Plum Tree Pests and Diseases: Problems and Fixes

Every plum tree pest or disease produces a specific, identifiable pattern on the leaves, bark, blossom, or fruit.

The main culprits are black knot, brown rot, silver leaf, bacterial canker, aphids, plum moth, and plum sawfly.

Correct identification matters, because treating the wrong problem often wastes a season, and some diseases spread fast if ignored.

I still remember the first summer I lost half a plum crop to what I assumed was just bad weather. It was not.

It was plum moth, and every single wormy fruit that dropped early was another generation of the pest setting up camp in my soil for next year.

That mistake taught me more about plum tree problems than any book did.

Plum trees have a reputation as one of the easier fruit trees to grow, and in fairness, that reputation is mostly deserved.

But easier does not mean problem free. Once you know what you are looking for, most plum tree pests and diseases are simple to catch early and simple to manage.

The trouble usually starts when a gardener assumes a spotty leaf or a sticky branch is nothing to worry about.

This guide walks through every common plum tree pest and disease, how to tell them apart, and exactly what to do about each one.

If your tree is newly planted, check our plum tree care guide first, since a well fed, well watered tree resists pests and diseases far better than a stressed one.

Quick Diagnosis Table

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

Understanding Why Plum Trees Get Pests and Diseases

Plum trees belong to the genus Prunus, the same group as cherries, peaches, and apricots, and this family is naturally more prone to a small set of specific fungal and bacterial problems than most other fruit trees.

Silver leaf, bacterial canker, and brown rot are all classic Prunus diseases, and if you have grown cherries or peaches before, you will recognize the pattern.

Most plum tree problems trace back to one of three root causes. Wet, humid weather during blossom and early fruit set encourages fungal spores to germinate and spread.

Wounds from pruning, storm damage, or careless mowing give bacteria and fungi an easy entry point into the wood.

Stress from drought, poor soil, or overcrowded branches weakens the tree and lowers its natural resistance to everything else on this list.

Older, neglected trees with a dense, unpruned canopy are far more vulnerable, since poor airflow keeps the leaves and fruit damp for longer after rain.

If your tree has not fruited well recently, it is worth reading our guide on why your plum tree is not fruiting, since several of the problems below reduce fruit set long before they become obvious.

Plum Tree Diseases

Black Knot

Black knot is one of the most recognizable and most serious plum tree diseases in the United States.

It starts as a small, olive green swelling on a twig or branch in spring, and by the following year it hardens into a rough, coal black knot that can wrap all the way around the wood.

The fungus, Apiosporina morbosa, releases spores during wet spring weather, and once a knot is established it keeps producing more spores every year until the wood is removed.

Left untreated, black knot girdles branches, stops sap flow beyond the knot, and can eventually kill the whole tree.

Prune out every knot as soon as you see it. Cut at least 8 to 10 inches below the visible swelling, since the fungus grows further into healthy looking wood than it appears.

Burn or bag the prunings rather than composting them, and always disinfect your pruning shears with rubbing alcohol between cuts so you do not spread spores to healthy branches.

Brown Rot

Brown rot is caused by the fungus Monilinia fructicola, and it is the disease most likely to cost you an actual harvest rather than just tree health.

It attacks blossoms first, turning them brown and wilted, then moves into twigs, and finally into ripening fruit, where it produces a soft brown rot that spreads fast in warm, humid weather.

Infected fruit often shrivels into a hard, gray mummy that clings to the branch through winter, and those mummies are the main source of infection the following spring.

This is the single most overlooked step in brown rot control: removing every mummified fruit and every fallen fruit from the ground before winter, since spores overwinter directly on that debris.

Thin the fruit early in the season so plums are not touching each other on the branch, since brown rot spreads fastest where fruit skins are in direct contact.

Water at the base of the tree rather than overhead, and prune the canopy to open it up, since better airflow dries the fruit and leaves faster after rain and gives the fungus less opportunity to take hold.

Silver Leaf

Silver leaf is the disease that worries experienced growers the most, and for good reason.

There is no chemical or home treatment that cures it once it is established in the wood.

The fungus, Chondrostereum purpureum, infects through pruning wounds and other damage, then grows through the wood, giving the leaves a silvery, almost metallic sheen as it disrupts normal water and sugar movement.

Cut across an affected branch and look for a dark stain in the center of the wood.

That stain is the clearest way to confirm silver leaf and rule out a lookalike condition called false silver leaf, which is caused by drought or cold stress and does not show any staining at all.

If you catch it early, cut back well into clean, unstained wood, ideally 4 to 6 inches past the last sign of staining, and burn the prunings immediately.

If the infection has already reached the main trunk, the tree usually cannot be saved, and removal is the safest option to protect nearby Prunus trees.

Bacterial Canker

Bacterial canker shows up as sunken, dark, gummy patches on the bark, often accompanied by small round holes in the leaves that gardeners call shothole, since the leaf looks like it has been peppered with tiny gunshots.

The inner bark beneath a canker often has a sour smell and a reddish streak when cut open.

This disease enters through wounds and through natural leaf scars in autumn, so it is most active in cool, wet conditions during dormancy and early spring.

Young trees are especially vulnerable in their first three to four years before the bark has fully matured.

Cut out cankers in summer when the bacteria are least active, cutting well into healthy wood, and apply a copper based fungicide spray in early autumn as a preventive measure before the wet season begins.

Good drainage and avoiding late season nitrogen fertilizer both reduce the soft, vulnerable growth that bacterial canker exploits.

Plum Pocket

Plum pocket, sometimes called pocket plum, is caused by a fungus in the Taphrina family and produces one of the strangest symptoms on this list.

Infected fruit grows abnormally large, pale, and hollow, with no proper stone inside, and the fruit eventually shrivels and turns leathery.

The infection actually happens at blossom time in spring, even though the deformed fruit is not obvious until it develops over the following weeks.

This timing is why spraying after you notice the symptom rarely does anything, since the infection has already happened weeks earlier.

Remove and destroy affected fruit before it can release spores, and apply a copper fungicide at the pink bud stage next spring, just before the blossom opens, since that is the only window when treatment actually prevents infection.

Powdery Mildew

Powdery mildew is one of the least serious problems on this list, and it is also one of the most common.

It appears as a white, dusty coating on the upper surface of leaves, usually starting on younger growth and spreading during warm, dry days followed by cool, humid nights.

Unlike most fungal diseases, powdery mildew does not need standing water on the leaf to spread, which is why it often shows up even during a dry summer.

It rarely kills a plum tree, but a heavy infection can reduce vigor and slightly affect fruit quality.

Improve airflow by thinning crowded branches, and avoid excess nitrogen fertilizer, which produces the soft, susceptible new growth that powdery mildew prefers.

A light application of a sulfur based fungicide or a diluted neem oil spray controls most infections without harming beneficial insects.

Plum Pox Virus (Sharka)

Plum pox virus, also known as Sharka, is the most serious disease on this list because there is genuinely no cure, and infected trees eventually need to be removed.

It is spread by aphids and produces yellow or brown ring shaped blotches on the fruit skin, along with pale rings or line patterns on the leaves.

This virus is a regulated pest in many US states, meaning suspected cases must be reported to your state department of agriculture rather than treated at home.

Confirmed outbreaks have historically triggered mandatory removal programs in affected regions, since the virus can spread through an entire orchard within a few years.

If you suspect plum pox virus, do not prune, propagate from, or move material from the tree.

Contact your local extension office for testing before doing anything else.

This is one of the very few plum tree problems where waiting to confirm the diagnosis matters more than acting quickly.

Plum Tree Pests

Aphids (Plum Leaf Curling Aphid and Mealy Plum Aphid)

Two specific aphid species target plum trees.

The plum leaf curling aphid attacks from April through June, causing new leaves to curl tightly around the colony and turn a distinctive shade of yellowish red.

The mealy plum aphid arrives slightly later and coats leaves and shoots with a powdery gray wax and sticky honeydew.

Both species weaken new growth and encourage sooty mold to grow on the honeydew they leave behind, though a light infestation rarely does lasting damage to an otherwise healthy tree.

Heavier infestations on young trees can distort growth badly enough to affect shape for a full season.

A strong jet of water knocks most colonies off within a week if you repeat it every two or three days.

For heavier infestations, spray insecticidal soap or neem oil directly onto the colonies in the early morning or evening, and encourage ladybugs and lacewings into the garden, since they will keep populations low for the rest of the season without any spraying at all.

Plum Curculio and Plum Sawfly

Plum curculio is the main fruit destroying pest across most of the United States, while plum sawfly plays a similar role in the UK and parts of Europe.

Both produce a very similar symptom: a small crescent shaped scar on young developing fruit, followed by premature fruit drop as the larvae feed inside.

Adult plum curculio beetles emerge in spring and feed on blossoms and young fruit before laying eggs just under the skin.

The classic crescent scar is actually the female cutting a flap to lay her egg, then curving the cut so the developing larva has room to grow without being crushed as the fruit expands.

Spread a light colored sheet or tarp under the tree and shake the branches firmly in the early morning, when the beetles are sluggish from the cold and simply drop rather than fly away.

Collect and destroy them daily during the two to three week period after petal fall, which is when adults are most active.

Removing and destroying every dropped fruit through the season is just as important, since each one contains the next generation.

Scale Insects

Scale insects attach permanently to stems and branches and look like small, motionless bumps ranging from pale tan to dark brown.

They feed by piercing the bark and sucking sap, and a heavy infestation slowly weakens the tree, causing dieback on individual branches over a season or two.

They are easy to miss because they do not move and do not look like typical insects at all.

The honeydew they excrete, along with the sooty mold that grows on it, is often the first visible clue that scale is present, well before the insects themselves are noticed.

Apply horticultural oil during the dormant season, ideally in late winter before buds swell, since this is when the oil smothers overwintering scale most effectively without harming the tree.

In early summer, treat again when young crawlers are active and have not yet formed their protective waxy shell, since this is the only stage where a lighter spray still works well.

Spider Mites

Spider mites thrive in hot, dry weather and are barely visible to the naked eye, but their damage is not.

Affected leaves develop a fine, stippled, bronzed appearance, and in severe infestations you may see fine webbing on the undersides of leaves and along the shoots.

Drought stressed trees are far more vulnerable to spider mites than well watered ones, since the mites reproduce faster on plants with reduced natural defenses.

A tree that is otherwise healthy and well watered through summer rarely develops a serious mite problem.

Increase watering during hot, dry spells and hose down the foliage periodically, since spider mites dislike moisture and a strong spray of water dislodges a large proportion of the population.

For established infestations, a miticide or horticultural oil spray applied to the undersides of leaves gives the most effective control.

Japanese Beetle

Japanese beetles are shiny, metallic green and copper insects that feed on plum leaves in groups, often skeletonizing them down to the veins within days during peak activity in midsummer.

They tend to feed on the sunniest, most exposed parts of the canopy first.

Adult beetles are attracted to the scent of already damaged leaves, which means an infestation can escalate quickly once a few beetles find the tree, since each one attracts more.

Handpicking beetles into a bucket of soapy water each morning, while tedious, remains one of the most effective non chemical controls available.

Avoid pheromone traps positioned near the tree itself, since they attract far more beetles to the area than they actually catch, often making the problem on your specific tree worse rather than better.

Kaolin clay sprayed on the foliage before beetles arrive makes the leaves unpalatable and significantly reduces feeding damage.

Peachtree Borer

The peachtree borer is a moth whose larvae tunnel into the trunk near the soil line, and despite the name, it attacks plums just as readily as peaches.

The clearest sign is a mass of gummy sap mixed with sawdust like frass at the base of the trunk, usually within the first foot above ground level.

This pest is far more serious than most on this list because the larvae feed inside the trunk itself, and a heavy infestation can girdle the trunk completely, cutting off the flow of water and nutrients and killing the tree outright.

Young and newly planted trees are especially vulnerable since their bark has not yet thickened.

Keep the base of the trunk free of mulch piled against the bark, weeds, and grass, since borers are drawn to the shelter and moisture these provide.

In late summer, when moths are laying eggs, an application of a registered trunk spray or a physical trunk wrap provides the most reliable protection for young trees.

Troubleshooting Summary Table

Seasonal Care Calendar for Preventing Plum Tree Problems

If your tree has struggled for more than one season despite following the steps above, it is worth reading how experienced growers assess a tree that may be beyond saving in our guide to telling if a tree is dead or dying, since the scratch test described there applies just as well to plums as it does to pines or Acer trees.

Frequently Asked Questions

What is eating the leaves on my plum tree?

Aphids, Japanese beetles, and occasionally caterpillars are the most common culprits behind chewed or skeletonized plum leaves.

Aphids cause curling and distortion rather than large holes, while Japanese beetles leave leaves looking lace like, with only the veins remaining.

Check the undersides of curled leaves for aphid colonies and look for visible beetles during midsummer feeding periods.

Identifying which pest is present matters because the treatment for each is different.

Can a plum tree recover from silver leaf disease?

Yes, but only if the infection is caught early and the affected wood is removed well before it reaches the main trunk.

Cut back into clean, unstained wood at least 4 to 6 inches past any visible dark staining, and burn the prunings immediately to prevent reinfection.

Once the fungus reaches the trunk, recovery becomes very unlikely and removal is usually the safest choice.

Regular inspection of branches for early silvering gives you the best chance of catching it in time.

Why is my plum tree dropping fruit early?

Early fruit drop is most often caused by plum curculio or plum sawfly, both of which lay eggs inside developing fruit and cause it to fall before ripening.

Brown rot can also cause fruit to drop after developing soft brown patches in humid weather.

A small amount of natural fruit drop in early summer, sometimes called June drop, is completely normal and not a cause for concern.

If drop is heavy and fruit shows scarring or soft rot, treat for the specific pest or disease responsible rather than assuming it is natural thinning.

How do I get rid of black knot on a plum tree?

Prune out every visible knot, cutting at least 8 to 10 inches below the swelling since the fungus extends further into the wood than it appears.

Disinfect your pruning shears with rubbing alcohol between every cut to avoid spreading spores to healthy branches.

Burn or bag the removed wood rather than composting it, since composting does not kill the fungus reliably.

Prune during the dormant winter months when knots are easiest to spot against bare branches, and check the tree again the following winter, since new knots often appear near old ones.

Is brown rot on plums dangerous to eat?

Fruit affected by brown rot should not be eaten, since the soft brown rot is caused by an active fungal infection that spreads through the flesh of the fruit.

Any fruit showing signs of brown rot should be removed from the tree and destroyed rather than composted, since composting does not reliably kill the spores.

Mummified fruit left hanging on the tree over winter is the main source of infection for the following spring, so removing every mummy before winter is one of the most effective steps you can take.

Healthy, unaffected fruit from the same tree is completely safe to eat.

What is the best natural spray for plum tree pests?

Neem oil and insecticidal soap are the two most broadly effective and least harmful sprays for common plum tree pests like aphids and spider mites.

Horticultural oil is the better choice specifically for scale insects and overwintering pest eggs, applied during the dormant season in late winter.

Kaolin clay is a good natural option against Japanese beetles, since it makes the leaf surface unpalatable without harming beneficial insects.

Whatever product you choose, always spray in the early morning or evening and never during blossom, since many natural sprays are still harmful to bees on contact.

Do I need to spray my plum tree every year?

Most healthy, well established plum trees do not need a full annual spray program if you focus on the highest impact steps instead.

Removing fallen and mummified fruit every autumn, pruning at the correct time of year, and applying a single dormant oil spray in late winter prevent the majority of common problems without ongoing chemical treatment.

Trees with a history of brown rot, canker, or curculio damage benefit from the targeted sprays described in the seasonal calendar above, timed to the specific vulnerable period for each problem.

Spraying on a fixed calendar regardless of actual pest pressure wastes product and unnecessarily harms beneficial insects.

How can I tell the difference between a plum tree disease and normal seasonal change?

Genuine disease symptoms tend to appear suddenly, follow a specific pattern such as a stain in cut wood or a ring shape on fruit, and often worsen over days or weeks rather than staying stable.

Normal seasonal change, such as autumn leaf drop or the natural yellowing of older lower leaves, happens gradually and evenly across the whole tree rather than in isolated patches or on specific branches.

If you are unsure, the scratch test used to check whether a branch is alive, cutting a small nick to look for green tissue beneath the bark, is a reliable first step before assuming the worst.

When in doubt, comparing the pattern against the quick diagnosis table earlier in this guide will usually point you toward the correct answer.

Key Takeaways

1. Identify the exact pattern of damage before treating anything, since the wrong treatment wastes time and can make some problems worse.
2. Remove and destroy every mummified and fallen fruit each autumn to break the life cycle of brown rot, curculio, and sawfly.
3. Prune plum trees in early spring or midsummer only, never in winter, to avoid the highest risk period for silver leaf infection.
4. Disinfect pruning tools between cuts whenever you are removing diseased wood, to avoid spreading infection to healthy branches.
5. Never spray insecticide during blossom, since this kills the bees your tree depends on for a full fruit set.
6. Apply dormant oil in late winter to control scale and overwintering aphid eggs before the growing season begins.
7. Shake branches over a sheet each morning during the three weeks after petal fall to catch plum curculio or sawfly before they cause fruit drop.
8. Keep mulch and grass clear of the trunk base to reduce the risk of peachtree borer infestation.
9. Report any fruit with ring shaped blotches to your local extension office, since this may indicate plum pox virus.
10. A stressed, poorly watered tree is more vulnerable to almost every pest and disease on this list, so consistent care is your best defense.

Final Thoughts

That first ruined plum harvest taught me something I still tell every new grower who asks about plum tree pests and diseases.

The tree itself is rarely the problem. Timing is.

Catching black knot in winter, catching curculio in the three weeks after petal fall, catching brown rot before the mummies overwinter, these small windows matter more than any spray you could buy.

Most plum trees that get consistent, well timed attention go on to produce heavy, healthy crops for decades with barely any serious disease pressure at all.

The goal is not a perfect, pest free tree. It is a tree that is strong enough, and watched closely enough, that small problems never become large ones.