Citrus are some of the most popular fruits found in our homes and 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 of which have devastating economic consequences for the citrus industry and are strictly regulated by governmental agencies in the U.S. and abroad.
Citrus canker or the citrus canker bacterial disease 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. In the U.S., this disease first arrived as imported rootstock seedlings and infected trees from Japan in 1910 and has had several outbreaks in citrus-producing states 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 16-year long 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 that were within 1900 feet (580 meters) of an infected tree, in an effort to eradicate the disease. The Florida state legislature recently approved compensation for the tree removal and an expected $42 million will be paid out to homeowners.
In addition to Florida, the USDA also reports cases found in Louisiana and Texas to date and are 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 to keep your citrus trees healthy and prevent the overall spread of the disease.
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 across many citrus growing regions. 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.
Life Cycle of The Citrus Canker Bacteria
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. X. axonopodis thrive in temperatures between 20-30 degrees Celsius. Once inside the plant tissue, the citrus canker bacteria multiplies and can inoculate other healthy tissue after 5 days of development. Infected plants will first start to show visible symptoms after 9 days of exposure. Bacteria are released from the infected plant lesions as they erupt and 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 throughout the year 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 causing physical damage to trees. Citrus leaf miner larvae that tunnel through the tissue of plants can greatly exacerbate citrus canker transmission and severity.
Symptoms Of Citrus Canker
As its name suggests, the citrus canker disease development causes lesions on leaves, stems, and fruit of citrus and 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 between 1-2 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, and keep a close eye on any leaves with that telltale yellow halo or any potential canker lesions on fruit.
Citrus canker lesions can also develop on twigs or on 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 and 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.
Controlling Citrus Canker
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 and 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.
Preventing Citrus Canker
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
Q: Is citrus canker harmful to humans?
A: No, citrus canker is not harmful to humans but can result in devastating economical losses for citrus producers.
Q: How can we stop citrus canker from spreading?
A: Transporting infected citrus trees and plant material is the top way of spreading the disease. Purchase citrus plants from certified sources and follow all regulations regarding transporting citrus material across state lines. If you suspect that you have infected trees, contact your local agricultural offices immediately so they can run diagnostics and help control the disease spread.