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7 Orange Tree Diseases: How to Identify and Treat Them

Any living thing can contract diseases; fruit trees are no exception. When growing and nurturing orange trees, you want to protect them from diseases that kill and spoil the fruit as well as the tree.

A healthy backyard orange tree.

The more you learn about these diseases, the easier it will be to protect your orange trees! Read on to learn about seven orange tree diseases and how to identify, treat, and prevent them.

Types of Diseases in Orange Trees

Citrus Canker

Citrus canker is a contagious, bacterial orange tree disease caused by Xanthomonas citri, a bacterium originating from southern Asia. Wet, warm temperatures enable it to thrive.

Closeup of citrus canker.

It is spread by rain, wind, and Asian leaf miner bugs that have fed on infected leaves. Contact via pruning equipment and other items that may have touched an infected tree can also cause citrus canker.

This disease causes blemishes in the oranges and makes the leaves drop prematurely (defoliation), harming the tree. Even the fruit will drop before they are ready for harvest.

Identifying Symptoms

Circular lesions with raised, scabby centers will show on both sides of the leaves and on the stems and oranges. They are small, watery, and brown or tan with yellow halos.

As the disease progresses, they will look like craters and darken in color.

Though the oranges are safe to eat despite infection, blemishes or spots of any kind will not market well for consumers.

Treating Citrus Canker

Unfortunately, if your orange tree has citrus canker, you will have to remove and destroy it to stop the spread. That is where treatment for preventing an orange tree disease like this comes in.

Liquid copper fungicide is a great way to keep your orange tree from contracting citrus canker. The spray has long-lasting effects when sticking to the fruit, foliage, and wood of the tree.

Be sure to follow its directions when applying the spray to your tree.

Also, remember to wash your hands and clothes, and disinfect your gardening and pruning tools before handling your trees. Good hygiene should always apply to stave off orange tree diseases!

Citrus Greasy Spot

Citrus greasy spot is a fungal orange tree disease that is spread by wind and rain.

Black spots on citrus tree leaves, a sign of several orange tree diseases.

Decomposing leaves and dead sticks at the tree’s base are where the fungus thrives, especially if it is summer and humid. It spreads throughout the tree and its grove, defoliating leaves, which will eventually kill the tree.

Identifying Symptoms

Symptoms of citrus greasy spot may not appear for months. Now, some orange tree diseases are asymptomatic at the start.

You may not notice symptoms of any disease, but keep watch as you grow and nurture your oranges.

When symptoms show, on the leaves, you may see yellow spots on the top and raised brown blisters on the bottom. These spots will also be seen between the oil glands of the infected oranges.

Later in progression, the spots will become black lesions with a greasy appearance.

Treating Citrus Greasy Spot

Just like for citrus canker, spraying Liquid Copper Fungicide can prevent or control citrus greasy spot and protect developing oranges. Depending on how heavy the infection is on the leaves and fruit, you may spray again when late summer arrives.

Removing fallen leaves and dead branches from the tree’s base can also prevent the fungus from spreading and infecting your orange tree. This is especially important when pruning.

Follow the directions of the spray and you may save your tree and oranges!

Citrus Leprosis

Citrus leprosis is a viral orange tree disease that is spread by false spider mites after feeding on infected leaves. Though the oranges are safe to eat, nobody wants oranges that have bugs crawling on or near them.

Closeup of citrus leprosis.

Identifying Symptoms

Similar to citrus canker, the lesions that form where the mites have fed are round and dark brown with yellow halos. The difference is that the centers of the lesions are not raised.

Treating Citrus Leprosis

Treating an orange tree disease like this requires controlling the spreaders. Setting up windbreaks (rows of shrubbery) can prevent spider mites from flying onto your trees through the wind.

Other ways include removing weeds and pruning dead branches and twigs when needed.

Check your trees regularly for mites, as spraying can be difficult due to their hiding beneath cracks in the tree. If you decide to spray them, use an insecticide like Neem Oil or a miticide like Horticultural Oil.

Citrus Melanose

Citrus melanose is a fungal orange tree disease that grows on dead and wet twigs or branches. This would be due to rain and overhead watering (watering above plant level).

Closeup of citrus melanose.

Identifying Symptoms

Small red or brown spots will appear on the leaves and become raised bumps. The fruits will also show brown spots that grow together, trailing downward where water was dripping, like tearstains.

Treating Citrus Melanose

Remove any dead or decaying plant material near your orange tree. When pruning the tree of any dead twigs or branches, make sure your tools are disinfected and avoid overhead watering.

If your tree is already infected, spray a copper fungicide according to its bottle’s directions.

Citrus Psorosis

Citrus psorosis is a viral orange tree disease that is transferred through infected bud grafts, grafting tools, or tree seeds. It affects orange trees that are about eight to ten years old and lessens their lives and produces fewer oranges.

A citrus leaf with yellow discolorations on it.

Identifying Symptoms

This is another orange tree disease that may be asymptomatic, being as long as ten years.

Once symptoms appear, you will notice patches of tree bark with small pustules, becoming scaly and forming gummy lesions later. The leaves will have yellow spots and blotches, and the fruit rinds will have sunken grooves.

Unlike citrus canker and leprosis, this disease will make the oranges inedible as they develop gray or yellowish rings.

Treating Citrus Psorosis

To prevent an orange tree disease like this, you can purchase disease-free budwood from a certified citrus nursery. If you graft an orange tree, make sure your tools are disinfected.

Should you have an orange tree with citrus psorosis, you may have to remove and replace it. Though scraping away infected bark stalls the disease, the tree will produce neither a good number nor quality of oranges.

Huanglongbing (HLB), or Citrus Greening

Huanglongbing (HLB), or citrus greening, is considered to be a very serious bacterial orange tree disease.

Citrus leaves showing signs of citrus greening, or Huanglongbing.

The wedge-shaped Asian citrus psyllid is the carrier of this disease, spreading it by feeding on the stems. It will slow down the tree’s nutrient intake and stunt the oranges’ taste and growth, eventually killing the tree.

Identifying Symptoms

HLB-infected orange trees may be asymptomatic for quite some time. Once the symptoms appear, you will find yellow veins and asymmetrical blotches on the leaves as they defoliate.

The oranges will also drop prematurely, appear deformed and discolored (green ends, even if ripe), and taste sour or bitter.

Treating HLB

Just like with citrus canker and leprosis, treating an orange tree disease such as this is not an option. You would have to remove the infected tree to stop the spread of the bacteria.

You have to be vigilant when inspecting your orange tree for any signs of HLB. Early detection will give you plenty of time to save nearby trees because removing more than one is overwhelming.

The Asian citrus psyllid is a major concern when preventing an orange tree disease like this. So you may want to put up a tightly woven screen house around your trees to keep out psyllids.

Additional Action

Should you see a psyllid or any signs of it on your tree, you are advised to catch the bug(s). Next, call the California Department of Food and Agriculture (CDFA) hotline at 1(800) 491-1899 or your local extension service.

Sooty Mold

Sooty mold is more of a fungus, a black leaf fungus at that, than an orange tree disease. Nonetheless, it should still be treated as such if you want to prevent your oranges from appearing sickly.

Closeup of sooty mold on leaves.

When insects like whiteflies, aphids, and leafhoppers secrete honeydew on the tree leaves and the fruit itself, mold will grow. With that, the fungus can grow as it feeds on the honeydew.

Identifying Symptoms

Blackened and powdery spots on orange tree leaves and the fruit are a surefire sign that sooty mold is present.

Treating Sooty Mold

To remove these bugs, you will want to spray the tree, especially on both sides of the leaves. An insecticide like Neem Oil will help you control these bugs.

After removing the insects, you can wash the blackened spots off with soap and water. You can even spray a steady or heavy stream of water with soap mixed in to speed up the extermination.

Once you accomplish that or just have the bugs’ numbers under control, sooty mold should go away very soon!

Taking Action Against Orange Tree Diseases

Oranges on a tree.

You now know of a few orange tree diseases to watch out for. With research and a green thumb for oranges, you can take action to protect your source of vitamin C!

Excited for more orange content? Check out our orange trees page to start learning everything there is to know about your favorite citrus!