What’s Wrong With My Oak Tree? 10 Most Common Oak Diseases

Large valley oaks with mistletoe growing on the branches, sunol regional wilderness, san francisco bay area, california

Oak trees are one of the most popular trees in the world. Most commonly known for their height (often growing between 100 to 150 feet) and their adaptability, oaks are not immune to all threats. 

The most common oak tree diseases are oak leaf blister, armillaria root rot anthracnose, oak wilt, bacterial leaf scorch, powdery mildew, hypoxylon cankers, actinopelte leaf spot, mistletoe, and galls. In general, you can save your oak tree by nourishing your tree and removing affected branches.

So, what common issues should you look out for on your oak tree? We’re here to equip you with the best info to tackle the problem.  Keep on readin’ on!

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What Kills Your Oak Trees?

In truth, oak trees, while some of the more disease resistant trees out there, can still be susceptible to a multitude of problems.

Oak tree saplings for instance, are more prone to problems than a full grown oak. However, full grown oaks can still develop problems during their lifespan.

Below is a chart of the most common oak tree diseases, how to fix them, and how severe the issue is.

COMMON OAK DISEASETREATMENTSEVERITY
Oak Leaf BlisterWatch and MonitorLow
Armillaria Root RotRemove Infected Trees and RootsHigh
AnthracnosePrune Infected Branches and MonitorMedium
Oak WiltDirect control with fungicides or prevention via regulation.Red Oak-High, White Oak-Low
Bacterial Leaf ScorchRemove Infected Trees and Replant Resistant SpeciesMedium
Powdery MildewPrune Infected Branches and Remove Dead LeavesLow
Hypoxylon CankerPrune or Remove Infected TreesMedium
Actinopelte Leaf SpotMinimize Stress or Remove Infected SectionsMedium
MistletoeCut Infected Limbs or Break Off The TopsLow
GallsRemove Infected TwigsLow-High

If you’re interested, you can read our guide to the 10 early symptoms of a dying oak tree here.

Oak Leaf Blister

While most oak trees are susceptible to this fungus-based disease, it affects red and black oaks more severely. The level of severity is still low, regardless of the fact that some oaks are more susceptible than others. In cases of minor afflictions, little harm is caused to the oak tree and the symptoms may even be difficult to spot. 

Speaking of spots, this is what occurs when oak leaf blister is ranging toward the more severe end. The leaves begin to display patches that look like light green blisters, that turn brown and resemble a type of sap. These patches occur when fungus causes an overgrowth on affected leaves. 

Leaves with oak leaf blister grow faster than those without the tissue because the spores have more time to germinate during the winter freeze. The fungus typically lives in these spores before leaves have a chance to bud, and cause mayhem the moment that growth restarts in the spring.

Spores are spread, including those infected with fungus, in the fall months. This is a cyclical process that fungicides typically don’t have much control over. If you do try a fungicide, we suggest looking at this Natria’s Neem Oil Spray which works for both insects and fungi. You’ll want to apply this before the buds break in the spring. 

Armillaria Root Rot

Group of armillaria on on an old wooden stump

Armillaria root rot is high severity issue for an oak tree, where everything from affected roots to entire trees need to be removed. 

You can expect to see a slow decline in the tree and, especially in wet weather, mushroom structures will appear around the base and roots of the tree. These mushrooms are a honey color that doesn’t look nearly as harmful as they really are- but now you know. This is not a fungus to be messed with.

Armillaria root rot can be most easily identified by the clusters of the mushrooms, but there are also other signs. Trees with this affliction will fight poor growth, have needles that are browning and leaves that are yellowing at incorrect times of the year, along with the possible overproduction of seeds and pinecones. 

Flat white sheets of fungal growth may also pair with thin black fungal strands that meet near the base of the trunk. The extra resin may be seeping from the tree, or the wood itself may be soft and stringy. 

The symptoms are endless- what we listed is just scratching the surface. This highly severe affliction not only will kill your oak tree, but it will make the process a long, messy one.  

So… how can you save your oak tree?

Reducing stress on your oak tree can help stop armillaria root rot. Mulching the soil that sits around the base of the tree to add nutrients and water trees, especially during a drought when stress is running high can help save your oak tree. 

Also, avoid wounding your trees. Wounds make trees more susceptible to getting armillaria root rot- so avoid that at all costs. Keep lawnmowers, darts, weed whackers, power tools, and other sharp tools away from your tree. 

Anthracnose

Anthracnose is not the most severe oak tree affliction on this list, but it should be a cause for concern. You’ll notice that your oak leaves may be turning a brown, rusty color along the veins along the foliage of your leaf. Typically, anthracnose is most prevalent during the fall to oak seasons.

If you want to avoid excessive pruning of affected branches, the worry that it will spread, or closely monitor your trees, it is time to nip this in the bud. 

If you are just planting your oak trees, space them far apart so that they have adequate access to fresh air and plenty of sunlight.

Additionally, when planting your oak tree, make sure you’re doing it at the correct time of year. You can read our guide on the best time to plant an oak tree here.

Chemical control is an option, but there are other ways to treat this issue if it does happen to arise in your tree. Pruning, sanitation, and monitoring are going to become the trifecta for you in your journey to manage this oak tree affliction.

Oak Wilt

This fungal affliction finds prominence in the Southeastern United States and may cause different symptoms in red oaks versus white oaks. 

Transmitted via fungal spores, or possibly one of the many varieties of bark beetles, oak wilt can cause quite the distruption to your trees.

Getting in to it, oak wilt affects red oak trees and white oak trees in very different ways.

In order to understand the sections below about oak wilt in red oak trees and white oak trees, you may want to read our guide on the key differences of red and white oak trees.

Red Oak Wilt

In red oaks, oak wilt manifests as a more severe and rapidly escalating issue.

Oak wilt in red oak trees is prevalent a few weeks from infection until symptoms appear, and the tree can perish in as little as a few months after that. Oak wilt in red oak trees is a much more severe process than their white oak counterparts will face.

At first, the leaves of your red oak tree will begin to wilt as a bronze color appears. This typically works its way from the edges of the tree, like small limbs and the ends of branches, before making it to the rest of the tree, such as the trunk. The leaves will begin to drop gradually as they simultaneously discolor. 

The oak wilt fungus continues to grow and it produces a sort of sticky layer underneath the bark of an infected tree that ranges from light grey to a tan color. With a strong, unpleasant smell, this layer may begin to split the bark as it expands and the tree perishes.

Most often, red oak trees never survive from oak wilt.

White Oak Wilt

White oaks display their resistance when it comes to oak wilt because they can live for years after the initial infection occurs.

Oak wilt in white oak trees can be managed in a variety of ways, from regulatory prevention to direct control using fungicides.

Bacterial Leaf Scorch 

Bacterial leaf scorch is an oak tree affliction that causes a halt in water, reddening, or yellowing of a tree that is followed by the browning of leaves. This loss of hardiness sees leaves drop eventually, leading up to passing of the tree as a whole. Symptoms most often appear after a summer drought, when water movement is already low.

Usually, bacterial leaf scorch is spread by tree-hopping insects. 

The severity is at a medium level due to the fact that regression is a gradual decline. This means that an oak tree could survive an additional 5 to 10 years after bacterial leaf scorch has been present until it needs to be removed. 

Bacterial leaf scorch is a less selective disease, affecting red, white, pin, bur, and shingle oaks, along with sycamore trees, maples, and other varieties.

To treat this bacterial leaf scorch, you will want to not only remove affected trees but replant resistant tree species in the same area to avoid another outbreak in the near future.

Look into insect control such as Organic Deet-Free Bug Repellent. Spray treatments are not currently available for the affliction itself, but controlling insects that may spread bacterial leaf scorch is a great first step. 

Powdery Mildew

A close up of an oak leaf with powdery mildew

In essence, powdery mildew is going to look like someone spilled some baby powder on the leaves of your tree, more than likely.

Powdery mildew is characterized by the way that many fungi combine to appear as a white dusting on the leaves of an infected tree. The Clemson Cooperative Extension tells us that this disease is not fatal, but can cause plant damage when the infection becomes severe. The severity level of powder mildew is medium.

White spots are only the first sign of powdery mildew on your oak tree, however. Other symptoms include leaf curling and discoloration, among other abnormal growths.

The powdery fungal growth is not even always visible on your oak. Though when it is, it tends to begin on the lower sections of the leaves. In severe cases, abnormal growth of the oak tree’s leaves will shift into yellow or brown colored leaves that are dried and may soon fall off the tree. 

Powdery mildew management can first be solved if you notice white spots or discoloration on just a few leaves. Simply removing these leaves may keep the fungus under control. 

If applicable, prune the dead branches as you pull off any dead leaves at the end of the growing season. This will help prevent powdery mildew from spanning into the next season, and the one after that, and so on.

Hypoxylon Canker

Hypoxylon cankers are common inhabitants of many varieties of oak trees, but only begin causing symptoms when trees are under stress. Much like canker sores that humans get- the bacteria and pathogens react to the stress the host organism is under and begin to take effect.

Thanks to Texas A&M’s Agrilife Extension, we know that symptoms of this hypoxylon canker manifest in a way that reflects that of most other declining trees.

From yellowing or browning leaves to reduced growth rates, thinning canopy and branches, and white stringy sap around the base, hypoxylon cankers aren’t great news. They also aren’t as severe as anthracnose or oak wilt in a red oak tree.

This is more of a medium concern when related to the other common oak tree threats we are discussing here today.

To control hypoxylon canker on your oak tree, eliminating stress (or reducing it at the very least) is going to be the best plan of action. Vertical mulching is a technique that ensures nutrients are properly added to the soil around a tree, enriching the roots and fortifying the tree against stress-fueled disease. 

Remedial pruning may also be your saving grace in this situation because you can remove the parts of the tree most affected by stress before the hypoxylon cankers spread to the rest of your oak tree. 

If all else fails, tree removal may be the way to go for you. This white-rot symptom quickly kills trees once it has spread, and may become a hazard to the vitality of other trees or even a safety hazard to people and pets. 

Actinopelte Leaf Spot 

Another affliction of medium severity, actinopelte is caused by fungus spores that last over the winter and are agitated by rain and wind in the spring months. Leaf spots will appear and may join together to form larger and more irregular blotches on the leaves. A dark, reddish-brown color makes green leaves look particularly sickly, and the more severely affected specimens will fall to the ground prematurely.

The management of actinopelte revolves around ideas of transplanting, minimizing stress, and properly fertilizing the area- all of which go hand-in-hand to create a better environment for an oak tree to flourish.

Mistletoe

Green mistletoe hanging on the branches of a valley oak tree, sunol regional wilderness, san francisco bay area, california

Surely you’ve heard of mistletoe. I.E. ‘kiss me under…’, ‘ho ho the mistletoe’, you know, that sort of holiday tradition in the western world. Did you know that mistletoe is actually quite the parasitic pest? 

That’s right, mistletoe wraps itself around healthy oak trees and takes part of their water and minerals. The severity is low, however, because when it comes to food sources like chlorophyll, mistletoe handles that one on its own. Though mistletoe is not fully detrimental to trees, it still needs to be monitored.

To work on this problem, you’ll just need to cut off branches that are too tightly wound with this vine. Breaking off the tops of the mistletoe itself can also help.

Galls

Macro photo of tree pests eggs on oak leaves isolated on white background. Chemically induced leaf blister galls of a gall wasp neuroterus numismalis

These spots created by insects may simply make an oak tree look less appealing. However, they could also do enough harm to cause the oak tree to perish. Their severity level ranges from low to high for this reason: it is difficult to know what to expect when these little pests (gallmakers) join the party.

Galls are growths or swellings that are abnormal and most often caused by small wasps. One of the issues with these is that there is no way to cure galls on your oak tree fully. Once galls are on trees, they cannot be removed. 

Typically, galls cause aesthetic issues more than anything truly severe but in extreme cases, they can lead to the death of a tree. Often, the symptoms lie somewhere in the middle with severe leaf deformities or premature leaf drops.

Manage galls on your oak tree by simply removing and destroying fallen leaves that have been impacted by gall-makers. Prune or remove twigs that are sporting galls before holes have a chance to form, ensuring that this issue does not quickly spread. 

That’s All For Now!

Now that we know a little more about some common diseases and other threats to oak trees, it’s clear that these durable giants do have their limitations.

Ultimately, it helps to pick the right oak trees to plant from the get go. You can read our guide on the best oak trees to plant here.

If you want to keep your oak trees healthy and clear of threats, try using fungicides to prevent and eliminate these sorts of issues. You may also find that removing infected roots (and/or branches) works better than spraying, depending on what type of affliction your oak has, and at what stage you catch it.

We hope this helps you know what to expect, and have a better idea of how to tackle the problem!

References

Everitt, J. H., Escobar, D. E., Appel, D. N., Riggs, W. G., & Davis, M. R. (1999). Using airborne digital imagery for detecting oak wilt disease. Plant disease, 83(6), 502-505.

Juzwik, J., Appel, D. N., MacDonald, W. L., & Burks, S. (2011). Challenges and successes in managing oak wilt in the United States. Plant disease, 95(8), 888-900.

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