White Spots On Tree Bark

If you’ve noticed white spots on tree bark, you’re right to be concerned. These white spots on tree bark are often one of the first visible signs of a fungal infection, and understanding what they mean is key to protecting your trees. This guide will help you identify the cause, take action, and prevent future problems.

That patchy, powdery, or crusty white substance on your tree isn’t just dirt or lichen. While some white growths are harmless, many signal an active fungal issue. Fungi are simple organisms that thrive in moist, poorly ventilated conditions. They feed on organic matter, and when they target a living tree, they can cause anything from cosmetic damage to severe decline.

Common Fungal Culprits Behind White Spots

Not all white fungi are the same. Correct identification is your first step toward an effective response. Here are the most frequent offenders you might encounter.

Powdery Mildew

This is one of the most recognizable fungal infections. It looks like someone dusted your tree’s leaves and sometimes young bark with white flour.

Appearance: White to grayish, powdery coating.
Common Targets: Dogwoods, crabapples, lilacs, and oaks.
Impact: Primarily weakens the tree by stressing it and reducing photosynthesis. Rarely fatal but makes the tree vulnerable.

Hypoxylon Canker

This is a serious disease often appearing on stressed trees, like oaks and aspens. The white spots are an early stage.

Appearance: Starts as a white, gray, or yellow fungal mat under peeling bark. Later turns to black, crusty patches.
Impact: Severe. It kills branches and can lead to the tree’s death. There is no cure.

White Rot Fungi

This group includes fungi like the artist’s conk. They decay wood, breaking down both lignin and cellulose.

Appearance: White, stringy or spongy growths, often as shelf-like brackets (conks) protruding from the bark.
Impact: Causes structural weakness in the heartwood. The tree may become a hazard.

Lichen: The Innocent Bystander

It’s crucial to distinguish lichen from harmful fungi. Lichen is a symbiotic mix of algae and fungus.

Appearance: Crusty, leafy, or shrubby growths in white, gray, green, or yellow. They sit on the bark, not in it.
The Truth: Lichen does not harm the tree. It simply uses the bark as a place to live. Its presence often indicates good air quality but can suggest the tree is growing slowly, which might be due to other stresses.

Step-by-Step: Diagnosing Your Tree’s Problem

Follow these steps to figure out what you’re dealing with before taking action.

Examine the Spot Texture: Gently touch the white material. Is it powdery (mildew), crusty (lichen or canker), or fleshy (rot fungus)?
Check the Bark Underneath: Is the bark beneath the spots intact and firm, or is it peeling, soft, or discolored? Fungal infections often damage the bark itself.
Look for Other Symptoms: Check the leaves. Are they spotted, wilting, or falling early? Look for oozing sap, cracks, or insect holes nearby.
Consider the Tree’s Health: Has the tree been under stress from drought, construction damage, or soil compaction? Stressed trees are far more suseptible to infection.
Consult an Expert: If you’re unsure, take clear photos and contact your local county extension office or a certified arborist. They can provide a definitive diagnosis.

Effective Treatment Strategies

Your treatment plan depends entirely on the specific fungus. Never apply a treatment without a good idea of the cause.

For Powdery Mildew and Similar Leaf/Bark Fungi

Prune affected branches: Improve air circulation by thinning the canopy. Always sterilize your pruning tools between cuts with a solution of one part bleach to nine parts water.
Apply fungicides: For severe cases, use a registered fungicide. Neem oil or potassium bicarbonate solutions are organic options. Apply as directed on the label, usually starting at bud break and continuing as needed.
Clean up debris: Rake and destroy fallen leaves in autumn to reduce spores that overwinter.

For Wood Decay Fungi (White Rot)

Once you see fungal conks, the internal decay is advanced. You cannot reverse it.

Evaluate safety: Have an arborist assess the tree’s structural integrity. It may need to be removed if it poses a risk to people or property.
Prevent spread: Avoid wounding the tree, as fresh wounds are entry points for spores. Maintain the tree’s overall health to slow decay.

For Hypoxylon and Other Cankers

There are no effective chemical controls for these diseases.

Focus on prevention: The best defense is keeping the tree healthy and unstressed with proper water and mulching.
Remove severely infected trees: To protect nearby healthy trees, an infected tree may need to be removed and the wood destroyed.

Prevention is Always Best: Cultural Practices

Healthy trees resist infection naturally. Make these practices part of your routine garden care.

Water Wisely: Water deeply but infrequently at the tree’s drip line (not at the trunk). Use soaker hoses or drip irrigation to keep foliage dry, which discourages fungal spores.
Mulch Properly: Apply a 2-4 inch layer of organic mulch (wood chips, shredded bark) in a wide ring around the tree. Keep the mulch a few inches away from the trunk to prevent moisture buildup against the bark.
Prune Correctly: Prune during the dormant season to remove dead or crossing branches and improve airflow. Make clean cuts just outside the branch collar.
Avoid Stress: Protect tree roots from construction compaction and grade changes. Use a balanced, slow-release fertilizer if a soil test indicates a need, but avoid over-fertilizing.
Choose Resistant Varieties: When planting new trees, select species and cultivars known for resistance to common fungal diseases in your area.

When to Call a Professional Arborist

Some situations are beyond DIY care. Call a certified arborist if:

The white spots are accompanied by large, dead branches or significant leaf loss.
You see shelf-like fungal conks or mushrooms growing from the trunk or roots.
The main trunk has large cracks, cankers, or is oozing sap.
The tree is leaning or you suspect root rot.
You are simply unsure of the diagnosis or the best treatment approach.

FAQ: Your Questions Answered

Q: Are all white spots on my tree bark bad?
A: No. Lichen, which is a mix of algae and fungus, is harmless and just uses the bark as an anchor. However, powdery or crusty spots that are part of the bark itself often indicate a problem.

Q: Can I save a tree with a fungal infection?
A: It depends on the type and extent of the infection. Superficial issues like mild powdery mildew can be managed. Internal wood decay or canker diseases are rarely curable, and the focus shifts to safety and prevention for other trees.

Q: What’s the difference between fungus and lichen?
A: Fungal infections actively feed on the tree’s tissues, causing damage. Lichen is a self-sufficient organism that sits on the surface without penetrating living tissue. Lichen feels attached but can often be pried off the bark.

Q: How do fungal spores spread to my tree?
A: Spores are microscopic and travel easily on wind, rain, insects, and even on your tools or clothing. They enter trees through natural openings or wounds in the bark.

Q: Will scraping the white spots off help?
A> For lichen, scraping is unnecessary but won’t hurt the tree. For true fungal infections, scraping the surface growth off does not remove the fungus inside the tree and can create wounds. It’s better to treat the underlying cause.

Q: Is there a general fungicide I can spray on any white spots?
A: No. Using the wrong fungicide is wasteful and can harm beneficial organisms. Always identify the issue first. For example, a fungicide for powdery mildew will have no effect on a wood decay fungus growing inside the trunk.

Spotting white marks on your trees can be worrying, but with careful observation and the right steps, you can address the issue effectively. Remember, the goal is not just to treat a single outbreak but to foster a resilient landscape where your trees can thrive for years to come. Regular care and early intervention make all the difference.