Citrus canker

Citrus canker is a serious disease of citrus. Once established, it can only be controlled by destroying all the susceptible plants. It's caused by the bacterium Xanthomonas citri subsp. citri.

The disease can reduce the growth of citrus trees and cause blemishes on otherwise healthy fruit. It can spread rapidly over short distances, particularly in tropical and subtropical climates.

Symptoms of citrus canker

Disease begins as small pimple-like spots that are about 1 mm in diameter and yellow in colour. As the spots enlarge, they become brown and corky with sunken centres and raised edges. Often each of these spots have a yellow ring surrounding them.

Older lesions can be up to 1 cm in diameter and in long-term infections, cankers of various sizes may appear at the same time (see Figures 1 and 2).

Similar symptoms appear on:

• fruit
• leaves
• stems.

Be careful not to confuse citrus canker with lemon scab (Elsinoe fawcettii). Lemon scab has drier lesions and no yellow ring surrounding them.

Dispersal of citrus canker

Xanthomonas citri can be splashed over short distances by rain and irrigation. If contaminated water lands on people, machinery, trucks or cars then the bacterium can spread even further.

Interstate or even international dispersal can occur by trade and movement of infected plants.

Xanthomonas citri is also carried inside parts of plants such as:

• fruits
• leaves
• branches.

Feeding by the citrus leaf miner (Phyllocnitis citrella) can increase the number of lesions of citrus canker.

Re-occurring outbreaks

Throughout the world, disease outbreaks occur after many years of absence. This could be as a result of a re-introduction of Xanthomonas citri, or because the bacterium has persisted in the environment at low and undetectable levels.

Xanthomonas citri has been reported on citrus plants without citrus canker and on plants not susceptible to the disease. It also survives in the soil and straw mulch.

Citrus canker in Australia

In Australia, citrus canker has been eradicated several times after a quick response and destruction of susceptible plants:

• two outbreaks in the Northern Territory in 1991 and 1993
• one in Queensland in 2004.

In those cases, 2 or more years of monitoring was required before eradication could be declared.

More recently there were outbreaks of the disease in Western Australia and the Northern Territory in 2018. Citrus canker was declared eradicated in Western Australia near the end of 2019 and from Northern Territory in early 2021.

Protect your property

To protect your property from citrus canker, be sure to maintain best practice on-farm biosecurity standards. Make sure propagation material is purchased from reputable suppliers and regularly check your orchard for citrus canker symptoms.

For more information about citrus canker visit Plant Health Australia.

Photo credits

Figure 1 from DAFF Queensland.

Figure 2 from Jeffrey W. Lotz, Florida Department of Agriculture and Consumer Services, Bugwood.org

References

• CABI, 2020. Xanthomonas citri (citrus canker). In: Invasive Species Compendium. Wallingford, UK: CAB International. www.cabi.org/isc
• Subcommittee on Plant Health Diagnostic Standards (SPHDS) (2016) National diagnostic protocol for asiatic citrus canker Xanthomonas citri subsp. citri NDP 9, V1.2. Australian Government, Department of Agriculture
• Plant Health Australia (no date) Citrus canker [web page]. Retrieved from https://www.planthealthaustralia.com.au/wp-content/uploads/2018/11/Citrus-canker-FS.pdf (Accessed 3 June 2020)