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Orange Tree Fungus: What You Need to Know

Have you ever had a frustrating problem with your orange trees that you couldn’t solve?

Growing your own oranges is fun and rewarding but you have to watch out for diseases. Orange tree fungus can devastate your trees, so it’s important to know what to look for and how to treat different types of orange tree fungus.

Orange tree fungus may show up as spots like the ones on this lemon tree leaf.

Since fungal infections are often caused by already existing fungus or excessive wetness, prevention can be challenging.

Keep reading to learn how to identify, treat, and prevent different types of orange tree fungus so you can enjoy the fruits of your labor!

Types of Orange Tree Fungus

There are several different types of orange tree fungus, each with its own characteristics and treatment methods.


Anthracnose, Colletotrichum gloeosporioides, is most likely to affect your orange trees in wet weather.

A pumelo infected with anthracnose.

This means you’ll want to keep a close eye out in spring and at the end of the season as well when rainy weather picks back up.

How to Identify Anthracnose

To determine if your orange trees have anthracnose, look to see if any trees are losing leaves before the end of the season. You’ll also notice spores all over the leaves.

Fully ripe oranges will have red, green, or black streaks on them as a telltale sign of this orange tree fungus.

How to Treat Anthracnose

The best thing you can do to treat anthracnose is to apply the fungicide of your choice to the entire tree.

Be sure to follow the directions on the label very carefully for the best results.

Armillaria Root Rot

Armillaria root rot, also known as mushroom root rot or Armillaria mellea, is typically transmitted from old pieces of wood or leftover stumps already carrying the fungus.

Mushrooms are a sign of armillaria root rot.

This allows the infection to remain in the ground where it can later affect new orange trees.

How to Identify Armillaria Root Rot

You might be dealing with a case of armillaria root rot if you’ve noticed any of your trees suddenly wilting and deteriorating.

Your tree will likely start dropping leaves. You may also notice rotting bark or mushrooms growing at the base of the tree, as well as lesions popping up on the trunk.

Upon closer inspection, there could be fungus growing between the inner wood and bark of the tree.

How to Treat Armillaria Root Rot

Unfortunately, armillaria root rot is an orange tree fungus that can be tough to get rid of once it sets in.

Start by removing any diseased trees and, most importantly, the entire root system. Wait at least a year before planting again in that space.

You can also try fumigating the soil, although it may not completely destroy all of the remaining spores.

Black Root Rot

Black Root Rot, Thielaviopsis basicola, is an orange tree fungus usually associated with trees grown in greenhouses.

A pea plant infected with thielaviopsis, black root rot.

Fortunately, it often resolves on its own once the affected trees are transplanted to their permanent area.

How to Identify Black Root Rot

Black root rot is characterized by spots on the roots that are brownish-black, and the roots may ultimately turn completely black.

Eventually, the outside layer of the root system may begin to come off. Because the roots are so profoundly affected, the tree will also start dropping its leaves.

How to Treat Black Root Rot

Treating black root rot is really about prevention.

Foster strong root growth while young trees are still in the greenhouse to ward off this orange tree fungus. This can be accomplished with adequate lighting, proper temperature throughout the space, and using quality potting soil that’s well-aerated.


Melanose, Diaporthe citri, is an orange tree fungus that spreads to nearby trees in splashed water or is carried further away by the wind.

A lime infected with melanose.

How to Identify Melanose

If you suspect you have a case of melanose, look for small brown spots that start out pushed in, then become raised with a yellow circle around them. They’ll also have a cork-like texture.

New leaves may be crinkled and deformed if the infection is extreme. Large spots or even pustules may appear on the oranges.

How to Treat Melanose

The best way to combat melanose is with consistent fungicide use.

You can usually prune away any infected parts on younger trees, but this doesn’t typically work for more mature trees.

Brown Rot

Brown rot, Phytophthora spp., becomes a threat when the weather is wet and cool.

Brown rot lesions on a tree.

How to Identify Brown Rot

To determine if your trees have a case of brown rot, check any nearly-ripe oranges for wet tan or dark brown lesions that smell.

This orange tree fungus will also cause browning of the blossoms and twigs.

How to Treat Brown Rot

Brown rot thrives in moist conditions, so be sure to manage the space around the trees to avoid the leaves getting wet.

Trim grasses around the trees to keep them short. Keep up with pruning and don’t allow any branches to reach the ground.

Proper irrigation maintenance is also crucial when dealing with this orange tree fungus.

When you see the infection popping up on oranges, don’t pick any from that tree until all the affected fruits fall on their own.

Finally, treat the leaves of the tree with copper fungicides.

Preventing Orange Tree Fungus

While you can’t guarantee you won’t end up with a case of orange tree fungus, good management practices can help you avoid problems.

Quality soil and a well-maintained area immediately surrounding your trees will go a long way.

Prune your trees well to allow air and sunlight to circulate through the branches, leaves, and fruit. Avoid injuries to the tree that might allow fungus to set in through any weak spots.

And of course, you should always regularly check your trees to catch any problems early on.

Don’t Let Orange Tree Fungus Ruin Your Season

Orange tree fungus comes in many forms, so keep a close eye out for any symptoms that show up.

Mandarins on a tree.

Now that you know the signs of the various types of orange tree fungus, you know when to take action to save your trees whenever possible.

To grow the best trees and most delicious oranges, visit our Orange Trees page for lots more orange-related blog posts and guides!