Citrus canker

Alert

Citrus canker has been detected in the Northern Territory and Western Australia.

The Biosecurity (Citrus Canker) Amendment Regulation 2019 under the Biosecurity Act 2014 is now in place for citrus canker and carriers of citrus canker. This means that host plants, plant material (including fruit) within the family (Rutaceae) and associated planting media and soil, machinery or equipment may not be brought into Queensland, unless they meet certain conditions as outlined in the Regulation and the Queensland biosecurity manual (PDF, 1.8MB).

An industry alert has also been issued.

These measures are to prevent the disease spreading into Queensland’s citrus production areas.

In Queensland, Citrus canker is prohibited matter under the Biosecurity Act 2014.

Be on the lookout, and report signs of Citrus canker to Biosecurity Queensland on 13 25 23.

Do not touch suspect citrus canker lesions or move plant material off your property. This can spread the disease.

Early detection and reporting are key elements in controlling this disease.

Citrus canker is a contagious plant disease caused by the bacterium Xanthomonas citri subsp. citri, affecting citrus and some other plant species. Citrus canker does not pose any risk to human health.

Citrus trees infected with citrus canker display unsightly lesions which can form on leaves, fruit and stems. Trees infected with the disease may have poor growth and a reduction in fruit quality and quantity. The disease has serious economic impacts on citrus production and is the subject of a number of control and eradication programs around the world.

In Australia, citrus canker is an exotic disease. Historically, there have been several outbreaks: in the Northern Territory in 1912, 1991 and 1993, and Queensland in 1984 and 2004. All were successfully eradicated.

Citrus canker was recently detected in the Northern Territory in April 2018. Following tracing and testing of nursery plants from the Northern Territory, there are confirmed, linked cases of citrus canker on a small number of properties in Western Australia’s north. A nationally coordinated response program is currently underway to eradicate the disease.

Other names

  • Citrus bacterial canker
  • Asiatic citrus canker

Description

Symptoms

The bacterium causes the development of blister-like lesions on host leaves, fruit and stems.

Citrus canker lesions

  • Usually raised, spongy, and coloured tan to brown, surrounded by an oily, water-soaked margin that can become a yellow ring or halo as the lesions age.
  • Gradually increase in size to 5–10mm over several months.
  • Large or older lesions may have a crater-like centre, which can fall out to create a 'shot-hole' appearance.
  • Occur in clusters where water pools on the leaf (such as along leaf margins or tips).
  • Can follow the feeding tracks of citrus leaf miners, where the wound provides an entry point for the bacteria.
  • Multiple lesions on fruit stems are typically only seen in cases where the foliage is severely affected.

The lesions can vary in size, shape and appearance depending on:

  • the citrus cultivar or host plant affected
  • the way the bacteria enters the plant (for example through stomata or entry wounds)
  • the age of the lesions
  • climatic conditions.

The disease causes abnormal leaf fall, poor tree health, dieback, blemished fruit and premature fruit drop.

Plant stage and plant parts affected

Leaf, fruit and stem tissue may be infected. Leaf tissue offers more opportunity for infection and as such typically displays the most numerous lesions over time.

The disease usually becomes more active in early spring. The highest risk for new infections is during active growing periods when fresh shoots are emerging.

Conditions for development of the disease are optimal in warm temperatures, and spread is highest in periods of high rainfall and strong winds.

May be confused with

Lemon scab caused by the fungus Elsinoe fawcettii also causes scab-like lesions, predominantly on lemons in coastal regions. Lemon scabs appear drier looking than citrus canker and to not have a yellow halo.

If you suspect citrus canker, don't hesitate to report it.

Distribution

The disease is widespread in many tropical and subtropical citrus-growing areas of the world. In early 2018, detections of citrus canker were made in the Northern Territory and in Western Australia. These detections are currently the subject of nationally coordinated response program.

Hosts

Affects orange, mandarin, lemon, lime, grapefruit, kumquat, calamondin, tangelo, pomelo, citrus rootstock and native citrus species such as Citrus australasica (finger lime), C. gracilis (Humptydoo lime) and C. inodora (North Queensland/Johnstone River lime).

All commercial citrus cultivars can be affected.

The Biosecurity (Citrus Canker) Amendment Regulation 2019 contains a full list of citrus canker host plants.

Life cycle

Bacteria ooze from the lesions and are spread predominantly by rain splash. In rain storms, bacteria can be carried between trees over distances up to 100m. The bacteria enter the plant through stomata or through wounds caused by wind driven rain, mechanical wounds caused by equipment, and wounds caused by insects such as citrus leaf miner. The disease can become less active (latent) when the weather is dry for long periods, and then become active again in periods of high rainfall and warm weather.

Impacts

Citrus is a significant crop in Australia, there is over 28,000 hectares of citrus planted and around 1,900 growers (Citrus Australia, 2016). For the year ending June 2017, citrus production was valued at $724.4 million, with Queensland the largest producer of mandarins. The high quality of Australia's citrus fruit results in citrus being Australia's largest fresh fruit exporting industry by volume, with exports of oranges, mandarins, lemons, limes and grapefruit, totalling 260,209 tonnes worth $427 million in 2017 (Plant Health Australia, n.d.).

Citrus canker causes plant defoliation, unsightly fruit blemishes and premature fruit drop. This leads to decrease in fruit production and a reduction in saleable fruit. Farmers can experience production losses and trade bans as the presence of citrus canker can affect domestic and international export market access. In order to prevent the disease from spreading, affected growers properties may be quarantined. Citrus canker can also have significant economic and emotional impacts on individual growers, their families and the communities that support them.

Home gardeners may also be affected as citrus is a common backyard plant.

Human health is not affected by infected plants and fruit.

How it is spread

The canker lesions ooze bacteria when wet. Over short distances, wind-driven rain, air currents, insects, birds, human movement and equipment such as overhead or spray irrigation systems can spread the bacteria.

Citrus canker can be moved and spread over longer distances on equipment (vehicles, tools, mechanical hedgers, sprayers, gardening equipment) and people (hands, shoes and clothing).

Movement of infected plant material, or airborne movement of bacteria as an aerosol or debris during severe weather events (where strong winds and rain are present), can also spread the disease further.

The disease is not transmitted by seeds.

Illegal importation of infected plant material poses the greatest risk of introducing this disease into Australia. The Australian Government closely monitors for illegal plant movements and regulates approved host plant imports.

Monitoring and action

Citrus canker is typically recognised by its characteristic symptoms, and can be detected during routine orchard inspections.

Refer to the Biosecurity Manual for Citrus Producers (PDF, 4MB) for specific advice on how to monitor for citrus pests and diseases:

If you see symptoms that resemble citrus canker infection, contact Biosecurity Queensland on 13 25 23 for advice on how to act.

Legal requirements

Citrus canker is a prohibited matter under the Biosecurity Act 2014.

Report suspected citrus canker to Biosecurity Queensland immediately on 13 25 23 or contact the Exotic Plant Pest Hotline on 1800 084 881.

If you think you have found citrus canker, you must take all reasonable and practical steps under your control to minimise any associated risks or spread of the disease. This is called a general biosecurity obligation (GBO). Do not touch or move infected plant material.

Interstate movement controls have been implemented to prevent entry of citrus canker hosts and carriers, such as fruit, plants and plant material, soil, equipment and machinery, from states where citrus canker has been detected into Queensland. For further information view the industry alert or movement restrictions in the Queensland biosecurity manual (PDF, 1.8MB).