The 11 Most Likely Reasons Your Oak Tree Looks So Sickly

Written by Katarina Betterton
Updated: September 18, 2023
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Oak trees remain a favorite, common tree in much of the world. Their hard wood provides the basis for much of the world’s crafted furniture and their longevity inspires resilience in those who use the tree as a symbol for themselves.

As wonderful as oaks are, they’re not invincible. Like any living thing, oaks become susceptible to various diseases, issues, and problems. If you have an oak and you think it looks a little worse for wear, you may not be wrong. 

Keep reading to discover 11 common reasons why your oak tree may look sick, the diseases that befall oaks, and how to prevent future sickness in this — or any other — oak tree. 

11 Reasons Your Oak Tree Looks So Sickly

Bur Oak

Oaks live a long time, even with disease. At McConnell Springs Park, a huge Bur Oak tree exists that’s over 250 years old.

©Steve Quinlan/

Disease, dense soil, and lack of nutrition make up just a few of the reasons your oak tree looks sick.


Did you know that oak trees can suffer specific diseases that not all tree species experience? That’s right — there’s even a disease called oak wilt. Watch out for the following diseases that your oak tree has the chance to contract.

Oak wilt. Caused by the fungal organism Bretziella fagacearum, oak wilt is a fast-acting and contagious disease that can kill an oak tree within three to six weeks of initial signs of rot.

Sudden oak death. The plant pathogen Phytophthora ramorum causes devastating oak disease, creating “bleeding” cankers on the oak’s trunk. While the name denotes a fast-acting pathogen, sudden oak death actually takes two or more years to fully kill its host.

Oak leaf blister. Another fungus, Taphrina caerulescens causes oak leaf blister. Nearly every oak subspecies contracts the fungus when there’s a cold, wet spring. 

Oak canker. Hypoxylon canker isn’t very aggressive (going after weak trees instead of those with the strongest roots). However, this disease remains a constant threat to oak owners because contracting it opens the tree up to many other dangers, too.

Anthracnose. Known broadly for a specific set of fungal infections, anthracnose appears on your oak’s leaves with dark lesions.

Leaf scorch. Like the burnt edges of paper, leaf scorch affects the outer rim of an oak leaf. This physiological disorder is actually a negative reaction to the environment. If your tree has leaf scorch, it’s telling you something is wrong with it. 

Armillaria root rot. Armillaria is a soil-borne fungus that will slowly chip away the life of your oak. It survives a while and can travel and infect new roots. Depending on how deeply the tree was affected, Armillaria root rot could spell death for your oak.

Soil Compaction

grass seeds begin to grow on new soil in the garden

Soil needs to breathe to get the nutrients from the roots to the leaves of an oak.


The way that soil surrounds your oak tree may contribute to its poor health. According to Gene Caballero, the co-founder of GreenPal and a landscaping expert with 20 years of experience, “Oaks need well-draining soil. Compacted soil can suffocate roots and hinder water and nutrient uptake.”

Soil compacts in a variety of ways, including construction when machinery rolls over it, or consistent, heavy foot traffic in places like hospitals, schools, museums, and government buildings. 

Improper Fertilization

If you’re seeing discoloration or a halt in the growth of your oak tree, you may have a problem of under or overfertilization. Oaks need fertilizer with balanced levels of potassium, phosphorus, and nitrogen. If you’re not using a fertilizer with those nutrients, or if you’re using too much of it, you may be making your oak tree sick.

Excessive Thinning

Avoid being too shrub-scissor-happy when pruning your oak. If you pruned your oak tree before its decline, there’s a chance that its poor health is from an over-pruning problem. Sure, it might look more tidy in your front yard with a trimmed-up oak — but oaks need as many leaves as possible to undergo photosynthesis. Cutting away too many branches may have robbed your oak of the nutrients it needs to thrive.

Inadequate Sunlight

Planting an oak tree in direct sunlight helps it grow — just like any other plant. If you’ve planted your oak tree somewhere that doesn’t get at least six hours of sunlight a day, your oak’s health problems might be Vitamin D related. Of course, it’s not actually Vitamin D that your tree needs. Because the sun’s rays assist trees in photosynthesis, getting adequate sunlight is the only way for your oak to feed itself.

Lack of Mulch

Oaks need mulch. It’s more than an aesthetic addition; mulch provides significant benefits to an oak for its health. Not only does it act as a protective layer over the soil, it regulates soil temperature and prevents extreme fluctuations that stress tree roots. 

Robert Silver expounded upon the use of mulch for oak trees in conversation with A-Z Animals. “It creates a barrier that reduces competition from weeds, which can rob the oak tree of essential nutrients and moisture. Without mulch, the soil around the oak tree may become dry, hot, and overrun with weeds, all of which contribute to tree stress and potential health problems.”

Insect Infestation

Borers remain the bane of oaks’ existence in the insect world. That’s not the only insect that could infest — and affect — your oak.

“Certain pests like borers, caterpillars, and beetles can cause significant damage to oak trees, affecting their overall health and vigor,” said Eoghan McHugh of Protect Earth, an environmental nonprofit that focuses on habitat restoration through creating greenspaces using native-only species.

Borers in particular pose a deadly risk to oak trees because their tunneling can cut off nutrient pathways that keep the tree healthy. This tunneling behavior, as well as reproduction, also can result in wilting, canopy thinning, and branches dying off. 

Physical Damage

You might not have meant to shoot your oak during archery practice, your kids didn’t mean to break a branch with their swing, and your new puppy didn’t know better than to eat the bark off the side. Still, the physical damage that comes with the wear and tear of an oak can depreciate its health quickly. Why? Because the inner bark’s exposure to the elements makes it more susceptible to disease and insect infestations.


Like other plants, oaks will fight for nutrients in the soil and sun in the sky. If there are a plethora of surrounding plants — especially larger or more established trees — your oak may suffer from a simple lack of resources.

Environmental Stress

Many environmental factors can impact an oak tree’s health. Outside of insects, external environmental situations like pollution, construction, and even extreme weather conditions can cause an oak to wilt or die. Severe weather events like tornadoes, hurricanes, and heavy snowfall can decimate branches, roots, or an oak’s trunk. After it’s safe to go outside, take full stock of the damage to your oak to figure out a plan forward.

Improper Watering

Overwatering and underwatering an oak tree have the potential to kill it. While you won’t drown your oak, you can make it more susceptible to disease by not watering it properly.  Do you know the right amount of watering for your tree’s size and age? Are you watering it near the trunk or the roots? For many native old oaks, owners should irrigate or water at least 10 feet away from the trunk of the tree. Usually, once a week is enough water for an adult oak tree.

Can You Cure a Sick Oak Tree?

Keep an eye on the leaves of your oak tree to know the difference between color changing for the season or for sickness.


While you can halt or slow the spread of a disease in its early stages, sick oaks probably can’t be fully cured if there’s any advanced damage. 

“Once an oak tree becomes sick, it cannot be cured in the way that humans can be treated for illnesses,” said Robert Silver, a gardening expert, writer, and speaker with over two decades of experience. “Trees don’t have an immune system like humans, and once they are diseased or damaged, they can only be managed or isolated to prevent further spread of the issue. Pruning affected branches, improving soil conditions, or using fungicides may help mitigate the problem, but it’s not a guaranteed cure.”

Other mitigation techniques include sanitation, fungicides, and insecticides. However, as Silver affirmed, these techniques are merely band-aids and do not fix the root problem.

Preventing Sickness for Your Oak Tree in the Future

When you suspect your oak tree may be suffering, there are a few different red flags to look out for. They include:

  • Exposed roots.
  • Early leaf loss.
  • Leaning to one side.
  • Yellow and/or brown leaves.
  • Growth halts.
  • Cracked bark.

On its own, one of these symptoms may not immediately spell a fatal diagnosis for your oak. However, the more of these signs you see, the more advanced a problem may be. It’s best to consult a professional arborist to get an accurate view of the extent of the issues.

So what can you do to prevent sickness from befalling your favorite oak?

Steve Wiltshire, a leading ecologist for Protect Earth, has worked in ecology for more than 15 years. In conversation with A-Z Animals, he let us know that total sickness prevention isn’t possible, and more and more threats bear down on natural species. The best line of defense is research before planting.

“One of the best strategies for planting oak trees is to match the right species to the right conditions (soil, altitude, temperature, light exposure) and to source acorns or saplings from as close to where the tree will be planted as possible,” he said. “This will mean the planted tree will be genetically acclimated to the local micro-environment.”

Other methods of keeping your oak tree healthy include: 

  • Keep an eye on your oak tree. Daily or weekly check-ins go a long way in preventing disease or problems from advancing to an unsalvageable stage.
  • Avoid using herbicides too close to the roots. Herbicides can kill or seriously injure the roots of a tree, so avoid spraying them whenever possible.
  • Avoid using lawn equipment near the tree. Running a lawnmower over an exposed root can do more than ding your lawnmower’s blade. Avoid using lawn, construction, or recreational equipment around your oak to preserve its health.
  • Change your care routine with the seasons. Following the seasons’ lead, care for your oak according to the temperature and precipitation of the current weather.
  • Follow the tree’s needs. During your check-ins with the tree, you’ll spot signs of any potential issues. Keep up with what your tree needs — and don’t be afraid to shift your care structure if it means you’ll have a healthier tree.

Keep Your Oak Trees Healthy

Despite the oak tree’s hardiness, some diseases and insects can significantly harm the health of it. As such, it’s vital to know the signs of sickness in your oak tree, which diseases may affect the health of your oak, and how to fix an oak’s health issues. While you may not be able to bring your oak back to its original state, you can take steps to mitigate the problem. 

The photo featured at the top of this post is © Smileus/ via Getty Images

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About the Author

Katarina is a writer at A-Z Animals where her primary focus is on dogs, travel, and unique aspects about towns, cities, and countries in the world. Katarina has been writing professionally for eight years. She secured two Bachelors degrees — in PR and Advertising — in 2017 from Rowan University and is currently working toward a Master's degree in creative writing. Katarina also volunteers for her local animal shelter and plans vacations across the globe for her friend group. A resident of Ohio, Katarina enjoys writing fiction novels, gardening, and working to train her three dogs to speak using "talk" buttons.

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