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Citrus Canker In-Depth
I. Economic Hosts
II. Pathogens
III. Disease
IV. Symptoms and Signs
V. Disease
VI. Identification
VII. Pathogenicity
VIII. Storage of Organism
IX. Reported Host Range
X. Geographical Range and Spread:
The disease occurs world-wide, primarily in regions where rainfall increases and
temperatures rise simultaneously. X. citri may be found in southern Asia, Japan,
the Middle East, Africa, South America and now North America. X. citri was present
in North America in the early 1900s and again in 1986. It was subsequently
eradicated in both cases. It was discovered again in 1995 and is now present
in both the Miami and Tampa Bay areas of Florida in the U.S., (to find current
maps of the quarantine zones in Florida,
click here) and attempts are being made
to eradicate it again. X. campestris pv. aurantifolii has been reported only in
S. America.
Because of increasing levels of international travel and in spite
of rigorous quarantine measures, the disease is likely to be reintroduced into
citrus growing areas repeatedly because of people inadvertantly bringing infected
citrus fruits and seedlings into the U.S.. Whether or not it will spread
naturally after introduction depends largely on climate in the area of
introduction. In the U.S., Florida is by far the state most at risk from citrus
canker. This is because of Florida's high humidity throughout the year, combined
with our seasonal hurricanes and frequent thunderstorms accompanied by high wind
gusts.
Citrus canker is spread locally primarily by wind-driven rain, overhead irrigation
and contaminated equipment. (As noted above, citrus canker is spread over long
distances primarily by movement of infected plant materials). Seedborne
dissemination has not been reported. X. citri is the only plant pathogen to have
been successfully eradicated (in Mozambique, South Africa, New Zealand, Australia
and twice in the U.S.A.), which reflects its dependence on optimum ecological
conditions for spread and poor survivability apart from its host. The extent of
natural spread depends entirely on local environmental conditions, including
presence or absence of the Asian citrus leaf miner (Phyllocnistis citrella
Stainton).
Eradication was successful in Florida in the mid 1910s (at a cost of
$6.5M) and the late 1980s (at a cost of ca. $70M). Much success was achieved
by implementing a policy of destroying infected trees and pruning all green wood
on trees within 50 feet of the infected trees. The policy in Florida was changed
in 1986 to cut and remove all "exposed" citrus trees within 125 feet of infected
trees. This policy failed only in certain grove situations where infestations
were heavy and equipment such as mowers appeared to have spread the infestation
mechanically. ("Exposed" is defined as citrus plants which are physically
located within a prescribed distance from an infected tree, and reflects a very
high probability that the trees will become diseased.)
The Asian citrus leaf
miner changed the eradication calculus by greatly increasing the availability
of the canker bacteria for spread by rain splash from the leaf surface. The
Asian leaf miner first appeared in Florida in 1993 and rapidly spread throughout
citrus growing regions of the State. The 125 ft. exposure zone was quickly
discovered to be inadequate to control the disease in Miami; a 1996 tornado
appears to have spread the disease 6-7 miles [23]. This indicates that all
the citrus in Miami may have been exposed and may ultimately have to be
destroyed. However, since 95% of actual disease spread in Miami seems to
occur within 1900 feet of infection, the new exposure zone is currently set
at 1900 feet.
Research on citrus canker disease in the U.S. is limited to areas of the country
which are far removed from citrus growing regions, and to highly specialized plant
quarantine and containment facilities such as the University of Florida's
ICBR Plant Containment Facility,
which provide a high level of physical containment of the disease.
XI. Suggested Taxonomic Keys
XII. References
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Copyright © October 2001 Integrated Plant Genetics, Inc. -- All Rights Reserved
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