Citrus canker is a bacterial disease that causes raised, corky lesions on the leaves, fruit, and stems of most citrus varieties. It doesn’t kill trees outright, but it blemishes fruit, weakens foliage, and can trigger premature fruit drop. In the United States, it’s a quarantine-level disease, meaning infected trees and fruit face strict movement restrictions enforced by federal and state agriculture agencies.
What Citrus Canker Looks Like
The earliest sign of infection is small, raised bumps on both sides of a leaf, though they’re more pronounced on the underside. As these lesions mature, they become crater-like: corky and rough-textured, with raised margins, a sunken center, and a distinctive yellow halo ringing each spot. That halo is one of the most reliable visual markers of citrus canker and helps distinguish it from other citrus diseases.
On fruit, lesions look similar but vary more in size. Because the rind stays susceptible over a long period, a single fruit can pick up infections from multiple cycles, resulting in clusters of spots at different stages of development. Twig and stem lesions are raised and corky as well, and they’re especially important because the bacteria can survive in them for months, serving as a reservoir for future infections. Over time, older lesions sometimes darken as other fungi colonize the damaged tissue, which can make diagnosis trickier on neglected trees.
How It Spreads
Wind-driven rain is the primary way citrus canker moves from tree to tree. When moisture hits an active lesion, bacteria ooze out of the surface and get carried by rain splash or wind to nearby leaves, fruit, or stems. The bacteria enter through natural pores on the leaf surface or through any wound or break in the tissue. This is why storms and hurricanes can cause explosive outbreaks, sometimes spreading the disease across entire groves in a single weather event.
Young, actively growing leaves are the most vulnerable. Damage from citrus leafminer larvae creates ideal entry points for infection, because the tunnels these insects carve through leaf tissue expose cells the bacteria can colonize easily. Temperature and humidity matter too. Warm, wet conditions accelerate the disease cycle, while cooler temperatures slow bacterial reproduction and extend the time between infection and visible symptoms. Dry climates are generally less hospitable to citrus canker, which is one reason California has been less affected than Florida or Texas despite growing enormous volumes of citrus.
Over longer distances, the disease travels through infected nursery stock, contaminated plant material, and even fruit carried by people. Moving a single infected seedling into a clean area can start a new outbreak.
Which Citrus Varieties Are Affected
Nearly all commercial and backyard citrus can get citrus canker, but susceptibility varies. Grapefruit and Mexican limes are among the most vulnerable, developing lesions readily even with light bacterial exposure. Sweet oranges, lemons, and most mandarins are also susceptible, though some cultivars show more resistance than others. No widely grown citrus variety is truly immune. If you’re growing any type of citrus in a region where canker is present, your trees are at some level of risk.
How to Tell It Apart From Other Diseases
Several common citrus problems can look superficially similar to canker, so knowing the differences matters if you’re trying to figure out what’s affecting your tree.
- Citrus scab produces raised, wart-like growths on leaves and fruit, but scab lesions are pale orange, irregularly shaped, and often cause leaves to become crinkled and distorted. A key difference: scab creates a bulge on one side of the leaf with a corresponding depression on the other, while canker lesions are raised on both surfaces. Scab also lacks the yellow halo that surrounds canker spots.
- Greasy spot starts as swelling on the lower leaf surface with a yellow mottle visible from above. Over time, the affected tissue collapses and turns brown or black. Greasy spot causes significant leaf drop but doesn’t produce the raised, crater-like lesions characteristic of canker, and it doesn’t affect fruit in the same way.
- Mechanical damage from thorns, wind, or insects can leave marks on fruit and leaves, but these wounds lack the uniform circular shape, corky texture, and yellow halo of canker lesions.
If you’re uncertain, your local agricultural extension office can confirm a diagnosis, and in quarantine areas they may need to be notified.
Managing Citrus Canker
There is no cure for citrus canker. Once a tree is infected, the bacteria persist in lesions and can reactivate whenever conditions are wet. Management focuses on slowing the spread, protecting new growth, and reducing the severity of symptoms.
Copper-based sprays are the standard protective treatment. They work by coating leaf and fruit surfaces with a barrier that kills bacteria on contact before they can enter the tissue. Timing is critical: sprays need to be applied to new flushes of growth while the leaves are still young and vulnerable, and reapplied after heavy rain washes the copper away. In commercial groves, this can mean multiple applications per growing season.
Cultural practices play an equally important role. Windbreaks, whether living hedgerows or constructed barriers, reduce the distance rain-splashed bacteria can travel between trees. Controlling citrus leafminer with appropriate insect management reduces the wound sites that bacteria exploit. Pruning out heavily infected branches during dry weather can lower the amount of bacteria present on a tree, though this needs to be done carefully to avoid spreading the disease on tools. Sanitizing pruning equipment between cuts and between trees is a basic but often overlooked step.
For backyard growers, keeping trees healthy with proper nutrition and irrigation helps them tolerate infection better. Removing fallen leaves and fruit from under the canopy reduces the bacterial load in the immediate area. Avoid working around citrus trees when foliage is wet, since you can inadvertently carry bacteria on your hands, clothing, or tools.
Quarantine Rules in the U.S.
Citrus canker is a federally regulated disease. APHIS, the agency responsible for plant health within the USDA, maintains quarantine zones in areas where canker has been detected. As of recent actions, quarantine areas exist in parts of Cameron and Hidalgo counties in south Texas, and Florida has dealt with canker regulations for years.
Within quarantine zones, interstate movement of citrus plants and plant parts is prohibited, with limited exceptions for commercially packed and disinfected fruit. Citrus nursery stock can only be shipped out of quarantined areas if it meets specific federal requirements. State agriculture departments often establish parallel intrastate quarantines that restrict movement even within the state.
These regulations exist because citrus canker is far easier to prevent than to control once established. The U.S. previously spent hundreds of millions of dollars attempting to eradicate canker from Florida through tree removal programs before ultimately shifting to a management approach. For home gardeners, the practical takeaway is straightforward: never move citrus plants or cuttings from a quarantined area, and if you’re buying citrus trees, purchase only from certified nurseries that comply with disease-free standards.