Regulatory control

Citrus canker

Citrus canker lesions on fruit.

Citrus canker is caused by a bacterium, Xanthomonas citri (synonyms: Xanthomonas axonopodis pv. citri, Xanthomonas campestris pv. citri). It probably originated in southern Asia and now occurs in Japan, the Middle East, central and southern Africa, and Central and South America.

The bacterium is highly contagious, especially in environments where high rainfall and high temperatures coincide. Short-distance spread of the bacteria (from plant to plant) is primarily by splashing water, either from rain or overhead irrigation. Intermediate distance spread (thousands of meters) is by wind-driven rain, by birds and animals, or by people carrying the bacterium on their hands, clothing or equipment. Long-distance spread (continent to continent) is by movement of infected plant material. The Asian leaf miner greatly enhances the infection, not as a vector, but by producing wounds in the leaves that facilitates the entry of the bacteria.

History

Citrus canker was introduced into Texas in 1910 and into Florida in 1914 on seedlings from Japan. The only known way to control the disease was to burn the infected trees, an effort initiated by the growers themselves and later taken up by the state departments of agriculture and the USDA. By destroying infected trees and removing all green wood on the trees within a 50-foot radius, citrus canker was finally declared eradicated in 1933, at the cost in Florida alone of $6.5 million (in 1933 dollars).

In 1986 citrus canker was again found in Florida, on one dooryard tree and in three commercial groves. Again eradication efforts were initiated, but this time the policy was to destroy all citrus within 125 feet of an infected tree. In 1994, after an expenditure of about $70 million, citrus canker was again declared eradicated.

In 1995 citrus canker was found in a residential area of Miami and in 1997 in the same area where the 1986 outbreak had occurred. (The latter proved to be a re-emergence of the 1986 outbreak that had not been completely eradicated.) In a collaborative effort between the Florida Department of Agriculture and Consumer Services, the Division of Plant Industry, and APHIS/PPQ, a strict quarantine/eradication program was established. The quarantine restrictions apply not only to commercial citrus producers but also to homeowners with dooryard citrus trees and anyone who might come in contact with citrus, such as lawncare personnel, utility workers, truck drivers, etc.

The current Florida quarantine

The following is a partial list of the quarantine restrictions:

• No citrus plants can be moved from quarantined areas
• No citrus plants can be planted in quarantined areas
• In quarantined areas fruit may be harvested only from trees previously approved by APHIS/FDACS inspectors
• All harvested fruit must be transported in enclosed vehicles
• Fruit harvested from a quarantined area must be treated with a hypochlorite solution or sodium ortho-phenyl phenol
• Fruit from a quarantined area can be shipped interstate only to states that do not produce commercial citrus
• The culls and debris from the packing station must be disposed of in a manner approved by APHIS/FDACS.
• All personnel and equipment having come in contact with citrus, whether inside or outside a quarantine area, must be decontaminated by an approved method before leaving the property

Destroying infected trees.

Citrus canker eradication

In the eradication effort, it was apparent that removing symptomless trees in a 125-foot radius around an infected tree was inadequate to stop the advance of the canker, much less eradicate it. An epidemiological study showed that 95% of the new infections occurred within 1900 feet (585 meters) of a known infected tree, and so the policy was changed to remove all citrus within a 1900-foot radius within 30-60 days after finding an infection. By 2002, over 1.6 million commercial citrus trees and 600,000 dooryard trees had been destroyed, and still the infected area had increased to over 650 square miles (1700 square kilometers). Nevertheless, there have been some limited successes, and in some areas the quarantine has been lifted after the required 2 years without any new canker detection.

Special problems in the citrus canker case

Citrus canker poses some difficult quarantine/eradication problems. Various citrus species are popular dooryard plants, both as ornamentals and as fruit trees, and there are large numbers of individuals who value their personal enjoyment of citrus far more than they value the health of the commercial citrus industry. These are the people who are most likely to smuggle a fruit or a seedling into the country, not knowing (or not caring) whether it is infected. The current citrus canker outbreak, according to DNA analysis, is the result of at least 3 independent introductions of the bacterium. Despite the quarantine restrictions, such repeated introductions are likely to continue to occur.

Early detection of low levels of disease is difficult. Once infection occurs, it can take more than 60 days for the symptoms to appear, by which time huge amounts of inoculum are being produced to initiate new infections. The bacteria are readily disseminated by windblown rain, and Florida not only periodically experiences hurricanes but also has a high frequency of severe thunderstorms accompanied by high winds and even the occasional tornado.

Citizen protesting removal of dooryard citrus trees

Since eradication requires destruction of symptomless trees, the eradication program has aroused the ire of countless property owners with dooryard citrus trees and even that of a few commercial citrus producers. They are unwilling to accept personal losses for the good of the citrus industry and the economy of the whole state. In Florida the political wrangling over the citrus canker eradication program and the endless rounds of litigation have weakened the eradication efforts, causing delays that have given the disease a head start.

A number of people, including homeowners, a few scientists, and even some citrus producers, have questioned the feasibility of eradication and whether the long-term outcome is worth the short-term costs. But for the moment, the regulatory agencies and the majority of the citrus producers are not yet willing to give in and try to live with citrus canker.


References

APHIS/PPQ. Citrus Canker Overview.
http://www.aphis.usda.gov/ppq/ep/citruscanker/index.html

Cirus Canker Eradication Jump Point. Stop Before You Chop.
http://fornits.com/canker/

Florida Department of Agriculture and Consumer Services. Citrus canker: The threat to Florida agriculture
http://doacs.state.fl.us/canker/menu1.htm

Gottwald, T.R., G. Hughes, J.H. Graham, X. Sun, and T. Riley. 2001. The Citrus Canker Epidemic in Florida: The Scientific Basis of Regulatory Eradication Policy for an Invasive Species. Phytopathology 91(1):30-34.
http://www.citruscanker.com/gottwald_article.pdf

Gottwald, T. R., Graham, J. H., and Schubert, T. S. 2002. Citrus canker: The pathogen and its impact. Plant Health Progress. http://www.apsnet.org/online/feature/citruscanker/

Historical Museum of Southern Florida. The Citrus Canker.
http://www.historical-museum.org/history/update/u051a.htm