How to Control and Prevent Citrus Canker

Citrus are some of the most popular fruits found in our homes, grown by gardeners who are lucky enough to live in warm climates. Two major diseases affect citrus plants: citrus canker and huanglongbing (citrus greening disease). Both have devastating economic consequences for the citrus industry and are strictly regulated by governmental agencies in the U.S. and abroad.

Citrus canker is caused by variants of the bacterium Xanthomonas axonopodis pv. citri. This disease is commonly found in tropical and subtropical climates where citrus is typically grown. Its geographic spread includes over 30 countries in Asia, the Middle East, Oceana, and the Americas.

It’s important to understand the signs of citrus canker and susceptible plants to keep your trees safe.

What Is Citrus Canker?

Citrus canker is a highly contagious plant disease caused by different pathotypes of the bacterium Xanthomonas axonopodis pv. Citri. The most common variant is the Asiatic citrus canker (CC-A) which impacts the broadest range of hosts and is the most economically damaging.

Different X. axonopodis variants, such as CC-B and CC-C, have different geographical distributions, disease severity, and host ranges. Agricultural regulators test for protein markers and use DNA analysis to determine specific citrus canker variants. 

In the U.S., this disease first arrived as imported rootstock seedlings and infected trees from Japan in 1910. Several outbreaks in citrus-producing states have occurred since then. Eradication efforts have been enacted in several states to control its spread. Citrus canker can spread rapidly if unchecked.

It has also taken center stage in a legal battle between 18,000 homeowners in central Florida and the state. Between 2002-2006, the Florida Department of Agriculture and Consumer Services removed over 60,000 citrus trees, including healthy residential citrus trees within 1900 feet (580 meters) of an infected tree in an effort to eradicate the disease.

In addition to Florida, the USDA also reports cases found in Louisiana and Texas. They are also closely monitoring the citrus industry in California. If you grow citrus in any of these states where the canker significantly affects the plant industry, stay on top of the latest regulations and recommendations from your state.

Life Cycle

Despite the different strains of X. axonopodis, they all share a similar pathogenicity.

Citrus canker spreads as the bacteria enters the leaf stomata, natural openings on the plant, or through wounds. Once inside the plant tissue, the citrus canker bacteria multiplies and can inoculate other healthy tissue after five days of development.

Infected plants will first start to show visible symptoms after nine days of exposure. Bacteria are released from the infected plant lesions as they erupt. They are then dispersed to infect healthy plant tissue through wind-driven rain.

The citrus canker bacteria can stay viable for a few days in the soil, a few months on infected plant material that’s been mixed into the soil, and even a few years on infected plants. X. axonopodis typically persists in warm climates and across multiple growing seasons. 

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

Symptoms

As its name suggests, citrus canker disease causes lesions on leaves, stems, and fruit of citrus. It can impact all above-ground parts of these plants. Infected plants may experience defoliation, fruit drop, and terminal die-back.

The citrus canker lesions first start developing on leaves within one to two weeks of infection. They start on the lower surface of leaves as tiny small bumps and develop into an irregular tan or yellow halo around the bump. Eventually, they take on a corky, crater-like appearance and drop, which is problematic for the overall health of infected citrus trees.

Monitor the leaves, stems, and fruit of your citrus tree closely to watch for disease spread. Keep a close eye on any leaves with that telltale yellow halo or any potential canker lesions on the fruit.

Citrus canker lesions can also develop on twigs or fruits. They are raised and also take on a cork-like appearance.

Citrus canker is a type of fruit rind blemishing disease that can greatly diminish the market value of these crops. On infected fruit, it can look like a water-soaked margin. The cankers can also erupt and ooze onto other plant surfaces.

A severe citrus canker infection may lead to premature fruit drop, which can also further spread citrus canker through the movement of plant material. Mature fruit infected by this citrus bacterial canker should be removed and disposed of.

Susceptible Citrus Species

Not all citrus relatives are equally affected by citrus canker since different strains of the bacteria have different hosts.

For example, the citrus cultivars sour orange, pomelo, and Mexican limes are susceptible to CC-B. Mexican lime and sour orange are also the only known hosts for CC-C. The most pervasive strain of citrus canker, CC-A, is most damaging to grapefruit, certain sweet orange cultivars (Pineapple, Hamlin, and Navel) and Mexican lime.

Lemon trees are moderately susceptible and mandarin trees are moderately resistant. Even with some resistance, most citrus fruit production is challenging when threatened by both citrus canker and citrus leaf miners. 

Control

As with the citrus greening disease, there is no cure for the citrus canker disease, which is why there is so much vigilance to monitor and control its spread. Disease prevention is paramount.

The standard practice for citrus tree producers around the world is to use copper-based bactericides. Copper fungicide has been shown to decrease the overall bacterial population on leaves. It must be sprayed onto citrus trees in three-week intervals throughout the growing season, starting with the first spring leaf flush.

The effectiveness of copper sprays depends on the weather. This control method is less effective under wet and rainy conditions, which are favored by the bacteria. 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.

Prevention

Take an integrated pest management (IPM) approach to residential citrus disease prevention by following regulations and sanitation best practices.

Purchase your citrus tree from one of the certified nurseries. Different states will have different certification entities. For example, if you live in Texas, the certification is issued by the Texas Department of Agriculture. Follow all regulations regarding moving citrus or citrus material across state borders.

Use good general sanitation practices to limit the risk of human and mechanical transmission. Protect your citrus trees from wind exposure which can transport the bacteria and avoid handling the plants when wet.  

Frequently Asked Questions