Common Tree Diseases And How to Treat Them
Just imagine. You walk out of your house and take in the view of the garden you’ve worked so hard to make beautiful. But something looks wrong. You look at your favorite tree and see it looks different.
It could be damage from an event, like a bad storm. Or from something climate-related: extreme cold or heat, for example.
Or it could be a disease.
If you rule out the first two, how do you figure out if your tree is diseased? And if it is, what do you do about it?
Below is a tree disease guide that gives you options and steps to take to save your trees, if possible.
Where to Look for Clues
The first thing you want to do is narrow down the potential problem. You can do that by looking at these four principal parts of your tree.
The Leaves
Look for discolored blotches or dead areas on the leaves; these could take the form of different shaped brown areas, some powdery mildew or dark, sooty mold.
The Trunk
A growth coming out of the trunk, usually from a wound or hole in the bark can take many shapes and sizes.
The Branches
You might see branches broken where they attach to the tree, or where they split. Check if the wood at the break is soft or crumbly. Or look for unusual growths coming out of the branch itself.
The Roots
While harder to see, check for visible broken roots or evidence of fungus on any exposed roots.
Take notes and pictures of everything you find, and jot down the exact kind of tree you have.
Sadly, because the disease-causing organisms such as bacteria or fungi are rarely visible to the naked eye, what you are looking for are symptoms that are only obvious once the tree is diseased.
REMEMBER
The sooner you identify a disease and treat your tree, the greater the chance of saving it. And if it’s contagious, you might be saving any nearby trees, too.
Do You D-I-Y or Do You Get Some Help?
Answering a few questions will help you decide when you can treat the tree yourself and when you need help.
- How important is the tree to me?
- If it has a contagious disease, how important are the nearby trees to me?
- Regardless of how I identify the disease, will I have the spraying, pruning or removal equipment needed to solve the problem?
- Do I know enough about different diseases or should I contact my local Cooperative Extension Service?
- Although it will cost money, am I better off having a Certified Arborist take over disease identification and whatever action is required?
How to Contact Your Cooperative Extension Service
The Cooperative Extension Service in your county is your friend when it comes to caring for your trees, whether your questions are about gardening pests or tree diseases. Most Extension Services have volunteers called Master Gardeners who help Extension agents answer questions.
Your local Extension Service can be located here:
http://npic.orst.edu/pest/countyext.htm
Steps to working with your Extension Service:
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Find your local Extension Service and call them.
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Tell them the type of tree and the symptoms you have seen.
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From your description, they may be able to tell you if the disease is parasitic (or caused by a living organism) or non-parasitic (age- or environment- related).
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If they need more information to help you, they might suggest you mail a sample of the diseased part of your tree to the state identification lab. Follow their exact instructions.
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Based on their information, identify the action plan to care for or if needed to remove the affected tree.
How to Find a Certified Arborist
If you decide to bring someone in to care for your tree’s health, look for an arborist. These professionals specialize in trees and other woody plants. You can choose between two types of professional arborists: Certified and Consulting.
Certified Arborists are more full-service, offering to plant, prune, transplant, fertilize, monitor and treat for disease and insects, and remove trees. You will want an ISA-certified (International Society of Arboriculture) arborist who is licensed, bonded and insured.
Cost Varies greatly with service provided.
You can find one here: https://www.treesaregood.org/findanarborist/findanarborist
And, you can certify an ISA credential here: https://www.treesaregood.org/findanarborist/verify
Consulting Arborists are less hands-on. They will typically do tree appraisals, diagnose a problem, recommend treatments and suggest a competent tree service. You will want a member of the American Society of Consulting Arborists (ASCA).
Cost Generally, $75-150 per hour, depending on complexity, level of liability, skill and seniority of the consultant.
You can find one here: https://www.asca-consultants.org/search/custom.asp?id=3818
Rather than just doing online research to find an arborist, you might want to ask knowledgeable neighbors or a trusted tree nursery owner for a recommendation.
Want to Be Your Own D-I-Y Tree Disease Doctor?
Want to figure it out yourself? Okay.
The list of tree diseases is long, but they come mostly from pathogens such as bacteria and fungi. Many of these are around all the time, but are ‘opportunists.’
If your tree is weakened or stressed, it cannot fight off the disease and your tree will become affected.
The Top Ten Diseases and What They Look Like
Tree diseases are named for the symptom you see or the type of damage they do, and some symptoms change names as they progress. For example, Leaf Spot (caused by fungi or bacteria) usually has distinct borders. Yet once it progresses, and spots begin to touch one another, the disease is called Leaf Blotch or Blight.
Here are the top tree diseases you will find in hardwoods, evergreens and palms.
Common Hardwood Tree Diseases
Canker
What it is
One of many different bacteria or fungi or bacteria that enter through a wound or injury on the bark and kill that area of the bark, branch or trunk. Ever-present, once pathogens enter the tree, they cause swelling.
What trees it affects
Many trees, including spruce, pine, poplar, willow, juniper, Russian olive, Douglas-fir, arborvitae, honey locust, oak and maple.
Outcome
If pruning the affected branch is impossible, remove the tree.
How to prevent
Avoid stressing a tree environmentally, injuring it mechanically (lawn mower or pruning wounds) or allowing damage by animal or insect.
How to treat or control
As soon as possible, cut off the affected branch using proper pruning methods. No chemicals effectively control the canker-causing disease.
Heart Rot
What it is
One of many fungal agents and pathogens that enter trees through open wounds. They expose bare wood and travel to the core of the tree; a conk or mushroom is the first sign of infection.
What trees it affects
Many deciduous trees, especially oaks, beech, birch, cedar, dogwood and maple. Resinous conifers have some resistance.
Outcome
Over time, it slowly destroys the tree’s support structure and can cause the tree especially large mature ones to fail and collapse.
How to prevent
Practice good tree care, minimize pruning wounds, shape young trees to avoid major branch removal later and remove broken branches after storms.
How to treat or control
No economically feasible fungicide is available.
Leaf Spot
What it is
One of many fungi and some bacteria that leave spots on affected leaves.
What trees it affects
Many different trees, including aspen, poplar and ‘stone’ fruits.
Outcome
Defoliation; only if defoliation occurs more than three years in a row will it adversely affect an established tree.
How to prevent
Practice good tree care, avoid crowding, thin out branches, do not top trees and rake up leaves in the fall (bury or compost them).
How to treat or control
Identify the exact leaf spot variety and specific pathogen (perhaps through your Extension Service) to know the most effective fungicide and spraying schedule.
Anthracnose
(an especially harmful version of Leaf Spot)
What it is
A very common fungus-caused disease that leaves discolored blotches or dead areas on the leaves. (Another version disrupts the vascular system of the tree.) It is widespread in the eastern U.S. and may become severe during years with long rainy spells.
What trees it affects
Many trees, especially dogwood, maple, ash, sycamore, oak and walnut.
Outcome
Repeated occurrences may weaken the tree and leave it susceptible to other diseases, but anthracnose will rarely kill a tree.
How to prevent
Remove and destroy any fallen leaves, twigs or branches.
How to treat or control
Fungicides have limited success. Prune dead or diseased branches and dispose of carefully. Prune during dry weather and sanitize pruning tools after each cut with a bleach-and-water 1:4 mixture.
Powdery Mildew
What it is
A white powdery substance found on the leaf surface. It travels by fungal spores carried by the wind and is most severe on young leaves and shoots.
What trees it affects
All kinds, but most commonly maple, oak, crabapple, dogwood, linden, lilac, catalpa, basswood and magnolia.
Outcome
It disfigures, but rarely kills the tree.
How to prevent
Minimize moist conditions by not planting trees in shaded areas, and keep trees pruned to allow for good air circulation.
How to treat or control
Spray with a chemical fungicide such as sulfur, or with baking soda and water for non-chemical solution.
Root and Butt Rot
What it is
The most common disease affecting hardwoods: one of many different fungi that rot the roots and cause decay of tree butts. It most commonly affects older trees or those that have suffered root or tree-base injury.
What trees it affects
Many hardwoods.
Outcome
Trees with extensive root rot will die in a few years.
How to prevent
Avoid root damage and wounds to lower trunks, keep soil healthy, do not plant where other trees died of root rot; contact your Extension Service about sterilizing your soil.
How to treat or control
No known effective treatment; pruning the crown and fertilizing the tree may extend its life.
Sooty Mold
What it is
A dark fungus that grows on the sap of some leaves or on the ‘honeydew’ that sucking insects leave behind.
What trees it affects
All kinds, but most commonly elm, box elder, linden and maple.
Outcome
It rarely damages the tree, but is unattractive.
How to prevent
Keep the tree well-watered and minimize the population of aphids, mealy bugs, etc.
How to treat or control
Wash the mold off the infested tree foliage with water, or spray with chemicals that control aphids and other sucking insects.
Verticillium Wilt
What it is
A fungus found in the soil that enters the tree through its roots, blocks the flow of moisture to the leaves and causes them to wilt. Sometimes it affects only one or several branches.
What trees it affects
Many trees, but most commonly ash, maple, redbud, yellow poplar, elm and catalpa, primarily when found in landscapes (versus natural forests).
Outcome
It is in incurable, but extreme care and pruning of affected branches may extend the tree’s life.
How to prevent
Keep the tree as healthy as possible and treat soil by solarization before planting.
How to treat or control
The pathogens can remain in the soil for years, and management of the disease is a challenge.
Common Diseases in Evergreens
In addition to many of the diseases found on hardwood trees, these two diseases are found in evergreens.
Needle Blight on Evergreens
What it is
Yellow or tan bands on needles, caused by fungi.
What trees it affects
Found in more than 35 pine species, but mostly Austrian, Monterey, Lodgepole and Ponderosa pine; not found in the western Great Plains or the central Rocky Mountains.
Outcome
Premature defoliation in mature trees and severe stunting and death in young trees.
How to prevent
Plant resistant pine strains.
How to treat or control
Apply a protective fungicide at budbreak; prune severely infected branches.
Tip Blight on Evergreens
What it is
Various fungi that develop at the lower part of evergreens and move upward. Dead shoots will appear with lots of resin and deformed ‘witches broom’ clusters.
What trees it affects
Various pines, arborvitae, cypress, fir, redwood and juniper.
Outcome
Most severely damages trees over 30 years old.
How to prevent
Practice good sanitation.
How to treat or control
Prune and destroy cones and branches, and apply fungicides starting at bud swell.
The Common Disease in Palms
While palms have a whole other set of diseases, the most common is this one.
Needle Blight on Evergreens
What it is
A bacteria-like organism that causes the lowest fronds to yellow, then brown, until the entire crown is affected. The growth tip dies after half the crown has yellowed.
What trees it affects
Date palm, coconut palm and other species.
Outcome
Trees usually die within 3-5 months of first symptoms.
How to prevent
It is hard to control the population of planthopper insect that transmits it. Plant only resistant species (that is, most natives).
How to treat or control
Injecting an antibiotic into the trunk can help manage the disease. Remove all moderately infected plants.
Diseases Common to Certain Types of Tree
If you have one of these trees, the first thing to look for are signs of the following diseases.
Dogwood Anthracnose can affect flowering and Pacific dogwoods.
Hazelnut Eastern Filbert Blight could be the culprit.
Elm Dutch Elm disease has been known to decimate elms, including American, winged, September, slipper, rock and cedar elms.
Oak Oak Wilt can affect many species of oaks, plus coast redwoods, Douglas firs and other trees and is transmitted by infected beetles. Sudden Oak Death kills many types of oaks.
Walnut Black walnut trees can be affected by Thousand Canker Disease.
Pine Pine Wilt can affect non-native pines such as Austrian, Scots and both red and black Japanese pines.
NOTE: Treating Trees with Chemicals
Are you using pesticides, fungicides, insecticides or herbicides? Always confer with your local Extension Service, a Certified Arborist or a qualified nursery professional.
Would you rather go organic? Check into friendlier solutions like Integrated Pest Management (IPM) or Plant Health Care (PHC).
Why You Should Try to Save the Tree
Unless the disease is contagious and you risk losing more trees, there are at least five reasons why you should do everything you can to treat and save a diseased tree.
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Trees lower energy bills A tree that blocks the sun from hitting your house or roof will lower your air conditioner bills. In winter, a tree can serve as a windbreak against the chilling effect of a north wind.
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Trees can raise a home’s curb appeal and selling price While landscapers will hesitate to put a fixed value on a tree, the right tree in the right place (especially mature ones) can increase the attractiveness of a house to a potential buyer.
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Trees can offer refuge for wildlife Having birds and small animals that feed off or inhabit a tree can add excitement and pleasure to the garden.
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Trees can provide privacy A tree that creates a visual barrier from neighbors can provide much-appreciated privacy.
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Trees can have sentimental value A tree may have witnessed your children’s infancy-or an entire lifetime-and have great sentimental value.
If You Cannot Save a Tree…
If all else fails, and you have to remove a tree, be sure to check out our guide to tree removal and cutting service.
Also, check with your local county to see if you need a permit to remove a tree (usually anything with a trunk diameter greater than three inches at chest height). In some counties, you do. Working only with certified, licensed professionals will ensure you break no regulations.