Citrus canker is a disease affecting Citrus species caused by the bacterium Xanthomonas citri. 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.
Biology
Xanthomonas citri 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:
- The Asiatic type of canker (canker A), X. citri pv. citri, caused by a group of strains originally found in Asia, is the most widespread and severe form of the disease.
- A* strains, discovered in Oman, Saudi Arabia, Iran, and India, only infect key lime.
- Cancrosis B, caused by a group of X. citri pv. aurantifolii strains originally found in South America is a disease of lemons, key lime, bitter orange, and pomelo.
- Cancrosis C, also caused by strains within X. citri pv. aurantifolii, only infects key lime and bitter orange.
Taxonomy
The pathovars citri and aurantfolii were originally classified in X. campestris (1960–1992), then in X. axonopodis (1995), then finally as its own species X. citri since 2016. As a result, sources may use any of the four species names. The classification as its own distinct species X. citri is supported by whole-genomic data.
Pathology
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.
right|thumb|160px|Citrus canker lesions on fruit
Pathogenicity
Xanthomonas citri 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. citri 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.
Disease cycle
Xanthomonas citri 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. 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.
Favorable environmental conditions
Wind-driven rain plays a major role in the dispersal of X. citri. The bacteria are said to be readily dispersed by splashed rain and wind and the quantity of X. citri 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.
Detection
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. Her research published in the 1915 Journal of Agricultural Research played a major part in saving citrus crops in multiple states.
Susceptibility
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.
{|border="2" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; border: 1px #aaa solid; border-collapse: collapse;"
|- bgcolor="#ffebcd"
!width="150"|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.)
|-
|colspan="2"| <small>Modified from: Gottwald, T.R. et al. (2002). Citrus canker: The pathogen and its impact. Online. Plant Health Progress</small>
|}
Management
Quarantine measures are implemented in areas where citrus canker is not endemic or has been obliterated to prevent the introduction of X. citri. 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.
Brazil
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. 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.
United States
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 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. 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.
See also
- Bacterial blight of cassava
- Clara H. Hasse
References
External links
- Species Profile- Citrus Canker (Xanthomonas axonopodis), National Invasive Species Information Center, United States National Agricultural Library. Lists general information and resources for Citrus Canker.
- Electronic Data Information Source - Citrus Canker, University of Florida IFAS Extension
- Type strain of Xanthomonas axonopodis at BacDive - the Bacterial Diversity Metadatabase