Integrated Plant Genetics Inc.
6911 NW 22nd St., Ste C
Gainesville, FL 32653

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Integrated Plant Genetics

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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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