una zona. Vergers de cítrics sencers s'han eliminat a Austràlia per intentar erradicar aquesta malaltia. Brasil i Estats Units en són focus actius en l'actualitat.
Xanthomonas axonopodis ist eine Bakterienart. Es handelt sich um einen wichtigen Pflanzenschädling.
Die Zellen von Xanthomonas axonopodis sind stäbchenförmig. Es ist eine einzelne, polare (am Zellende liegende) Flagelle vorhanden. Die Art ist in der Regel gelb-pigmentiert, allerdings treten auch nicht-pigmentierte Stämme auf. Hierzu zählen z. B. die auf bestimmte Pflanzen spezialisierten (pathovare) Stämme X. axonopodis pathovar manihotis und einige Stämme von X. pathovar pv. ricini.[1] Die Pigmente sind sehr charakteristisch für die Gattung, es handelt sich um bromierte Arylpolyene, hier auch als "Xanthomonadine" (engl. "xanthomonadins") bezeichnet. Arylpolyene schützen Bakterien vor oxidativen Stress. Ein weiteres typisches Merkmal für Xanthomonas-Arten ist die Bildung von Xanthan. Es handelt sich um ein extrazelluläres Polysaccharid (EPS). Es wird von Xanthomonas-Arten nach dem Eindringen in die Pflanzenzellen gebildet und außen um die Bakterienzelle angelagert. Es dient dem Schutz des Bakteriums.[2]
Xanthomonas axonopodis ist, wie alle Arten der Gattung, auf Sauerstoff angewiesen (aerob). Der Stoffwechsel ist die Atmung, Sauerstoff (O2) dient als der terminale Elektronenakzeptor. Der Katalase-Test verläuft positiv.
Xanthomonas axonopodis zählt zu der Familie der Lysobacteraceae. Bis 2005 wurde Xanthomonas der Familie Xanthomonadaceae zu geordnet. Aufgrund von biochemischen Untersuchungen wurde diese Familie aufgelöst.[3][4]
Es folgt eine Liste einiger Pathovare und Beispielen von den entsprechenden Pflanzenwirten:
Das von Xanthomonas-Arten produzierte Polysaccharid Xanthan wird auch industriell genutzt. Zur Produktion werden neben X. axonopodis noch z. B. X. arboricola, X. campestris, X. citri und X. vasculorium eingesetzt. Entdeckt wurde die industriell nutzbare Bildung von Xanthan zuerst bei der Art X. campestri.[5] Die erste industrielle Produktion begann in den 1960ern. Nach Dextran war es das zweite Polysaccharid, welches von Mikroorganismen extrahiert und industriell genutzt wurde.[6] Es wird z. B. als Verdickungsmittel, Geliermittel und Klebstoff genutzt. In der Medizin wird es u. a. bei der Produktion von Brausemitteln, Retardtropfen und Säften eingesetzt. Nach Cellulose, Stärke und Arabicum stellt es das in der Industrie am häufigsten genutzte Polysaccharid dar (Stand 2005). Hierbei ist auch der Name Xanthangummi (engl.: xanthan gum) üblich.[5]
Wie viele Arten der Gattung Xanthomonas ist Xanthomonas axonopodis ein Pflanzenschädling. Die verschiedene Pathovare von Xanthomonas axonopodis verursachen Krankheiten bei verschiedenen Pflanzen, wie z. B. Luzerne (Medicago sativa), verschiedenen Arten der Gattung Axonpus und Citrus. Der Artname X. axonopius bezieht sich auf die Gräsergattung Axonpus, welche von X. axonopodis vs. axonopodis befallen wird. So befällt z. B. das Pathovar X. axonopodis pv. citri verschiedene Arten der Zitruspflanzen (Gattung Citrus) und löst den Zitruskrebs aus.[7] Der Wirtsbefall beginnt sobald das Bakterien mit Öffnungen der Oberfläche (wie z. B. Spaltöffnungen, Hydathoden oder Läsionen des Pflanzengewebes) in Kontakt kommt, dringt es in das Mesophyll der Pflanze ein und verursacht die Krebssymptome. Zu den Wirtspflanzen zählt z. B. die Orangenart Citrus sinensis. Im Jahr 2005 waren über 40 verschiedene Pathovare bekannt.[1]
Es gibt Hinweise, dass die Aminosäure Methionin eine Rolle bei der Pathogenese von Xanthomonas-Arten spielen könnte. So haben Studien über das von X. axonopodis pv. manihotis produzierte Phytotoxin Methylthiopropionsäure (MTPA) gezeigt, dass Methionin eine Vorstufe von MTPA ist.[1]
Xanthomonas axonopodis ist eine Bakterienart. Es handelt sich um einen wichtigen Pflanzenschädling.
Citrus canker is a disease affecting Citrus species caused by the bacterium Xanthomonas (X.axonopodis; X. campestris). Infection causes lesions on the leaves, stems, and fruit of citrus trees, including lime, oranges, and grapefruit. While not harmful to humans, canker significantly affects the vitality of citrus trees, causing leaves and fruit to drop prematurely; a fruit infected with canker is safe to eat, but too unsightly to be sold. Citrus canker is mainly a leaf-spotting and rind-blemishing disease, but when conditions are highly favorable, it can cause defoliation, shoot dieback, and fruit drop.[1]
The disease, which is believed to have originated in Southeast Asia,[2] is extremely persistent when it becomes established in an area. Citrus groves have been destroyed in attempts to eradicate the disease.
Countries like Brazil and the United States also suffer from canker outbreaks.[3]
Xanthomonas axonopodis is a rod-shaped Gram-negative bacterium with polar flagella. The bacterium has a genome length around 5 megabase pairs. A number of types of citrus canker diseases are caused by different pathovars and variants of the bacterium:[4]
Plants infected with citrus canker have characteristic lesions on leaves, stems, and fruit with raised, brown, water-soaked margins, usually with a yellow halo or ring effect around the lesion. Older lesions have a corky appearance, still in many cases retaining the halo effect. The bacterium propagates in lesions in leaves, stems, and fruit. The lesions ooze bacterial cells that, when dispersed by windblown rain, can spread to other plants in the area. Infection may spread further by hurricanes. The disease can also be spread by contaminated equipment, and by transport of infected or apparently healthy plants. Due to latency of the disease, a plant may appear to be healthy, but actually be infected.
Citrus canker bacteria can enter through a plant's stomata or through wounds on leaves or other green parts. In most cases, younger leaves are considered to be the most susceptible. Also, damage caused by citrus leaf miner larvae (Phyllocnistis citrella) can be sites for infection to occur. Within a controlled laboratory setting, symptoms can appear in 14 days following inoculation into a susceptible host. In the field environment, the time for symptoms to appear and be clearly discernible from other foliar diseases varies; it may be on the order of several months after infection. Lower temperatures increase the latency of the disease. Citrus canker bacteria can stay viable in old lesions and other plant surfaces for several months.
Xanthomonas axonopodis has the capability to form a biofilm for attachment on the host. The biofilm is the result of the production of extracellular polysaccharides (xanthan). The biofilm ensures the virulence and epiphytic survival of X. axonopodis pv. citri prior to the development of citrus canker. In addition, the bacteria secrete transcriptional activator-like (TAL) effectors through the type III secretion system. The effector interacts with host machinery to induce transcription for genes that regulate plant hormones such as gibberellin and auxin.[5][6]
Xanthomonas axonopodis pv. citri overseason in an infected area which appears as a canker lesion on leaf or stem. Canker lesions start out as pinpoint spots 2 to 10 millimeters in diameter.[1] The bacteria ooze out of the lesions when there is free moisture. During rainy weather, wind-blown rain carries the inoculum to new susceptible hosts. The bacteria infect new plants through stomata and wounds. Pruning or hedging can cut open mesophyll tissues, creating wounds through which the plant may be directly infected. The rain can also cause water congestion on the leaf surface, form columns of water through the stomata and promote infection through natural openings. Infections can form on fruit, foliage and young stem. Leaves and stems are most susceptible to infection within the first six weeks of initial growth. Infection of fruit is most likely to occur during the 90 day period after petal fall during fruit formation.[7] The varied size of lesions on citrus fruit is because of the multiple cycle of infections and can reflect different-aged lesions on the same fruit.[8][9]
Wind-driven rain plays a major role in the dispersal of X. axonopodis. The bacteria are said to be readily dispersed by splashed rain and wind and the quantity of X. axonopodis declines after the first event of wind-blown rain dispersal. Apart from that, the bacteria also favor warm weather. The cases of citrus canker are more acute in areas that receive high rainfall and have high mean temperature, such as Florida. Often, cankers emerge briskly during fall, slowly during winter and most rapidly in mid to late spring.[10]
The disease can be detected in groves and on fruit by the appearance of lesions. Early detection is critical in quarantine situations. Bacteria can be tested for pathogenicity by inoculating multiple citrus species with them. Additional diagnostic tests (antibody detection), fatty-acid profiling, and genetic procedures using polymerase chain reaction can be conducted to confirm diagnosis and may help to identify the particular canker strain. Clara H. Hasse determined that citrus canker was not of fungoid origin but was caused by a bacterial parasite.[11][12] Her research published in the 1915 Journal of Agricultural Research played a major part in saving citrus crops in multiple states.
Not all species and varieties of citrus have been tested for citrus canker. Most of the common species and varieties of citrus are susceptible to it. Some species are more susceptible than others, while a few species are resistant to infection.
Susceptibility Variety Highly susceptible Grapefruit (Citrus x paradisi), Key lime (C. aurantiifolia), Kaffir lime (C. hystrix), lemon (C. limon) Susceptible Limes (C. latifolia) including Tahiti lime, Palestine sweet lime; trifoliate orange (Poncirus trifoliata); citranges/citrumelos (P. trifoliata hybrids); tangerines, tangors, tangelos (C. reticulata hybrids); sweet oranges (C. sinensis); bitter oranges (C. aurantium) Resistant Citron (C. medica), Mandarins (C. reticulata) Highly resistant Calamondin (X Citrofortunella), kumquat (Fortunella spp.) Modified from: Gottwald, T.R. et al. (2002). Citrus canker: The pathogen and its impact. Online. Plant Health Progress[13]Quarantine measures are implemented in areas where citrus canker is not endemic or has been obliterated to prevent the introduction of X. axonopodis. On the other hand, in regions where citrus canker occurs, Integrated Pest Management (IPM) is utilized. The most notable feature of this management program is the transposition of susceptible citrus plants to field resistant citrus cultivars. Apart from using resistant cultivars in fields, there are several measures that are taken to control citrus canker from causing failed crop. The measures can be divided into three major categories: exclusion, eradication and sanitation.[8][4]
Citrus trees or fruits from outside of the country are inspected to ensure they are bacteria-free trees. Under the management program, the production of Xac (X. axonopodis pv. citri)-free nursery trees for exclusion of canker from orchard is also mandatory. Because the bacteria can be introduced from countries with endemic canker or canker outbreaks, strict restrictions on citrus importation are implemented in citrus-growing countries. Citrus trees will only be grown on canker-free fields at least one year after effective eradication. Planting sites are also chosen to minimize favorable environmental conditions for the spread of X. axonopodis. For example, areas with strong wind are avoided to decrease the dispersal of bacterial inoculum to the susceptible citrus trees.[8][4]
Once citrus canker is introduced into a field, removal of the infected trees is enacted to halt further spread of the bacteria. For instance, in Florida between 2000 and 2006, all citrus trees within 1,900 feet (580 m) of infected trees were required to be eradicated.[14] In the process, the infected trees are uprooted and burned, as the bacteria can survive on the lesions of woody branches for years. In urban areas, the trees are cut down and chipped, then disposed of in landfills.[8][4]
X. axonopodis pv. citri can be transmitted by mechanical means such as humans and machinery. As a sanitation measure, the workers in citrus orchards are required to do thorough decontamination of personnel and equipment to prevent the spread of bacteria from the infected areas. Aerosol inoculum is able to cause infection in wetted foliage in the zone of bacterial dispersal. Vehicles can also become contaminated by contacting the wet foliage. Contaminated equipment and machines can be disinfected by spraying bactericide.[8][4]
Citrus canker is thought to have originated in the area of Southeast Asia-India. It is now also present in Japan, South and Central Africa, the Middle East, Bangladesh, the Pacific Islands, some countries in South America, and Florida. Some areas of the world have eradicated citrus canker and others have ongoing eradication programs, but the disease remains endemic in most areas where it has appeared. Because of its rapid spread, high potential for damage, and impact on export sales and domestic trade, citrus canker is a significant threat to all citrus-growing regions.
The citrus industry is the largest fresh-fruit exporting industry in Australia. Australia has had three outbreaks of citrus canker, all of which have been successfully eradicated. The disease was found twice during the 1900s in the Northern Territory and was eradicated each time. In 2004, an unexplained outbreak occurred in central Queensland. The state and federal governments ordered all commercial groves, all noncommercial citrus trees, and all native lime trees (C. glauca) in the vicinity of Emerald to be destroyed rather than trying to isolate infected trees. Eradication was successful, with permission to replant being granted to farmers by the biosecurity unit of the Queensland Department of Primary Industries in early 2009.
Citrus canker was detected again in April 2018 and confirmed in May 2018 in the Northern Territory and Western Australia.[15]
Citrus is an important domestic and export crop for Brazil. Citrus agriculture is the second-most important agricultural activity in the state of São Paulo, the largest sweet orange production area in the world.[16] Over 100,000 groves are in São Paulo, and the area planted with citrus is increasing. Of the estimated 2 million trees, greater than 80% are a single variety of orange, and the remainder is made up of tangerine and lemon trees. Because of the uniformity in citrus variety, the state has been adversely affected by canker, causing crop and monetary losses. In Brazil, rather than destroying entire groves to eradicate the disease, contaminated trees and trees within a 30-m radius are destroyed; by 1998, over half a million trees had been destroyed.
Citrus canker was first found in the United States in 1910 not far from the Georgia – Florida border. Subsequently, canker was discovered in 1912 in Dade County, more than 400 mi (600 km) away. Beyond Florida, the disease was discovered in the Gulf states and reached as far north as South Carolina. It took more than 20 years to eradicate that outbreak of citrus canker, from 1913 through 1931, $2.5 million in state and private funds were spent to control it—a sum equivalent to $28 million in 2000 dollars.[17] In 26 counties, some 257,745 grove trees and 3,093,110 nursery trees were destroyed by burning. Citrus canker was detected again on the Gulf Coast of Florida in 1986 and declared eradicated in 1994.
The most recent outbreak of citrus canker was discovered in Miami-Dade County, Florida, on September 28, 1995, by Louis Willio Francillon, a Florida Department of Agriculture agronomist. Despite eradication attempts, by late 2005, the disease had been detected in many places distant from the original discovery, for example, in Orange Park, 315 miles (500 km) away. In January 2000, the Florida Department of Agriculture adopted a policy of removing all infected trees and all citrus trees within a 1900-ft radius of an infected tree in both residential areas and commercial groves. Previous to this eradication policy, the department eradicated all citrus trees within 125 ft of an infected one. The program ended in January 2006 following a statement from the USDA that eradication was not feasible.[14][18]
Citrus canker is a disease affecting Citrus species caused by the bacterium Xanthomonas (X.axonopodis; X. campestris). Infection causes lesions on the leaves, stems, and fruit of citrus trees, including lime, oranges, and grapefruit. While not harmful to humans, canker significantly affects the vitality of citrus trees, causing leaves and fruit to drop prematurely; a fruit infected with canker is safe to eat, but too unsightly to be sold. Citrus canker is mainly a leaf-spotting and rind-blemishing disease, but when conditions are highly favorable, it can cause defoliation, shoot dieback, and fruit drop.
The disease, which is believed to have originated in Southeast Asia, is extremely persistent when it becomes established in an area. Citrus groves have been destroyed in attempts to eradicate the disease.
Countries like Brazil and the United States also suffer from canker outbreaks.
Le chancre bactérien des agrumes, ou chancre asiatique des agrumes, ou chancre citrique, est une maladie bactérienne d'origine asiatique, causée par Xanthomonas axonopodis pv. citri. Cette maladie affecte principalement les plantes de la famille des Rutaceae et notamment les agrumes (plantes du genre Citrus sp.).
L'infection cause des lésions sur les feuilles, les tiges et les fruits des arbres du genre Citrus tels que limettier, oranger et pamplemoussier. Bien qu'il ne soit pas dangereux pour l'homme, le chancre affecte de manière significative la vitalité des arbres, ce qui provoque la chute prématurée des feuilles et des fruits ; les fruits infectés par le chancre sont comestibles mais peu esthétiques donc non commercialisables.
La maladie, qui aurait pris naissance en Asie du Sud-Est, est extrêmement persistante quand elle est établie dans une zone. Des vergers d'agrumes ont été entièrement détruits pour tenter d'éradiquer la maladie en Australie, au Brésil et aux États-Unis où des foyers sont actuellement actifs.
Les agrumes infectés par le chancre bactérien présentent des pathologies caractéristiques : leurs feuilles sont écorchées. Les fruits et les feuilles présentent des tâches noires et jaunâtres et des pustules. L'infection peut aussi causer la déchirure du fruit, dégradant l'intérieur. Les plus anciennes lésions peuvent avoir un aspect liégeux. La bactérie caractéristique de cette maladie se propage dans les lésions des feuilles, des tiges et des fruits. De plus, lorsqu'il y a des mineuses, des larves, sur la surface des feuilles ou des fruits, la propagation de l'infection est facilitée. En effet, ces mineuses font des lésions dans les organes, ce qui facilite la pénétration de la bactérie[1]. La période de latence entre l'infection et l'apparition des symptômes est de 5 à 6 jours à 30 °C et de plus d'un mois à 15 °C.
L'agent pathogène, Xanthomonas axonopodis pv. citri est une bactérie Gram-négative, en forme de bâtonnet, à un seul flagelle polaire. La plage de température optimale pour la croissance est comprise entre 28 et 30 °C, avec un maximum de 39 °C. Cultivées en laboratoire sur un milieu de culture, les colonies produisent un pigment, la xanthomonadine, qui leur donne une couleur jaune[2].
Selon CABI[3] :
Pour s'établir, la maladie nécessite un climat chaud et humide. En effet, les températures doivent être comprises entre 14 et 38 °C (avec une température optimale de 28 °C), et la présence d'eau libre est indispensable à l'infection. Les attaques du chancre sont donc plus importantes dans les régions offrant un climat tropical.
La propagation des bactéries est favorisée par les climats chauds, la pluie et le vent. En effet, ces facteurs peuvent altérer les plantes en créant des déchirures ou des lésions. De plus, la bactérie se développe plus facilement dans les plaies en présence d'humidité. Le vent permet la propagation des bactéries d'une plante à une autre par voie aérienne, généralisant la maladie.
Le facteur humain n'est pas négligeable dans la propagation, car l'homme transporte le matériel végétal infecté d'un pays à un autre[4].
La maladie du chancre des agrumes est probablement originaire de l'Asie du Sud-Est et de l'Inde. Elle s'est répandue dans de nombreuses région agrumicoles du monde au climat chaud et humide. Son aire de répartition comprend désormais également l'Australie, le Japon, le Moyen-Orient, l'Afrique, la Papouasie-Nouvelle-Guinée et diverses régions d'Amérique et d'Océanie[5]. Concernant l'Australie, la maladie a été repérée en 2004 à Emerald et a été déclarée éradiquée début 2009 à la suite d'un programme strict de quarantaine[6].
Les plantes hôtes de l'agent pathogène appartiennent toutes à la famille des Rutaceae, à l'exception de Lansium domesticum (Meliaceae). Parmi les Rutaceae, les espèces infectées naturellement appartiennent aux genres Citrus, Poncirus, Fortunella, Severinia et Swinglea.
Le genre Citrus est de loin le plus important sur le plan économique. Parmi les Citrus, le pamplemoussier (Citrus paradisi), le limettier (Citrus aurantiifolia) ainsi que Poncirus trifoliata sont très sensibles ; le bigaradier (Citrus aurantium), le citronnier (Citrus limon) et l'oranger (Citrus sinensis) sont modérément sensibles ; le mandarinier (Citrus reticulata) est moyennement résistant[7].
Le chancre bactérien des agrumes, ou chancre asiatique des agrumes, ou chancre citrique, est une maladie bactérienne d'origine asiatique, causée par Xanthomonas axonopodis pv. citri. Cette maladie affecte principalement les plantes de la famille des Rutaceae et notamment les agrumes (plantes du genre Citrus sp.).
L'infection cause des lésions sur les feuilles, les tiges et les fruits des arbres du genre Citrus tels que limettier, oranger et pamplemoussier. Bien qu'il ne soit pas dangereux pour l'homme, le chancre affecte de manière significative la vitalité des arbres, ce qui provoque la chute prématurée des feuilles et des fruits ; les fruits infectés par le chancre sont comestibles mais peu esthétiques donc non commercialisables.
La maladie, qui aurait pris naissance en Asie du Sud-Est, est extrêmement persistante quand elle est établie dans une zone. Des vergers d'agrumes ont été entièrement détruits pour tenter d'éradiquer la maladie en Australie, au Brésil et aux États-Unis où des foyers sont actuellement actifs.
O cancro cítrico é uma doença de plantas cítricas que ataca os tecidos vegetais, causada por bactérias do gênero Xanthomonas.
De um modo geral, ocorre de forma severa em regiões onde o clima no verão é quente e úmido, manifestando-se em folhas, frutos e ramos através da formação de lesões necróticas. Os sintomas geralmente são muito característicos, mas podem variar de acordo com o órgão afetado e idade de quando infectado pela bactéria. Desde a sua introdução no Brasil em 1957[carece de fontes?], diversas medidas de controle vêm sendo empregadas no intuito de reduzir os prejuízos causados por esta doença. Exclusão e erradicação têm sido as medidas básicas adotadas para prevenir e controlar o cancro cítrico em vários países, principalmente no Brasil e nos Estados Unidos, os dois maiores produtores mundiais de citros. No Brasil essa doença está restrita aos Estados de São Paulo, Paraná, Santa Catarina, Rio Grande do Sul e Mato Grosso do Sul. Porém, já foi relatada no Nordeste, no Rio Grande do Norte.
