Citrus canker is a disease affecting
Citrus species caused by the
Xanthomonas axonopodis. 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.
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.
Brazil and the
United States are currently suffering from canker
4 Disease cycle
5 Favorable environmental condition
7 Distribution and economic impact
7.3 United States
8 See also
10 External links
Xanthomonas campestris pv. citri
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:
The Asiatic type of canker (canker A), X. axonopodis pv. citri, caused
by a group of strains originally found in Asia, is the most widespread
and severe form of the disease.
Cancrosis B, caused by a group of X. axonopodis 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. axonopodis pv.
aurantifolii, only infects key lime and bitter orange.
A* strains, discovered in Oman, Saudi Arabia, Iran, and India, only
infect key lime.
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.
Citrus canker lesions on fruit
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 type III
secretion system. The effector interacts with host machinery to induce
transcription for genes that regulate plant hormones such as
gibberellin and auxin.
Xanthomonas axonopodis pv. citri overseason in infected area which is
canker lesion on leaf or stem. The bacteria ooze out of the lesions
when there is free moisture. During the rainy weather, wind-blown rain
carries the inoculum to the new susceptible hosts. The bacteria infect
new plants through stomata and wounds. The wound can be caused by
pruning or hedging that could cut open mesophyll tissues for direct
infection. The rain can also cause water congestion on leaf surface,
form column of water through stomata and promote infection through the
natural opening. The infection can form on fruit, foliage and young
stem. The varied size of lesions on citrus fruit is because of the
multiple cycle of infections.
Favorable environmental condition
Wind-driven rain plays 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 high mean temperature such as Florida.
Often, cankers emerge briskly during fall, slowly during winter and
most rapidly in mid to late spring.
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 caused by
bacteria. Her research published in the 1915 Journal of Agricultural
Research played a major part in saving citrus crops in multiple
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.
Citrus x paradisi),
Key lime (C. aurantiifolia), Pointed
leaf hystrix (C. hystrix), lemon (C. limon)
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)
Citron (C. medica), Mandarins (C. reticulata)
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
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 the regions where citrus canker
outbreak occurs, Integrated Pest Management(IPM) is utilized. The
significant features of this management program is the transposition
of susceptible citrus plants to field resistant citrus cultivars.
Apart from using the 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 which
are exclusion, eradication and sanitation.
In exclusion method, citrus trees or fruits from outside of the
country are inspected to ensure they are bacterial-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 the countries
with canker issue, strict restrictions on the citrus importation are
implemented in citrus-growing countries.
Citrus trees will only be
grown on canker-free fields for at least one year after effective
eradication. The planting sites are also chosen to minimize the
favorable environmental condition for the introduction of X.
axonopodis. For example, areas with strong wind are avoided to evade
the dispersal of bacterial inoculum to the susceptible citrus
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, all citrus trees within 579 m of infected trees must be
eradicated.In the process, the infected trees are uprooted and burned.
In urban areas, the trees are cut down and chipped, then disposed in
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. The contaminated equipments and machines
can be disinfected by spraying bactericidal compound.
Distribution and economic impact
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
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 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
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. In 26 counties, some
257,745 grove trees and 3,093,110 nursery trees were destroyed by
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 Sept. 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.
Bacterial blight of cassava
Clara H. Hasse
^ a b c d e Gottwald, T.R, Graham, J.H. and Schubert, J.S. (2002).
Citrus canker: The pathogen and its impact. Online. Plant Health
^ Pereira, Andre; Carazzolle, Marcelo; Abe, Valeria; Oliveira, Maria;
Domingues, Mariane; Silva, Jacqueline; Benedetti, Celso (2014).
"Identification of putative TAL effector targets of the citrus canker
pathogens shows functional convergence underlying disease development
and defense response". BMC Genomics (15).
^ Rigano, Luciano; Siciliano, Florencia; Enrique, Ramón; Sendín,
Lorena; Filippone, Paula; Torres, Pablo; Qüesta, Julia; Marano, Maria
Rosa (2007). "
Biofilm formation, epiphytic fitness, and canker
Xanthomonas axonopodis pv. citri". The American
Phytopathological Society. 10 (20). doi:10.1094/MPMI-20-10-1222.
^ a b c d e Gottwald, Tim R; Graham, James H (2000). "
The Plant Health Instructor. doi:10.1094/PHI-I-2000-1002-01.
^ Bock, Clive H; Parker, P.E; Gottwald, Tim R (2005). "Effect of
simulated wind-driven rain on duration and distance of dispersal of
Xanthomonas axonopodis pv. citri from canker-infected citrus tree".
Plant Disease (89): 71–80. doi:10.1094/PD-89-0071.
^ PSEUDOMONAS CITRI, THE CAUSE OF CITRUS CANKER (archive.org book
reader)PSEUDOMONAS CITRI, THE CAUSE OF CITRUS CANKER (archive.org text
version), Clara Hasse, Journal of Agricultural Research, 2015-10,
Volume 4, p. 97.
^ CITRUS CANKER, Frederick Wolf, Journal of Agricultural Research,
2016-10, Volume 6, p. 68.
Xanthomonas axonopodis pv citri GENOME PROJECT Archived 2005-08-30
at the Wayback Machine.
Citrus Canker Animal and Plant Health Inspection Service
United States Department of Agriculture USDA Secretary Deputy Chuck
Conner's Memo to Commissioner Bronson Archived 2006-09-25 at the
Florida Department of Agriculture and Consumer Services Press
Release 2006 - USDA Determines
Citrus Canker Eradication Not Feasible
Citrus Canker (
Xanthomonas axonopodis), National
Invasive Species Information Center,
United States National
Agricultural Library. Lists general information and resources for
Florida Learn to Live with Canker - Redorbit
Vogel, Mike (2001-02-01). "Grapefruits of Wrath.(management of citrus
canker in Florida's citrus fruit industry)(Statistical Data
Florida Trend, HighBeam Research. Retrieved
Type strain of
Citrus canker at
BacDive - the Bacterial Diversity