Citrus canker: A widespread bacterial drawback

Citrus fruits are among the most popular fruits to be found in our homes, grown by gardeners lucky enough to live in warm climates. Two major diseases afflict citrus plants, citrus canker and huanglongbing (citrus blight), both of which have devastating economic consequences for the citrus industry and are tightly regulated by government agencies in the US and abroad.

Citrus canker, or the bacterial citrus canker disease, is caused by variants of the bacterium Xanthomonas axonopodis pv. Lemon. This disease is common in tropical and subtropical climates where citrus fruits are typically grown. Its geographic distribution spans over 30 countries in Asia, the Middle East, Oceania and the Americas. This disease first arrived in the US in 1910 as imported rootstock seedlings and infected trees from Japan and has had several outbreaks in citrus-growing states since then. Eradication efforts have been enacted in several states to control its spread. Citrus canker can spread quickly if left unchecked.

It was also the focus of a 16-year legal battle between 18,000 homeowners in central Florida and the state. Between 2002 and 2006, the Florida Department of Agriculture and Consumer Services removed over 60,000 citrus trees, including healthy citrus trees, that were within 1,800 feet of an infected tree to eradicate the disease. The Florida state legislature recently approved compensation for tree removal, and homeowners are expected to receive $42 million.

In addition to Florida, the USDA has also reported cases in Louisiana and Texas and is closely monitoring the citrus industry in California. If you grow citrus in one of these states where the canker is significantly affecting the crop industry, keep up to date with your state's latest regulations and recommendations to keep your citrus trees healthy and prevent the general spread of the disease.

What is citrus canker?

Citrus canker is a serious problem for citrus growers. Source: mmmavocado

Citrus canker is a highly contagious plant disease caused by various pathotypes of the bacterium Xanthomonas axonopodis pv. Lemon. The most common variant is Asiatic citrus canker (CC-A), which affects the widest range of hosts and is the most economically damaging in many citrus growing areas. Different X. axonopodis variants such as CC-B and CC-C have different geographic distributions, disease severity, and host ranges. Agricultural authorities test for protein markers and use DNA analysis to identify specific citrus canker variants.

Life cycle of citrus canker bacteria

Despite the different strains of X. axonopodis, they all share similar pathogenicity. Citrus canker spreads when the bacteria get into leaf stomata, natural openings on the plant, or through wounds. X. axonopodis thrives in temperatures between 20-30 degrees Celsius. Once the citrus canker bacteria are in the plant tissue, they multiply and after 5 days of development are able to inoculate other healthy tissue. Infected plants only show visible symptoms after 9 days of exposure. Bacteria are released from the infected plant lesions as they erupt and then spread to infect healthy plant tissues by wind-driven rain. The citrus canker bacteria can remain viable in soil for a few days, for a few months on infected plant material that has been mixed into the soil, and even for a few years on infected plants. X. axonopodis typically persists throughout the year in warm climates and through multiple growing seasons.

Major weather events like hurricanes create the perfect storm for both moisture needed for citrus canker to proliferate and physical damage to trees. Citrus leaf miner larvae tunneling through the tissues of plants can greatly exacerbate the transmission and severity of citrus canker.

Symptoms of citrus canker

Cancer on citrus stalkDetection of citrus canker on stem tissue. Source: JIRCAS

As the name suggests, the development of citrus canker disease causes lesions on the leaves, stems and fruits of citrus fruits and can affect all aerial parts of these plants. Infected plants can experience leaf loss, fruit drop and eventual death. Citrus canker lesions begin to develop on the leaves 1-2 weeks after infection. They start out on the underside of the leaves as tiny little bumps and develop into an irregular brown or yellow halo around the bump. Eventually they take on a corky, crater-like appearance and fall off, which is problematic for the overall health of infected citrus trees. Monitor your citrus tree's leaves, stems, and fruit closely to watch for the spread of disease, and keep an eye on any leaves with that tell-tale yellow halo or potential canker lesions on fruit.

Citrus canker lesions can also develop on branches or fruit. They are raised and also take on a corky appearance. Citrus canker is a type of fruit peel disease that can severely reduce the market value of these plants. It can appear as a water-soaked rim on infected fruit, and the cankers can also break out and seep onto other plant surfaces. Severe citrus canker infection can result in premature fruit drop, which can also further spread the citrus canker through movement of plant matter. Ripe fruit infected with this citrus bacterial canker should be removed and discarded.

Susceptible citrus species

Not all citrus relatives are equally affected by citrus canker because different strains of bacteria have different hosts. For example, citrus cultivars tart orange, grapefruit, and Mexican lime are susceptible to CC-B. Mexican lime and tart orange are also the only known hosts for CC-C. The most common strain of citrus canker, CC-A, is most harmful to grapefruit, certain sweet orange cultivars (pineapple, hamlin, and navel), and Mexican lime. Lemon trees are moderately susceptible and tangerine trees are moderately resistant. Despite some resistance, citrus production is challenging in most cases when threatened by both citrus canker and citrus leaf miners.

Fight citrus canker

Cancers on leaves and stemsCitrus canker as seen on both leaf and stem tissue. Source: JIRCAS

As with citrus greening disease, there is no cure for citrus canker disease, which is why so much vigilance is needed to monitor and control its spread. Disease prevention comes first. Standard practice for citrus tree producers around the world is the use of copper-based bactericides. Copper fungicide has been shown to reduce the overall bacterial population on the foliage and must be sprayed on citrus trees at three-week intervals throughout the growing season, beginning with the first leaf flush in spring. The effectiveness of copper sprays depends on the weather. In wet and rainy conditions, which the bacteria prefer, this control method is less effective. There are additional long-term concerns with using copper sprays, including the development of resistant strains of citrus canker and the accumulation of excess copper in the soil.

Prevent citrus canker

Use an integrated pest management (IPM) approach to prevent residential citrus diseases by following regulations and sanitation best practices. Buy your citrus tree from one of the certified nurseries. Different states have different certification bodies. For example, if you live in Texas, the certification is issued by the Texas Department of Agriculture. Comply with all regulations regarding the transportation of citrus fruit or citrus materials across state lines. Use good general hygiene practices to limit the risk of human and mechanical transmission. Protect your citrus trees from wind, which can carry the bacteria, and avoid handling the plants when they are wet.

frequently asked Questions

Q: Is citrus canker harmful to humans?

A: No, citrus canker is not harmful to humans but can result in devastating economic losses for citrus growers.

Q: How can we prevent citrus canker from spreading?

A: Transporting infected citrus trees and planting material is the best way to spread the disease. Purchase citrus from certified sources and follow all regulations regarding the transportation of citrus materials across state lines. If you suspect you have infected trees, contact your local agricultural offices immediately so they can diagnose and limit the spread of the disease.

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