Identifying Common Tree Pests & Diseases: How To Protect Your Trees

Any number of problems can befall a tree, but these 6 pests and diseases are at the top of the list.

Spongy Moth
(Image credit: phototrip / Getty Images)

Just like diagnosing your own medical matters can be challenging, diagnosing issues with trees isn’t always easy. Of course there are some issues a gardener can figure out right away, but many require expertise. Some common tree pests and diseases are look-alikes for other problems, and some issues can only be determined after laboratory analysis.

While identifying tree diseases and pests can be tricky, the first step is to identify signs and symptoms. It’s important to keep in mind that most plant issues are not caused by diseases or insects, but by cultural issues like drainage, improper irrigation, or improper mulching.

Are Some Trees More Susceptible to Diseases & Pests?

Some trees are more vulnerable to attack from diseases and pests. Trees that have been improperly planted - e.g. planted in shade when they require sun - will not thrive. That makes them more susceptible to attack by pests or diseases.

Trees that are not given proper care can be stressed. This is particularly true of trees that are not given water when it is needed. Drought stress weakens the tree and lowers its defenses. Trees that are not properly pruned to create a strong branching structure may lose limbs, leaving them susceptible to disease or pests entering the trunk.

In addition, some trees have been developed to resist certain diseases. It is particularly important to look into disease resistance when you are planting a tree in an area where diseases in trees are prevalent.

And some tree species are more susceptible to certain diseases than others. For example, beech trees are vulnerable to bleeding canker and Laetiporus root rot, while spruce trees often are attacked by fungal diseases. Boxwood often is the victim of boxwood leafminer, a pest not likely to attack a cherry tree. The emerald ash borer kills millions of ash trees each year but has never been known to touch a redwood.

Do Tree Problems Depend On Hardiness Zone Or Climate?

A tree in any climate or hardiness zone can suffer from disease or be attacked by pests. The key is to select and plant trees that are appropriate for your area. It makes sense that a palm tree planted in Hawaii is going to have less problems than a palm tree planted in Alaska. To that extent, tree problems can depend on climate.

Common Tree Pests

1. Spongy Moth

Spongy Moth

(Image credit: phototrip / Getty Images)

The spongy moth is a tree killer. Its larvae are caterpillars that eat the foliage from over 500 tree species and is said to be one of the most destructive pests of hardwood trees. It kills by defoliating trees and destroys more than a million forested acres each year.

If you have a hardwood tree, look for the egg masses in springtime on tree trunks. They are yellow masses between 1 and 2 inches across, and each can have 500 eggs. The eggs hatch into larvae that eat through the leaves of oak, birch, elm and maple. A single larva can eat a square foot of tree leaves per day.

These pests are hard to stop, but if you find egg masses, you can drown them in kerosene or burn them. Using a horticultural spray in late fall and early spring can be helpful. Your best bet is to keep your trees healthy enough to withstand an attack with adequate water, fertilizer and pruning.

2. Western Tent Caterpillar

A cluster of tent caterpillars on a tree

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The western tent caterpillar attacks a wide range of trees including poplar, willow, cottonwood, quaking aspen and especially fruit trees. Look for their “signature” in spring: white silken tents in the branches. This will cause foliage to turn brown and leaves to fall, beginning at the top of the tree and the outer branches.

These caterpillars start as eggs left to overwinter on the tree. When they hatch into larvae, they spin a white web that expands as they grow. They turn into moths that emerge in summer and lay eggs. Even if you only have one tent caterpillar colony, the larvae can defoliate a small tree. Large trees are only harmed with multiple colonies.

Monitor your trees for this and, when you see a tent colony, cut out the tents and destroy them. Insecticides are also available for these pests.

3. Douglas-Fir Tussock Moth

Douglas-fir tussock moth caterpillars are the larvae of moths. The eggs are laid on spruces, Douglas-fir and true firs, and when the larvae hatch out, they feed on the needles. Don’t think this is a one-and-done event. The tussock moth stays in one place and trees are often attacked annually and suffer considerable damage.

Look out for these trees in late spring. You will see the the tree’s foliage will turn russet, starting at the top or outer branches, then moving throughout the tree. Keep your eyes out for egg masses on the canopy. If left unchecked, the tops of trees may die in one season, and the trees are weakened and can be vulnerable to bark beetles.

Fortunately, the natural enemies of Douglas-fir tussock moth often control these outbreaks after a few seasons. But monitor for egg masses. There are several insecticides that are effective against these pests.

Common Tree Diseases

1. Anthracnose

Leaves of a mango tree infected with anthracnose

(Image credit: Chantawit Chaytaweep / Getty Images)

Do you have any deciduous hardwoods in your landscape? These trees are vulnerable to a group of fungal diseases that attack tree leaves, generally termed anthracnose. It first appears on foliage as irregular spots, yellow or brown, that darken and expand over time. They become sunken lesions. This can result in the death of tips of young branches as well as defoliation.

Anthracnose spreads very easily, even by rain. The fungus survives on the debris of infected tree leaves, so one way to prevent spread is cleaning up the plant debris. It also helps to use drip irrigation, be sure your soil drains well and buy disease resistant plants.

For trees, prune out any infected branches and destroy the cuttings. prune out the dead wood and destroy the infected leaves. Using a dormant spray of Bordeaux mix is recommended.

2. Fire Blight

A young tree infected with fire blight

(Image credit: Andrey Maximenko / Getty Images)

Fire blight is a bacterial disease that attacks some 75 species of trees and shrubs, including apple trees, pear trees, crabapple trees and mountain ash trees. Fire blight kills flowers and young shoots, and can result in the tree’s death. If many shoots are infected, the tree looks as if has been scorched by a fire.

What to look for? If your tree has flowers, they start looking as if they were soaked with water, then they shrivel and die. New foliage turns gray and wilts, bending downward, forming a hook. leaves and shoots wilt and turn gray-green. Fruits can dry up and bark can peel.

Handle this disease by pruning out infected branches to prevent the disease from attacking the trunk. Do this while the tree is dormant and dispose of the clippings carefully. If you are not successful and the disease reaches the main trunk, the tree is doomed. Remove it from the landscape.

3. Armillaria Root Rot

Armillaria Root Rot mushrooms at the base of a tree

(Image credit: Elizabeth Fernandez / Getty Images)

Armillaria root rot is a fungal root rot. Many trees are vulnerable. In fact, because Armillaria results from many species of fungus, its host range is broad and includes almost every tree. The symptoms can include stunted foliage, dieback of branches, and needles turning yellow. The classic symptom is a white, rotting fungus in white fans on the trunk near the soil line. Look for slow or no growth on the top of the tree and rotting roots.

If possible, purchase species that are resistant to the fungus. Otherwise, prune out infected branches and dispose of them carefully, removing all infected material from the ground as well.

Note that the trees that are most likely to get Armillaria root rot are those under stress. So be sure to irrigate your trees during dry periods, deal with insect pests as you identify them, and avoid root injury.

Frequently Asked Questions

When are Pests Fatal for Trees?

Some pests can kill trees, others are just nuisances. Your best bet is to keep your trees healthy by proper planting and cultural care, then monitor them for pests.

Can a Really Diseased Tree Be Saved?

While some trees are destroyed by disease and must be removed, it may be possible to save a tree. Call in an arborist to advise you.

Teo Spengler

Teo Spengler has been gardening for 30 years. She is a docent at the San Francisco Botanical Garden. Her passion is trees, 250 of which she has planted on her land in France.