Dooryard Citrus Production: Asiatic Citrus Canker Disease1
Asiatic citrus canker is a bacterial disease of citrus caused by the pathogen Xanthomonas citri subsp. citri. The bacterium causes necrotic lesions on leaves, stems, and fruit of infected trees. Severe cases can cause defoliation, premature fruit drop, twig dieback, and general tree decline. Considerable efforts have been made to prevent its introduction or limit its spread in citrus-growing areas throughout the world.
History of Citrus Canker in Florida
Not surprisingly, citrus canker is believed to have originated in the native home of citrus, Southeast Asia and India. From there, the disease has spread to most of the subtropical citrus-producing areas of the world, including Japan, parts of Africa, the Middle East, Australia, South America, and Florida. Eradication efforts were successful in South Africa, Australia, the Fiji Islands, and New Zealand.
The first known outbreak of citrus canker in Florida occurred around 1910. It is believed that the disease was brought into the United States on infected rootstock material from Japan. The causal organism was unknown at the time, and it was not until 1915 that the pathogen was determined to be a bacterium. After the failure of all control measures, an eradication effort was begun in 1913, by which time the disease had spread throughout the Gulf States. In 1915, a quarantine banned the import of all citrus plant material. The last known infected tree was removed from Florida in 1933, and the disease was declared eradicated from the United States in 1947. Since that time, citrus canker has been the focus of regulatory rules to prevent its re-introduction into the United States.
Despite regulatory efforts, citrus canker was found on residential trees in Hillsborough, Pinellas, Sarasota, and Manatee Counties in 1986. Shortly after these detections, the disease was found in nearby commercial groves. Infected trees were removed immediately. The last tree with citrus canker from this outbreak was detected in 1992. Citrus canker was once again declared eradicated in 1994.
Citrus canker was detected for the third time in a residential area near the Miami International Airport in September 1995. An initial survey found thousands of infected trees within a 14-square mile area. Eradication efforts began immediately. New molecular genetic tools enabled researchers to determine that the specific strain of X. citri subsp. citri responsible for this new outbreak was different from the 1986–1992 outbreak. State and federal eradication efforts proved inadequate and by 2001, the disease had moved northward from Dade County and was detected in Broward and Palm Beach Counties.
In 1997, a separate outbreak of citrus canker occurred in Manatee County in commercial and residential citrus. Again, molecular genetic tools were used to determine the strain of X. citri subsp. citri responsible for this outbreak. It was the same stain as the 1986–1992 outbreak in the same area. These data suggested some infections had gone undetected during the earlier eradication efforts.
Another simultaneous outbreak was detected in the Immokalee area of southwest Florida in 1998. As new outbreaks occurred, the bacterial isolates were genetically characterized by researchers. With the exception of the 1986 and 1997 Manatee County outbreaks, all of these new outbreaks were caused by the strain of the pathogen detected in 1995 near the Miami airport.
Although eradication efforts continued, legal battles over the forced removal of infected trees hampered the effort. The paths of hurricanes during the very active hurricane season of 2004 (Figure 1), followed by more hurricanes in 2005, spread the disease from the original affected areas in southeast Florida throughout most of the citrus-growing areas of the state. In January 2006, it was determined that eradication of citrus canker from Florida was no longer feasible, and eradication efforts ceased. Containment within Florida to minimize further spread became paramount.
As of January 11, 2008, Florida was placed under a statewide citrus quarantine by the United States Department of Agriculture (USDA) and no citrus (trees, fruit, rootstocks, leaves, potted citrus plants, etc.), whether commercial or dooryard, may leave the state unless the USDA has issued a limited permit. Quarantine regulations change frequently but can be accessed through the Citrus Health Response Program website (see Resources), including information on which packinghouses will grade homeowner fruit. These regulatory efforts are in place to limit the spread of citrus canker beyond those areas already infected and to control the severity of infections within affected areas. Much of Florida now has citrus canker, but the northern third of the peninsula and the eastern panhandle remain canker free for now.
Citrus Canker Symptoms
All above-ground tissues of citrus trees are susceptible to infection by citrus canker when young. Canker symptoms appear on fruit, leaves, and stems and vary in appearance depending on the age and severity of the infection (Figure 2). Severe infections can cause defoliation, stem dieback, fruit drop, general tree decline, and result in very unappealing fruit. Trees with persistent, severe infections can become weak and unproductive.
The bacteria enter plant tissues through natural openings (e.g. stomates—pores in leaf surfaces) and wounds. Early leaf symptoms appear as tiny, slightly raised, blister-like lesions (Figure 3). These lesions can develop water-soaked margins. The time from infection to symptom appearance will depend on environmental conditions. Under ideal conditions, that is, the presence of moisture and temperatures between 68–86°F (20–30°C), lesion development will occur 10–14 days after inoculation. Under very unfavorable conditions, symptoms may take as long as 60 days to appear. As young, blister-like lesions age, they change color, turning tan to brown, and the area immediately surrounding the lesion can have a water-soaked appearance with a yellow halo (Figure 4). The centers of the lesions become raised and corky (Figure 5) and lesions are visible on both sides of the leaf. The centers of large, old lesions may fall out, leaving holes in the leaf, a symptom known as shot-hole (Figure 6).
The disease severity of citrus canker can be greatly increased by the citrus leafminer (Phyllocnistis citrella), a tiny moth that provides entry points for the bacterium. Leaves damaged by citrus leafminer become more sensitive to canker infection because leafminer wounds allow easy penetration of the bacterium into the leaf tissue and because they heal more slowly than other types of wounds (Figure 7). Citrus canker lesions caused by the infection of leafminer feeding galleries are larger and more numerous than when infections occur through natural openings. These large lesions result in the production of very high inoculum levels for more infections as the bacteria reproduce. Additional information can be found in the EDIS publication on Citrus Leafminer available online at https://edis.ifas.ufl.edu/IN165.
Stem and Twig Symptoms
Because stems and twigs are less susceptible to infection than leaves, the presence of citrus canker lesions on stems and twigs generally indicates an infection has been present for some time. Stem lesions can serve as a reservoir of inoculum that can continue to re-infect new tissues as they are produced. Symptoms on stems and twigs are generally dark brown, raised, corky lesions which when young, may have an oily or water-soaked appearance around the edges (Figure 8, top). Stem lesions do not usually have the yellow halo typical of leaf lesions. As the lesions on stems age they begin to appear more dried out and take on a scabby appearance (Figure 8 bottom).
Lesions on fruit, similar to those on stems, tend to be dark brown and raised, and are frequently surrounded by a yellow halo (Figure 9). The appearance of fruit symptoms differs slightly depending on the citrus variety (Figure 10). Canker lesions on fruit may cause premature fruit drop, thus reducing fruit yield and giving the fruit an unappealing appearance. However, canker lesions on fruit are only "skin deep", and the fruit is still edible.
Other diseases of citrus, such as citrus scab, melanose, greasy spot, citrus black spot, and alternaria brown spot, have symptoms that may at first be mistaken for citrus canker (Figure 11). However, upon closer inspection these diseases can be distinguished quite easily from citrus canker.
Scab infections begin as small, pale pink, somewhat circular, elevated spots on leaves and fruit. As the infection develops, the spots become wart-like and, depending on variety, can become highly raised. The crests of the wart-like growths usually become covered with a corky pale tissue and can flatten as the fruit matures, especially on grapefruit. Scab is very rarely seen on sweet oranges and is most common on tangerine or mandarin varieties.
Melanose lesions are small (~1/32"; > 1 mm), raised, and irregularly shaped spots on the fruit or leaf surface, ranging from brick red to black, that feel like sandpaper when touched. They commonly will cover a quarter to one half of the surface of the fruit. Melanose occurs on any citrus variety that has dead wood in the canopy but is particularly common on grapefruit varieties.
Alternaria brown spot infections on fruit appear as small, slightly depressed black spots that can cause the young fruit to fall from the tree. Alternaria lesions may also be surrounded by a yellow halo, similar to canker; however, the depressed lesions distinguish it from canker. Alternaria brown spot is mainly found on tangerine varieties, especially Minneola (Honeybell), and much less frequently on grapefruit. It is never seen on sweet orange.
Citrus black spot has four symptom types, but the most common and most likely to be confused with citrus canker is the hard spot symptom. The hard spot lesions on fruit are sunken with pycnidia (small, pencil-sized dots) in the center of the lesions. Sometimes green halos surround the hard spot lesions. The hard spot lesions most commonly occur on mature fruit and are most common on sweet oranges and lemons but all varieties can get the disease.
Citrus Canker Infection and Spread
For X. citri subsp. citri to cause an infection, it must be able to enter plant tissues. For this reason, young leaves, stems, and fruit (Figure 12) are highly susceptible to natural infections because they have not yet fully developed their natural protective layer, called the cuticle. Once these tissues have matured and "hardened-off," infection is unlikely. However, wounding any of these tissues, young or old, can provide a point of entry for the bacterium and lead to an infection. Wounds can be caused by physical means, such as pruning, thorn punctures, or by plant pests, such as citrus leafminer.
The disease cycle of citrus canker, showing infection and spread, is illustrated in Figure 13. Wind-driven rain is the primary mechanism for movement over short to medium distances. A rain drop can carry millions of bacteria. These rain drops can then flow into leaf pores called stomates, introducing the bacteria to the internal leaf tissues where it causes infection.
Long-distance spread of citrus canker occurs primarily through human activity, via the movement of infected plant material, equipment, clothing, or even cell phones and cell phone cases. Other natural events, such as hurricanes, can transport bacteria long distances.
Citrus Canker Management
As of January 2006, citrus canker eradication efforts have been suspended. This means the removal of infected citrus trees and all other citrus trees within a 1,900 foot radius of infected trees is no longer required. However, under the USDA statewide quarantine, no citrus may leave the state of Florida without a USDA-issued limited permit. These restrictions apply to commercially grown as well as residential citrus, and apply to all plant material, not just fruit. These rules may change in the future and the Florida Department of Agriculture and Consumer Services, Division of Plant Industry (FDACS, DPI) Citrus Quarantine Information website (https://www.fdacs.gov/Agriculture-Industry/Pests-and-Diseases/Plant-Pests-and-Diseases/Citrus-Health-Response-Program/Citrus-Quarantine-and-Disease-Detection-Maps) should be consulted for the most current quarantine information.
When working in your yard, you should take precautions to avoid infecting your trees or spreading the disease from one tree to another. Your hands and gardening tools should be disinfested while working in your yard to help prevent the spread of the disease if one of your trees is infected. If you borrow gardening tools from a friend, disinfest prior to using them on your citrus trees and before you return them. It is best to practice personal and equipment decontamination between trees.
To remove the bacterium from the skin, wash hands and arms with soap and water, hot or cold, for a minimum of 20 seconds. The use of an alcohol-based hand-sanitizer after hand-washing will make personal decontamination more effective. Tools should be decontaminated with a solution of 1 ounce household bleach to 1 gallon of water. To reduce the risk of rusting, tools should be rinsed afterwards. The bleach solution will not be effective if used on dirty equipment and should be made fresh daily because it loses effectiveness quickly. Disease spread can also occur via clothing, so it is best to wash or change clothing after handling citrus trees and when switching between yards.
At this time, homeowners may replant citrus trees anywhere in Florida, regardless of past citrus canker quarantine areas. However, in an effort to control citrus canker and other diseases, the FDACS has implemented strict regulations for propagating and selling citrus trees in Florida. New citrus trees must be purchased from certified nurseries registered with the state. Every citrus tree sold in Florida must have a tag on it that identifies the nursery that produced the tree by name and state registration number. Do not buy a citrus tree without this certification tag.
Florida residents should be aware that UF/IFAS is not encouraging the planting of new residential citrus trees at this time because another, more serious disease, Huanglongbing (HLB; citrus greening) causes the young trees to rapidly decline. It is very difficult for a homeowner to protect their citrus trees from HLB and these trees, which will almost assuredly become infected, contribute to the further spread of HLB that is imperiling the Florida citrus industry.
Controlling Existing Infections
Once a canker lesion occurs, the lesion cannot be cured. However, because the bacterium is not systemic within the plant, the bacteria can be prevented from spreading beyond that lesion and creating a more severe infection on the rest of the tree. That is, the bacteria do not move within the plant; they are restricted to the lesion itself unless moved by water or some other physical means.
If you find citrus canker on your trees, the best thing you can do is try to suppress the disease if it is limited to a relatively small area (e.g. a single branch or a few fruit). This can be easily done by pruning away the infected portion of the tree. The remaining portion of the tree should be monitored carefully for any new infections and they should be removed as soon as they are detected. Citrus canker bacteria can easily survive from one season to the next in old lesions on fruit, leaves, and stems, especially in Florida's climate. Thus, removing infected plant material reduces inoculum sources and the potential for new infections. Any material removed from a plant that has citrus canker should be sealed inside a double plastic bag before being disposed of in the trash. Do not place canker-infected plant material in compost piles or horticultural waste because this will not kill the bacteria and may allow for easy spread of the disease.
The only chemical products that are effective for canker control are copper-based fungicides/bactericides. However, these products act only as a chemical barrier against canker infection, not systemically. Therefore, if one of these products is applied to a young leaf or fruit, that leaf or fruit will only be protected as long as the copper coating remains intact. The copper only coats the surface, and when the plant surface expands, the coating is no longer intact and the new exposed tissue is again susceptible to infection. Because of the rapid expansion of the citrus tissue, it is difficult to protect the fruit with copper, especially grapefruit. It is nearly impossible to protect leaves to which copper is applied less often than once every three weeks during the early summer growing period (Table 1). Often, homeowners find they are not able to apply copper as frequently as needed and do not bother. However, if you choose to use chemical control, you must follow and comply with all label directions for the products you are using.
Copper management program indicating citrus fruit types and periods in which to implement or schedule copper sprays.
Diagnosing Citrus Canker
If you are unsure whether a disorder on your citrus tree is citrus canker, please contact your local UF/IFAS Extension office (visit https://sfyl.ifas.ufl.edu to find your local office) or the FDACS, DPI Helpline (1-800-282-5153). They may ask you to e-mail them a digital photo of the symptomatic tissue for positive identification. If a positive identification cannot be made in this way or you are unable to provide them a photo, you may be asked to bring a sample to the office. If you bring a sample to the county extension office it should be sealed inside two clear plastic zip-top bags to prevent the possibility of spreading the disease. Do not transport suspect material without being specifically asked to do so and without first double-bagging the sample.
Citrus canker affects all types of citrus, but there is some variability in the disease severity depending on the variety of citrus grown. A general ranking of varietal susceptibility, from most susceptible to least susceptible is shown in Table 2.
General ranking of cultivar susceptibility to citrus canker from most susceptible to least susceptible (resistant).
Although it is no longer governed by the strict rules that were in place during eradication, citrus canker is still a serious disease of citrus. The disease has become endemic in many parts of the state, but it is still possible to restrict its spread beyond these areas. North Florida and parts of the Panhandle are still canker free. Citrus canker is highly contagious and can easily be spread by human activity, further threatening what is a hallmark of the Florida landscape—citrus. Homeowners should be familiar with disease symptoms and regularly inspect their dooryard trees for citrus canker infections. No citrus, including fruit, potted trees, or cuttings, should be moved into or out of Florida without first checking the most current quarantine regulations on the FDACS, DPI website. Through vigilance on the part of homeowners and commercial citrus growers alike, citrus canker can be controlled and Florida's citrus trees can be protected.
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Citrus Health Response Program https://www.fdacs.gov/Agriculture-Industry/Pests-and-Diseases/Plant-Pests-and-Diseases/Citrus-Health-Response-Program
Shipping Home Grown Citrus Fruit from Florida https://www.fdacs.gov/content/download/24077/file/Homeowner_Fruit_Packinghouses.pdf