Orange Slime on Trees: What You Need to Know

Trees are an important part of the ecosystem. They do a lot for the environment, including providing oxygen, cleaning the air, and stabilizing the soil.

They also provide shelter and growing space for many different species including animals and funguses.

If you notice orange slime on the trunk or branches of your trees, don’t be alarmed.

This is most likely a harmless fungus known as “Orange Slime Mold”, or its scientific name “Fusicolla Orange Slime”.

orange slime on tree

This type of mold is often seen on woody plants like trees during or immediately after rainy weather. The mold itself is not harmful to the tree but can be unsightly.

In this post, we will go over what causes orange slime mold, whether it is harmful, and how to get rid of it.

What is Orange Slime Mold?

Orange slime mold is a fungal growth that often appears on trees during humid or wet weather. It looks like, well, orange slime! And it can range in color from pale yellow to bright orange.

The orange slime mold can grow on any type of tree as long as the conditions are right. However, trees that produce a lot of sap are more likely to be affected. Especially if the sap is running down the trunk or branches.

The orange slime mold is often mistaken for a disease, but it is not harmful to the tree.

In fact, mold is believed to break down organic matter and release nutrients back into the soil. So, in a way, it can actually be beneficial to the tree!

Having said that, we don’t know much about the symbiotic relationship between the orange slime mold and the tree, but it’s thought to be beneficial or at the very least non-harmful.

Why Does Orange Slime Mold Appear on Trees?

The mold appears on trees because the tree provides the perfect environment for the mold to grow. The tree gives the mold a place to live as well as food in the form of sugar from the tree’s sap.

Here are the most important reasons why the orange slime mold appears on trees:

Sap Flow

The sap is the lifeblood of trees. It is a watery solution that contains nutrients and hormones that the tree needs to grow. The sap also contains sugars which the tree uses for energy.

Once the sap leaks from the tree, it provides a food source for the mold. The sugars in the sap are used by the mold to grow and reproduce.

There may be many reasons why the sap leaks from the tree including:

The sap pressure in the trees is typically higher during the spring season. This is why we can effectively tap trees for syrup and why mold is often seen during this time of year.


Seasonality is an important factor that may boost the growth of the mold. The mold is most often seen in the spring and summer when the weather is warm and humid. However, it can also be seen in the fall and winter depending on the region.

Every tree goes through a natural process of seasonality. During certain times of its cycle, the tree may be more likely to develop orange slime mold.

A lot of times, the mold will appear after a heavy rainstorm when the humidity is high. However, mold can also appear during a dry spell if the conditions are just right.


Overwatering is an important contributor to the growth of orange slime mold. When trees are overwatered, the soil stays wet for long periods of time. This creates a humid environment that is perfect for the mold to grow.

Although there may not be sap flowing from the tree, the humid environment and organic matter on the bark may provide enough food for the mold to survive.

You can avoid overwatering your trees by:

If there are frequent rains in your area, you may not need to water your trees as often.

Always, check the forecast before watering to see if rain is in the forecast. By reducing the amount of water your trees receive, you can also reduce the chances of orange slime mold.


Underwatering can also cause orange slime mold. When trees are underwatered or cannot access water, they will start to stress.

This stress can cause the tree’s bark to become cracked and creates an environment for fungal spores to enter the tree. Once the spores are inside the tree, they can start to grow and form the orange slime mold.

Mold doesn’t need much water to grow. Even a small crack in the bark and a little bit of moisture can cause the mold to grow.

Therefore, it’s important to water your trees regularly with a planned watering schedule. This will help to ensure that your trees are healthy and not stressed.

Poor Health of the Tree

Although any type of tree can be affected by the orange slime mold, the mold is more likely to affect trees that are already in poor health.

Trees that are stressed from drought, pests, or disease are more likely to be affected by mold.

Although mold can appear on healthy trees, this is less common. The mold is more often found on trees that are already old or dying. This is because the bark of the tree is softer and easier for the mold to penetrate.

Older alders, maples, oaks, and willows are particularly susceptible to orange slime mold.

This is because these trees have a higher number of dead branches with cracks in the bark. The mold can enter through these cracks and quickly spread throughout the tree.

Insufficient Sun Exposure

Trees need sunlight to produce food through photosynthesis. If a tree is not getting enough light, it will be less able to fight off the orange slime mold. This is because the tree will be weaker and have less energy to fight the mold.

In addition, the lack of sunlight creates a prime environment for mold to grow. If your tree is not getting enough light, you may need to:

Incorrect use of Fertilizers & Pesticides & Fungicides

Fertilizers, pesticides, and fungicides change the chemistry and ecological balance of the soil and the plants.

If you are randomly using these products, you may be harming your tree without even knowing it.

Because incorrect use of these products can be stressing the tree which makes it more susceptible to the orange slime mold. Fertilizers change the nitrogen levels in the soil which may alter the delicate balance that is required for healthy trees.

Pesticides and fungicides can also be harmful to trees. These products are designed to kill living organisms, so they can kill beneficial organisms that help keep the mold in check.

If you are using any of these products, make sure you are following the instructions on the label. It is always a great idea to speak with an expert arborist or tree service before using any products on your trees.

How to Get Rid of Orange Slime Mold?

Although the mold is not harmful to the tree, you may want to remove it for aesthetic reasons. That is a perfectly understandable desire especially if the mold is on a tree in your front yard.

Here are a few methods you can use to remove orange slime mold from your trees:

Use a garden hose or pressure washer

You can use a garden hose or pressure washer to blast the mold off of the tree. This is the easiest and most effective method that doesn’t involve any chemicals.

By watering the bark of the tree, you will not only remove the mold but also the sap on the bark that is the food source for the mold.

If the mold doesn’t seem to be coming off with the water, you can add a small amount of vinegar or lemon juice to the water. This will help to break down the mold and make it easier to remove.

Avoid using cleaning products such as dishwasher soap, or bleach as these can be harmful to the tree and the surrounding environment.

If the mold still doesn’t seem to be coming off, you can try using a soft brush to scrub the bark.

We already know that fungal organisms thrive in wet conditions, so this method may not be completely effective and the mold could return. Still, it is worth trying this method first since it is the safest for the environment and your tree.

Even if the mold does come back, you will still be able to remove it with one of the other methods listed below. Therefore, it is best to start with the least invasive method and work your way up.

Prune affected branches

If the mold is localized to a few branches, you can remove the affected branches. This is a good option if the mold is on small branches that are easily accessible.

Pruning the branches will also help to improve the tree’s appearance and prevent the spread of mold. This is a good option if the mold is not too widespread on the tree, especially on the larger branches.

To prevent pruned branches from being exposed to mold, you can use a wound dressing or sealant. These products will help to protect the tree by creating a barrier between the pruned area and the environment.

Use a fungicide

Fungicides are a last resort option for removing orange slime mold from trees. This is because fungicides will not only kill the mold but also other beneficial microorganisms.

In addition, fungicides will have some impact on the health of the tree although the most modern fungicides are much safer than those used in the past. If you decide to use a fungicide, make sure to follow the directions on the label carefully.

Important Precautions

When removing mold with pressurized water, you can release spores into the air when disturbed, so it’s important to wear a mask. This will prevent you from breathing in the spores.

Don’t touch the mold with your bare hands, as it can cause allergic reactions in some people. If you need to inspect the mold up close, wear gloves.

Don’t forget to also protect your eyes! If you’re pruning branches, be sure to disinfect your tools afterward. You don’t want to accidentally spread the mold to other parts of the tree, or to other trees.

If you have pets or small children, it’s best to keep them away from the area until the mold has been removed.

Once you’ve removed the mold, take measures to prevent removed spores from coming into contact with the tree again. You can do this by placing a physical barrier between the tree and the source of spores, such as a tarp.

Immediately remove any fallen leaves or debris that may be contaminated with mold spores. If you have a compost bin, make sure the mold-contaminated material is not added to it. Rather, try to bury it in the ground if possible.

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