Potentially Misidentified Species:
There are multiple strains or pathotypes of the Xanthomonas spp.
citrus canker bacterium. The most devastating of these is X. axonopodis
pv. citri pathotype A, also known as Asiatic citrus canker. Also
present in Florida are pathotypes B and C of X. aurantifolia pv.
aurantifolia and at least two pathotypes of the congeneric bacterium X.
campestris, responsible for bacterial leaf spot disease in citrus
(Integrated Plant Genetics Inc. undated, ISSG). An outbreak of bacterial leaf
spot disease in Florida in 1984 was previously misidentified as a citrus canker
Recent molecular studies have suggested that X. aurantifolii pv.
citri should be reinstated to species status as X. citri.
Florida citrus trees are now additionally under threat from citrus greening disease (also called huanglongbing, HLB, or
yellow dragon disease), also a bacterial disease, that is believed to be
capable of worse damage than canker. The responsible pathogen (unnamed as of
2007) is also not native to the U.S., and is considered by some to be the most
serious worldwide citrus disease. Citrus greening disease was first reported
from Florida in 2005. Disease symptoms are dissimilar to those of citrus
canker, and include yellowing of shoots, leaf mottling or discoloration, and
the production of lopsided poor-quality fruit.
II. HABITAT AND DISTRIBUTION
Citrus canker is likely native to southeast Asia and India, but has now spread
worldwide into to warm, moist, citrus-growing coastal regions. The pathogen is
now also established in Australia, Japan, the Middle East, Africa, Papua New
Guinea and elsewhere in the Pacific, and the Americas (Gotwald et al. 2002,
Kumar et al. 2004).
In Florida, citrus canker had been reported from 24 counties as of January
2006. Recent northward spread of citrus canker into central Florida is
believed to be a consequence of the hurricanes of 2004 and 2005 (Gaskalla
As of January 2006, citrus canker had been reported in the IRL region from
Brevard, Indian River, St. Lucie, Martin, and Palm Beach counties (Gaskalla 2006). Most
reports of citrus canker in the IRL region north of Martin County did not occur
until after the 2004 hurricanes.
III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
X. axonopodis pv. citri is a microscopic bacterium.
Citrus canker was known to occur in 24 Florida counties as of January 2006.
Reproduction in genus Xanthomonas is similar to that of other bacteria,
occurring through asexual binary fission in which each dividing clonal daughter
cell receives an identical copy of the parental genome (Campbell et al. 1999).
IV. PHYSICAL TOLERANCES
Xanthomonas axonopodis pv.citri thrives in subtropical regions of the
world where citrus grows. It's invasive range , like the range of it's
various host species, is temperature limited, although the first reported
incidence of the pathogen in Florida in 1910 was reported in the northern part
of the state from near the Florida-Georgia border.
Citrus canker thrives in warm regions experiencing high humidity and heavy
rainfall. Gottwald et al. (2002) note the exposed bacterium on contaminated
surfaces perishes when the surfaces dry out, suggesting a humid environment is
important for the organism.
V. COMMUNITY ECOLOGY
Xanthomonas axonopodis pv.citri is a heterotrophic bacterium that derives nutrition from the tissues of the plants it infects.
The larval form of a moth known as the Asian citrus leaf
miner (Phyllocnistis citrella), non-native in Florida, facilitates
spread of the citrus canker bacterium through its leaf-burrowing activities
which expose disease-causing X. axonopodis pv.citri to transport
through wind and rain.
VI. INVASION INFORMATION
Citrus canker was first identified in Florida approximately a century ago in
1910. Aggressive measures were taken to combat the disease and it was declared
eradicated from the state in 1933. After an apparent absence of more than 50
years, citrus canker was again detected in Florida, in Manatee County in 1986.
Once again, steps were taken to contain the disease and it was in 1994 again
declared eradicated (Gaskalla 2006).
Canker-infected plants were in 1995 again reported from south Florida, in the
vicinity of Miami International Airport. An aggressive USDA-administered
eradication program was established but proved unsuccessful. The Florida 2004
and 2005 hurricanes facilitated the spread of citrus canker in the state to the
point that eradication was no longer deemed possible. Federal funds for
eradication were withdrawn in 2006 and the response strategy shifted to a
management and containment plan (Gaskalla 2006).
Although individual plant quarantines have been eliminated, a USDA statewide
quarantine remains in effect which places restrictions on the transport of
harvested citrus or plant stocks out of the state. Transport of material to
other citrus-producing states is prohibited. Prior to the 2006 lifting of
individual quarantines, more than 600,000 ha of Florida land had been placed
under enforced quarantine (FDOACS
undated, Gaskalla 2006).
The citrus canker pathogen was first reported in Palm Beach County in 1999 and in Martin County from a commercial citrus
grove in 2001. Citrus canker in the IRL region north of Martin County largely
did not occur until after the 2004 hurricanes. The post-hurricane appearance of canker was first reported from
St. Lucie County in December 2004, from a sentinel survey site in
Port St. Lucie. The first reports from Indian River County were
post-hurricanes, December 2004, in a grove near the Indian River/St. Lucie
county line. Confirmed reports of the disease in Brevard County as of 2005
were confined to a handful of residential sites.
The citrus canker pathogen has been spread through the transport of fruit,
plants, and equipment, and dispersal appears to be greatly facilitated by wind
and rain. Overhead irrigation systems may also facilitate movement of citrus
canker as does the leaf-burrowing activity of larval P. citrella.
Potential to Compete With Natives:
The biological impact of X. aurantifolii pv. citri is primarily
as a plant pathogen; competition with native microbial species remains
Possible Economic Consequences of Invasion:
X. aurantifolii pv. citri is a serious disease agent of citrus
crops in Florida and elsewhere. Infected trees exhibit a gradual decline in
health and fruit production until they eventually succumb to the disease. As
of January 2006, more than 15 million commercial trees in groves and nurseries
had been destroyed in Florida as a result of efforts to eradicate citrus
canker. Nearly 1 million residential trees were destroyed as well (Gaskalla
Transport of live citrus plants in and out of Florida is currently prohibited.
Homeowners who wish to plant citrus trees must purchase certified symptom-free
citrus plants through nurseries registered with the state (FDOACS 2006).
Campbell N.A., Reece J.B. and L.G. Mitchell. 1999. Biology. Fifth Edition.
Addison Wesley Longman, Inc., CA. 1,175 p.
Florida Department of Agriculture and Consumer Services (FDOACS), Division of
Plant Industry. 2006. Citrus Canker Quarantine Information: Schedule 20.
Gaskalla R. 2006. Comprehensive Report on Citrus Canker Eradication Program in
Florida Through 14 January 2006 Revised. Florida Department of Agriculture and
Consumer Services, Division of Plant Industry. 25 p.
Gottwald T.R., Graham J.H., and T.S. Shubert. 2002. Citrus canker: The pathogen
and its impact. Plant Health Progress doi:10.1094/PHP-2002-0812-01-RV.
Integrated Plant Genetics Inc. Undated. Citrus Canker In-Depth. Available
Kumar S., Mackie A., and N. Burges. 2004. Citrus canker: Exotic threat to
Wewstern Australia. State of Western Australia Department of Agriculture
Factsheet No. 13/2004.
Starr M.P, and W.L. Stephens. 1964. Pigmentation and taxonomy of the genus
Xanthomonas. Journal of bacteriology 87:293-302.
J. Masterson, Smithsonian Marine Station
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Page last updated: December 1, 2007