Acer Pests and Diseases: A Complete Guide

Acer pests and diseases include aphids, scale insects, spider mites, vine weevil, verticillium wilt, powdery mildew, and phytophthora root rot. Correct identification depends on symptom location, timing, and pattern.

Most problems respond to targeted treatment within one growing season. Untreated verticillium wilt and phytophthora root rot can kill a mature acer, so accurate diagnosis matters.

I once watched a client’s ten year old Bloodgood acer go from healthy to skeleton in about six weeks, and the culprit turned out to be something nobody had checked first: the roots.

By the time the leaves showed real damage, the problem underground had been building for months.

That is the frustrating thing about acer pests and diseases.

The visible symptom rarely matches the real timeline of the cause, and treating the leaf instead of the source is the single biggest reason acer treatments fail to work.

This guide walks through every common acer pest and disease, how to tell them apart, and what actually fixes each one.

If your tree is losing leaves fast rather than showing pest damage, our acer tree dying guide covers the wider list of causes including drought and frost.

This article focuses specifically on the insects and diseases that attack acers, from the mild and cosmetic to the genuinely serious.

Quick Diagnosis: Match Your Symptoms to the Cause

Use this table to narrow down the likely cause before reading the full sections below.

It is based on where the damage appears and how quickly it develops, which are the two most reliable clues.

Why Acers Get Pests and Diseases in the First Place

A healthy acer growing in the right spot is genuinely tough.

Most of the serious problems in this guide take hold after a tree has already been weakened by something else, usually drought stress, waterlogged roots, a pruning wound made at the wrong time of year, or a container that has become too small.

This matters because it changes how you should respond. Spraying a pest off the leaves treats the symptom.

Fixing the drainage, adjusting the watering routine, or repotting a root bound tree treats the reason the pest found an easy target in the first place.

Trees under stress produce weaker chemical defences and softer tissue, both of which make them easier for insects to feed on and easier for fungal spores to penetrate.

If your acer has been struggling generally rather than showing a specific pest or disease pattern, it is worth checking our acer tree not growing guide alongside this one, since underlying stress is often the missing piece.

Common Acer Pests

These are the insects most likely to be found on an acer in a UK or US garden, ordered roughly from mildest to most serious.

Aphids

Aphids are small, soft bodied insects that cluster on the undersides of young leaves and on soft new shoots in spring and early summer.

They feed by piercing the tissue and sucking sap, which causes leaves to curl, pucker, and sometimes turn slightly yellow.

The clearest sign is not the insect itself but the honeydew it leaves behind.

This sticky residue coats leaves and anything beneath the tree, and it often grows a black sooty mould on top, which looks alarming but is only a secondary cosmetic problem, not a disease in its own right.

On an established, healthy acer, aphids are more unsightly than dangerous.

A strong jet of water aimed at the undersides of affected shoots dislodges most of them within a few applications.

For heavier infestations, a diluted neem oil spray applied in the evening, when beneficial insects are less active, works well without the phytotoxic risk that stronger chemical insecticides carry on acer foliage.

Scale Insects

Scale insects are a slower, more persistent problem than aphids.

They attach permanently to stems and the undersides of leaves, appearing as small oval or rounded bumps that range from pale cream to dark brown.

Once attached, they do not move, which is why they are so easy to miss until numbers build up.

Cottony maple scale and horse chestnut scale are the two species most often found on acers.

Both feed on sap in the same way aphids do, but because they are protected by a waxy or cottony coating, contact sprays struggle to reach them once they are mature.

Timing is everything with scale. The young crawlers, which are the mobile juvenile stage, hatch in late spring to early summer and have no protective coating yet.

A thorough spray of horticultural oil during this short window is far more effective than trying to treat armoured adults later in the season.

On dormant winter wood, a tar oil wash can knock back established populations before spring growth starts.

Spider Mites

Spider mites are barely visible to the naked eye, but the damage they cause is not.

Affected leaves develop a fine, dry speckling and take on a dull, slightly bronzed appearance rather than their normal healthy sheen.

In heavy infestations, faint webbing appears on the undersides of leaves and between leaf clusters.

Spider mites thrive in hot, dry conditions, which is why outbreaks are far more common on acers in containers or in sheltered, warm positions during a dry summer than on trees growing in cooler, more humid garden settings.

Raising humidity around the tree with regular misting, combined with a forceful water spray to the leaf undersides every few days, controls most outbreaks without chemical intervention.

Persistent infestations respond well to a horticultural soap spray applied to the undersides of the leaves, where the mites actually live.

Vine Weevil

Vine weevil causes two completely different kinds of damage depending on the life stage, and beginners often treat one while missing the other entirely.

Adult vine weevils are active at night and chew distinctive C shaped notches out of leaf edges.

This damage looks dramatic but rarely threatens an established tree on its own.

The real danger comes from the larvae, which live in the soil or compost and feed on roots through autumn and winter, completely out of sight.

A container grown acer that suddenly collapses in spring, despite looking fine the previous autumn, has often lost most of its root system to vine weevil grubs over winter.

Tipping the tree out of its pot will usually reveal cream coloured, C shaped grubs with brown heads among the roots if this is the cause.

Biological control using pathogenic nematodes, watered into the compost in late summer or early autumn while soil temperatures are still above 12 degrees Celsius, is the single most reliable treatment for the larval stage.

For adults, checking the tree and surrounding foliage at night with a torch and removing weevils by hand is tedious but effective, since they cannot fly and have to walk between plants.

Caterpillars and Moth Larvae

Various moth caterpillars feed on acer foliage, chewing irregular holes in leaves that are larger and less uniform than vine weevil notching.

Damage typically appears in clusters, since caterpillars often feed from the same spot for several days before moving on.

On an established tree, caterpillar damage is almost always cosmetic.

Hand picking is the most practical control for the small numbers usually involved, and checking the tree at dusk or with a torch after dark catches the nocturnal feeders that do most of the damage.

Gall Mites

Gall mites cause raised, velvety or pimple like growths on the leaf surface, sometimes described as looking like small worm like protrusions depending on the species involved.

Despite the strange appearance, gall damage is almost entirely cosmetic and has very little effect on the overall health or vigour of the tree.

There is no effective chemical treatment for gall mites, and none is usually needed.

Removing and disposing of the worst affected leaves in autumn, rather than composting them where the mites can overwinter nearby, is the most sensible response for a tree where the appearance bothers you.

Squirrels, Rabbits, and Bark Damage

Rabbits will strip bark from the base of young acers over winter when other food is scarce, and squirrels sometimes strip bark from more mature trees, particularly in spring when sap is rising.

Both can cause serious damage if the bark is removed all the way around the trunk, a condition called girdling, which cuts off the flow of water and nutrients and can kill the tree above the damaged section.

A simple guard of hardware cloth or a proprietary tree guard wrapped loosely around the lower trunk over winter prevents almost all rabbit damage.

For squirrel damage higher up the tree, there is less that can be done beyond general deterrents, though squirrel damage severe enough to girdle a branch is relatively uncommon.

Common Acer Diseases

Diseases tend to be more serious than pests because several of them have no cure once established.

Catching the early signs, and knowing which ones are manageable versus which ones are not, makes a real difference to the outcome.

Verticillium Wilt

Verticillium wilt is the disease most likely to kill an otherwise healthy looking acer, and it is also the one most commonly misdiagnosed as simple drought stress in its early stages.

The fungus lives in the soil, sometimes for many years, and enters the tree through the roots, usually through a wound caused by cultivation, transplanting, or root damage nearby.

Once inside, it colonises the vascular tissue that carries water through the tree.

The telltale pattern is one sided or patchy dieback. A single branch, or one side of the canopy, wilts and browns while the rest of the tree still looks completely healthy.

This is different from drought stress, which tends to affect the whole tree fairly evenly.

Cutting into an affected branch reveals streaks of brown or greenish staining just beneath the bark, which is the clearest diagnostic sign available without a lab test.

There is no cure for verticillium wilt once it is established.

Management means removing affected branches promptly with clean, sterilised tools, keeping the rest of the tree as healthy as possible through correct watering and mulching, and avoiding root disturbance nearby that could spread the fungus.

Mildly affected trees can survive for years with some branches lost gradually. Severely affected trees are often not recoverable.

For a full breakdown of species specific detail, the RHS page on verticillium wilt is a reliable reference if you want to confirm a diagnosis before removing significant branch structure.

Phytophthora Root Rot

Phytophthora is a soil borne pathogen that thrives in waterlogged, poorly drained conditions.

It attacks the roots first, which is why the above ground symptoms are often confusing: the tree looks thirsty and stressed even though the soil around it may be saturated.

Leaves yellow and then brown, usually starting with the lower branches and working upward.

The base of the trunk may show discolouration or a slightly sunken, discoloured area at soil level, and a sour, unpleasant smell from the soil near the base is a strong practical indicator when you are trying to tell root rot apart from ordinary drought stress.

The most important first step is to stop watering immediately and assess drainage. For a container tree, remove it from the pot and look at the roots directly.

Healthy roots are firm and pale cream. Rotted roots are soft, brown, and collapse under light pressure.

Cut away all affected roots with clean tools, and repot into fresh, free draining compost in a clean container.

For a tree in the ground, improving drainage, whether through a French drain or by planting on a slightly raised mound in future, is essential, since the underlying wet conditions will keep reintroducing the problem otherwise.

Powdery Mildew

Powdery mildew shows up as a white, dusty coating on the leaf surface, usually starting on the upper leaves and spreading if conditions stay favourable.

It is most common in late summer, particularly where air circulation around the tree is poor or the position is shaded and humid.

This is one of the least serious diseases covered here. It rarely threatens the health of an established acer and is largely cosmetic, though a heavy, repeated infection over several years can reduce vigour slightly.

Improving air circulation by thinning overcrowded growth, and avoiding overhead watering that wets the foliage in the evening, prevents most cases.

Where treatment is wanted, a sulphur based or potassium bicarbonate fungicide applied at the first sign of white coating is effective.

Tar Spot

Tar spot produces distinctive black, raised, tar like spots on the upper surface of the leaves, usually appearing from mid to late summer onward.

It looks concerning but is almost entirely cosmetic and has no meaningful impact on tree health.

The fungus overwinters in fallen leaves, so raking up and disposing of leaf litter in autumn, rather than leaving it to compost under the tree, reduces the spore load available to reinfect the tree the following spring.

No spray treatment is generally necessary or worthwhile for tar spot.

Anthracnose

Anthracnose causes irregular brown to black blotches that tend to follow the leaf veins, often appearing after a wet, cool spring. In a bad year, affected leaves can curl, distort, and drop early, which looks dramatic but rarely does lasting harm to a mature, otherwise healthy tree.

Improving air circulation and avoiding wetting the foliage during watering are the most effective preventive steps.

Clearing fallen leaves in autumn reduces the amount of fungal spore material available to reinfect the tree the following spring, in the same way it helps against tar spot.

Coral Spot

Coral spot appears as small, raised, orange pink pustules scattered across dead or dying wood, most often on branches that have already been weakened, wounded, or killed by something else such as frost damage or a poor pruning cut.

It is a genuine warning sign that the wood beneath is dead, and left unaddressed it can spread into adjacent healthy wood through open wounds.

Pruning out affected wood well back into clean, healthy tissue, and disposing of the prunings rather than leaving them near the tree, is the standard response.

Always make pruning cuts during dry weather and avoid pruning acers in late winter or early spring, when the sap is rising strongly and cuts bleed heavily, since fresh wounds at this time are more vulnerable to infection.

Bacterial Leaf Scorch (Pseudomonas)

Pseudomonas bacterial infection produces symptoms that closely resemble physiological leaf scorch, with browning and dieback along leaf margins, which is exactly why it gets missed so often.

The distinguishing feature is that bacterial infection tends to be more localised to one area or side of the tree, similar in pattern to verticillium wilt, rather than the more even, whole canopy browning typical of heat or drought scorch.

There is no chemical cure for bacterial infections in trees.

Management focuses on removing affected material, avoiding wounding the tree during wet weather when bacteria spread most easily, and supporting overall tree health through correct watering and feeding to help it tolerate the infection.

Sooty Mould

Sooty mould is not a disease in the true sense.

It is a black fungal coating that grows on the sticky honeydew left behind by aphids and scale insects, rather than attacking the plant tissue directly.

Its presence is really a signal that a sap sucking pest problem exists somewhere on the tree, and treating the underlying insect is what actually resolves it.

Once the pest causing the honeydew is under control, sooty mould can be gently wiped or washed off the leaf surface with a soft cloth and diluted mild soap solution, and new growth will emerge clean.

Tools and Treatments Worth Having on Hand

You do not need everything on this list at once. Start with the first two or three items and build from there as needed.

Seasonal Pest and Disease Calendar for Acer Trees

Most acer problems are seasonal rather than random, so knowing what to look for and when saves a lot of guesswork.

Troubleshooting the Most Common Acer Problems

Leaves Are Sticky and Slightly Curled

This is almost always aphids, particularly if the sticky residue appeared over the space of a week or two on new growth.

Check the undersides of the youngest leaves and shoot tips first, since that is where aphids concentrate.

White Powder Appeared on the Leaves in Late Summer

This pattern points to powdery mildew rather than anything more serious.

It is worth checking whether the tree is in a shaded, still position with poor airflow, since improving circulation is usually more effective long term than repeated spraying.

One Branch Is Dying While the Rest of the Tree Looks Fine

This localised, one sided pattern is the classic sign of verticillium wilt or, less commonly, bacterial infection.

Cut into the affected branch and look for staining under the bark before assuming it is simple dieback from a physical wound, since the treatment approach differs significantly.

Bumps on the Stems Will Not Brush Off

Firmly attached bumps that do not move or brush away are scale insects, not a growth abnormality.

Scrub gently with a soft brush and diluted neem oil, and mark the date to treat again with horticultural oil the following early summer when the young crawlers are vulnerable.

A Container Tree Collapsed Suddenly in Spring

Sudden collapse after apparently healthy autumn growth is the classic vine weevil larvae pattern.

Tip the tree from its pot and check the roots for grub damage before assuming it is a watering or feeding issue, since no amount of watering will fix a root system that has already been eaten away.

Prevention: Why Spraying First Is the Wrong Habit

A lot of general advice online treats spraying as the default first response to any acer problem.

In practice, this often does more harm than good, since acer foliage tolerates chemical treatments poorly and a stressed tree treated with the wrong product can end up worse off than if nothing had been done at all.

The far more reliable long term strategy is prevention through growing conditions, not routine spraying.

Correct watering, avoiding waterlogged soil, mulching to protect the shallow root system, and pruning only in the correct season all reduce the stress that makes a tree vulnerable in the first place.

A tree grown well rarely needs more than the occasional targeted, well timed treatment described above.

If you are setting up a new acer or reworking an existing planting position, our acer tree care guide and when and how to prune an acer tree cover the foundational habits that prevent most of the problems in this guide from developing at all.

• Water deeply and infrequently rather than little and often, so roots grow deep and resilient.
• Mulch every year to protect the shallow root system and reduce moisture stress.
• Prune only in mid to late summer, never in late winter or early spring when cuts bleed heavily.
• Repot container trees before they become root bound; see our growing an acer tree in a pot guide for timing.
• Clear fallen leaves in autumn to reduce fungal spore carryover into the following year.

When to Call In a Professional

Most of what is covered in this guide can be handled at home.

There are a few situations where bringing in a qualified arborist or tree surgeon is genuinely the better call rather than continuing to try home treatments.

• Verticillium wilt has spread through more than a third of the canopy, since large scale branch removal on a mature tree needs proper structural assessment.
• Any disease symptom appears on a tree near a boundary, path, or building, where falling dead wood could be a safety risk.
• Phytophthora root rot is suspected on a large, established garden tree, since assessing root damage without killing the tree needs experience.
• You are not confident distinguishing verticillium wilt from bacterial infection or simple drought stress, since the wrong response can waste a full growing season.

Frequently Asked Questions

What is eating the leaves on my acer tree?

The most common culprits are caterpillars, adult vine weevils, and occasionally slugs or snails on very low branches.

Caterpillar damage tends to be irregular holes within the leaf, while vine weevil damage shows up as neat C shaped notches along the leaf edge.

Checking the tree at night with a torch usually catches the insect responsible, since most leaf chewing pests on acers feed after dark.

Why does my acer have black spots on the leaves?

Small, raised, tar like black spots are most likely tar spot, a fungal disease that is cosmetic and does not seriously harm the tree.

Flatter, blotchy black or brown marks that follow the veins are more likely anthracnose, particularly after a wet spring.

Both are managed the same way, by clearing fallen leaves in autumn to reduce the spore load for the following year.

Why is my acer tree dying suddenly?

Sudden, one sided dieback in an otherwise healthy looking tree points strongly to verticillium wilt, which should be confirmed by checking for staining under the bark of an affected branch.

If the whole tree is failing evenly rather than in a localised pattern, drought, waterlogging, or root damage are more likely, and our acer tree dying guide covers those broader causes in more depth.

Is verticillium wilt in acer trees curable?

No, there is no cure once verticillium wilt is established in the vascular tissue of the tree.

Management involves removing affected branches promptly with clean tools and supporting the overall health of the remaining tree through correct watering and mulching.

Some trees survive for years with gradual, partial dieback, while others decline more quickly, and the outcome depends heavily on how much of the vascular system is affected at diagnosis.

What are the sticky black spots under my acer tree?

Sticky patches on paving, furniture, or plants beneath an acer are honeydew, a sugary excretion from aphids or scale insects feeding higher up in the canopy.

The black coating that often develops on top of it is sooty mould, a secondary fungus that grows on the honeydew rather than attacking the tree directly.

Treating the pest causing the honeydew resolves both problems together.

Can I save an acer with root rot?

Yes, in many cases, provided the rot is caught before it has destroyed the majority of the root system.

Stop watering immediately, remove the tree from its pot if container grown, cut away all soft brown roots back to firm white tissue, and repot into fresh, free draining compost.

For a tree in the ground, improving drainage is essential, since the underlying wet conditions will otherwise cause the problem to return.

Do acer trees get powdery mildew?

Yes, acers are susceptible to powdery mildew, particularly in shaded, humid positions with poor air circulation.

It appears as a white, dusty coating on the leaf surface, usually from mid to late summer onward.

It is one of the least serious problems covered in this guide and rarely threatens an established tree, though improving airflow around the canopy is the most effective long term prevention.

How do I get rid of vine weevil in acer pots?

Water pathogenic nematodes into the compost in late summer or early autumn while soil temperature is still above 12 degrees Celsius, since this targets the larvae before they can do serious root damage over winter.

For adult weevils, checking the tree and surrounding foliage at night with a torch and removing them by hand controls the population feeding on the leaves, since adults cannot fly between plants.

Key Takeaways

1. Match the symptom pattern to the cause before treating anything, since one sided dieback, whole canopy browning, and localised bumps or spots all point to very different problems.
2. Treat vine weevil in two stages, nematodes for the root feeding larvae in late summer and hand removal for the leaf chewing adults at night.
3. Confirm verticillium wilt by cutting into an affected branch and checking for staining under the bark before removing large sections of canopy.
4. Stop watering immediately if phytophthora root rot is suspected, and check root colour and texture directly rather than guessing from the leaves alone.
5. Treat scale insects in early summer while the crawlers are still exposed, since mature armoured scale is far harder to control.
6. Prevent disease pressure through correct watering, mulching, and seasonal pruning rather than relying on routine spraying as a default response.
7. Clear fallen leaves every autumn to reduce the fungal spore load available to reinfect the tree the following spring.
8. Test any spray on one small branch first and wait 48 hours, since acer foliage is genuinely sensitive to phytotoxic leaf burn.

Final Thoughts

That ten year old Bloodgood I mentioned at the start did eventually recover, but only after we stopped treating the leaves and started treating the roots.

It lost about a third of its canopy that year and looked genuinely rough by August. The following spring it came back close to full.

That is the pattern worth remembering with almost every problem in this guide. The leaves tell you something is wrong, but they rarely tell you the whole story on their own.

Take the time to check the pattern, the timing, and where the damage actually started, and the right treatment usually becomes obvious.

For ongoing care once your tree is back on track, our acer palmatum care guide and best acer trees for small gardens are good next steps if you are planning future planting.