Plum Tree Diseases: Black Knot, Brown Rot, and What to Do About Them

Plums are easier than peaches and harder than apples. They escape some of the worst peach problems (no peach leaf curl, no Cytospora-driven Peach Tree Short Life) but they pick up two diseases that are almost exclusively plum-specific: black knot and plum pockets. Black knot will gradually disfigure a tree over several years if you ignore it. Plum pockets is more shocking to look at than dangerous, but it’ll wipe out a season’s fruit.

Five diseases account for almost everything that goes wrong on backyard plums. Three of them (brown rot, bacterial spot, Cytospora-related canker) overlap with peach and cherry, so the management protocols transfer. Two of them, black knot and plum pockets, need plum-specific strategies.

Here’s how to identify what’s wrong with your plum tree, what to do about it, and which problems mean rethinking your cultivar choice.

How to tell which disease you’re looking at

Quick visual triage. Find the most obvious symptom and jump to the section below.

• Black, hard, warty galls growing on branches: black knot
• Plums grown 10x normal size, hollow inside, looking like bladders: plum pockets
• Brown fuzzy spore tufts on rotting plums, blackened wilted blossoms in spring: brown rot
• Small dark spots on leaves that fall out leaving “shothole” holes, pitted cracked fruit: bacterial spot
• Orange or yellow pustules on the underside of leaves in late summer, premature leaf drop: plum rust

The plum-specific diseases (black knot and plum pockets) both launch their infections in spring during wet weather, and both are best controlled by a combination of dormant copper sprays and winter pruning. Get those two habits right and you’ll handle most of what plum throws at you.

Black knot

Black knot (Apiosporina morbosa, formerly Dibotryon morbosum) is the most distinctive and most damaging plum disease. The galls are unmistakable: hard, black, warty, knobby growths on branches, sometimes only an inch long, sometimes a foot or more. Heavy infections gradually kill the productive scaffold limbs, year by year, until the tree stops producing meaningful fruit.

Plum is black knot’s preferred host. Cherry gets it, chokecherry gets it, but plum and especially Japanese plum (Prunus salicina) gets the worst of it.

What it looks like

The first-year infection is subtle: a swollen olive-green area on a small branch, often missed because it looks like normal growth oddities. By year two, the swelling has matured into a black, hard, distinctly knobby gall, typically 1-6 inches long. Heavily infected trees can have galls every few feet along the branches, with branches dying back above each gall.

Why it happens

The fungus releases spores from existing galls during spring rains from April through June. Per the University of Minnesota Extension, spores can infect new shoots during wet periods as short as 6 hours at the right temperature. Infections happen on the current season’s growth, but the visible gall doesn’t appear until the following year. That’s why black knot feels like it appears out of nowhere: by the time you spot a knot, the infection is 12 months old.

What to do about it

This is the most pruning-intensive disease in this guide. Plan on a winter pruning pass every year for at least the first three years after you notice the disease.

• Cut at least 4-6 inches below every visible knot, into clean wood. The fungus grows beyond the gall, often 4 inches or more. Cuts right at the visible edge leave the disease behind. Some extension sources recommend 6-8 inches for safety.
• Sterilize between cuts with 70% alcohol or 10% bleach (1:9 with water). The blade carries spores from branch to branch otherwise.
• Use anvil pruners on dry galls: Felco F31 Anvil Pruners cut the hard woody knots cleanly, where bypass pruners would crush.
• Prune in dry late-winter weather only. February or early March, before bud break, when the temperature stays below 40°F and spores aren’t being released.
• Burn or bag the prunings. Don’t compost. The spores survive composting and re-infect the tree from the brush pile next spring.
• Plan two pruning passes in year one. Small first-year swellings get missed on the first pass and show up obvious by the next winter.
• Remove wild plum or chokecherry within 500 feet if you have access. Wild Prunus are spore factories that re-infect cultivated trees every spring no matter how clean you keep your own tree.

For backyard trees with light infection, an early-spring fungicide application can supplement pruning. Captan or chlorothalonil applied at bud break, repeated at 7-14 day intervals through bloom, reduces new infections. For heavy infections or large trees where complete spore-release coverage isn’t practical, the spray rarely pays for itself.

How to keep it from coming back

Cultivar choice matters if you’re starting fresh. Among European plums (Prunus domestica: Stanley, Damson, President), Damson and President show better field resistance than Stanley. Among Japanese plums (Prunus salicina: Methley, Santa Rosa, Shiro), there’s less variation and almost all are susceptible. If you live in a black-knot-heavy region (Midwest, Northeast, mid-Atlantic), Japanese plums are the riskier choice.

Plum pockets

Plum pockets is the strangest disease in this guide. The pathogen (Taphrina communis) is a close relative of the peach leaf curl fungus, but its effect on plums is dramatic: infected fruit swells to 10 times its normal size, hollows out inside, turns reddish, then dries to a black husk. The first time you see it, you’ll think your tree is producing alien artifacts.

What it looks like

The disease starts as a small blister on a developing plum, typically a few weeks after bloom. The blister rapidly enlarges until it covers the entire fruit, which grows wildly out of proportion to normal plums. According to the University of Minnesota Extension plum pockets page, infected fruit can reach 10 times normal size, with a spongy texture, gray fungal spores on the surface, and an empty hollow interior where the pit should be.

Affected shoots may also thicken and curl. Leaves on infected twigs sometimes show curl symptoms similar to peach leaf curl. The fruit eventually shrivels into black mummies that may hang on the tree all winter.

Why it happens

The fungus overwinters on bark and bud surfaces. Cool wet spring weather (the same conditions that drive peach leaf curl) lets the spores germinate at bud break and infect developing flowers and young fruit. Once the symptoms appear, there is no in-season treatment.

The disease is much worse on American plums (Prunus americana) and wild plums than on European or Japanese plums. Per the U Minnesota Extension, “most of the dessert plums grown in Minnesota are unlikely to be affected.” Plum pockets remains a real issue for native-species growers and for anyone with American or wild plums in their landscape.

What to do about it

This is a dormant-spray disease, like peach leaf curl. Once you see bladder-fruit, it’s too late for that year.

• Single dormant copper spray just before bud swell in late winter (typically February to early March). Bonide Copper Fungicide concentrate at full label rate, applied to drench the bark and bud scales. One spray is enough.
• Remove infected fruit promptly when you see the bladders, before they sporulate. Bag and trash. Don’t compost.
• Light infections need no treatment. The disease doesn’t harm the tree itself. If you’re only losing a handful of plums per year, leave the spray budget for brown rot and bacterial spot, the two diseases that threaten the harvest itself.

How to keep it from coming back

If you have a wild American plum thicket nearby that’s heavily infected, it’ll keep reinfecting your cultivated tree year after year. Removal of nearby wild plums helps. For homeowners replanting, dessert plum cultivars (European Stanley, Japanese Santa Rosa) are safe from plum pockets in most regions.

Brown rot

Brown rot is the same disease on plums as on peaches and cherries: caused by Monilinia fructicola (and sometimes M. laxa), with three symptom phases (blossom blight, twig dieback, fruit rot). The complete management protocol is covered in our peach tree diseases guide and cherry tree diseases guide, and the same approach works on plum.

A few plum-specific notes:

• Japanese plums (Santa Rosa, Methley, Shiro) crack easily in rain near harvest, and brown rot moves through cracked fruit overnight. European plums (Stanley, Damson) are tougher-skinned and resist brown rot better at harvest.
• The mummified fruit on plums is harder to spot than peach mummies because they’re smaller and often hidden in the canopy. Winter cleanup needs more careful inspection.
• Blossom blight is less common on plums than on peaches or cherries, but twig dieback from spring infections is just as serious. Cut out blighted spurs as soon as you see them.

What to do about it

Same protocol as peach and cherry. Spray at full bloom, every 10-14 days through fruit set, tighten to 7-day intervals as fruit colors. Remove every mummified fruit in winter. Prune out dieback with sterilized Felco F2 Bypass Pruners in late winter.

The combination of dormant copper spray (which also handles plum pockets) plus a spring fungicide schedule covers both diseases with the same calendar.

Bacterial spot

Bacterial spot (Xanthomonas arboricola pv. pruni) hits plums just like it hits peaches. The full disease background is in our peach tree diseases guide. The differences on plum:

• Japanese plums are highly susceptible (the most disease-prone in the genus by some research)
• European plums are more tolerant
• The fruit lesions on plums tend to look like sunken dark pits rather than the cracking common on peach fruit
• Bacterial spot on plum leaves often produces clean shothole holes, larger and more obvious than on peach

What it looks like

• On leaves: small dark spots, often along midrib or veins, with the dead tissue falling out to create shothole holes. Heavy infections cause yellowing and early leaf drop.
• On fruit: small sunken dark pits, sometimes cracking. The flesh underneath is edible but cosmetically ruined.
• On twigs: small dark cankers that may girdle and kill the twig.

What to do about it

Like on peach, this is mostly a cultivar problem. Plant European or hybrid plums in regions with bacterial spot pressure. Japanese plums in the Southeast, mid-Atlantic, and humid Midwest are usually a battle. For trees already in the ground, fall copper sprays at leaf drop reduce overwintering populations. There is no in-season chemical control that’s practical for the home grower.

Plum rust

Plum rust (Tranzschelia discolor) is usually the last disease of the season, showing up in August and September on backyard plums. It produces small yellow-orange pustules on the underside of leaves, followed by early defoliation. The fruit itself is rarely affected.

What it looks like

Bright yellow-orange to brownish pustules, about a millimeter across, on the underside of mature leaves in late summer. The upper leaf surface shows a corresponding light-colored spot. Heavy infections cause leaves to turn yellow and drop several weeks early, weakening the tree for the next season.

Why it happens

Tranzschelia discolor is a rust fungus with a two-host life cycle, alternating between Prunus trees (its primary host) and Anemone species (its alternate host). Unlike some rust diseases, T. discolor can complete its life cycle without the alternate host, so removing anemones from your yard rarely prevents the disease.

Almond, peach, and prune are the most susceptible Prunus hosts, with plum and other stone fruits getting milder infections. The disease develops late in the season and most extension sources rate the damage as minor.

What to do about it

For most backyard plums, plum rust is not worth chasing with a dedicated spray. The damage is minor and shows up after the harvest. Two passive controls handle it:

• A copper fungicide applied for brown rot control during summer also suppresses rust
• Raking and removing fallen leaves in autumn reduces overwintering spores

If rust regularly defoliates your tree before September, consider whether the tree has other stresses (drought, root damage, nematodes) making it more susceptible than usual. Healthy vigorous plums resist rust without intervention.

When to call an arborist

Most plum diseases are home-garden problems and the homeowner can handle them. Two situations warrant professional help.

First, a heavily black-knotted tree where deciding between two years of aggressive pruning and full removal needs an experienced eye. A mature plum with 20+ galls scattered through the canopy may not be worth saving.

Second, an unfamiliar canker pattern on a mature tree (15+ years) where you can’t distinguish between Cytospora canker, bacterial canker, and black knot. The treatments differ and getting the diagnosis right matters.

An ISA-certified arborist consultation runs $75-200 in most US markets. Find one through the Trees Are Good arborist locator or read our guide to what an arborist does before you call. If the tree is past saving, tree removal on a mature plum typically runs $300-1,200.

Prevention checklist

Six habits handle almost everything that goes wrong on backyard plums:

• Plant European plums in disease-heavy regions. Stanley, Damson, and President resist black knot and bacterial spot far better than Japanese plums. The Japanese plums have better flavor, but the disease management cost is higher.
• Single dormant copper spray at late winter bud swell. Handles plum pockets, reduces bacterial canker and brown rot overwintering inoculum.
• Walk the tree every February with pruners. Cut out every visible black knot (4-6 inches below into clean wood) and any dead or cankered branches. Sterilize between cuts.
• Remove every mummified fruit every winter. Highest-leverage brown rot prevention.
• Rake fallen leaves in autumn. Reduces leaf spot and rust spore loads.
• Prune in late winter only. February or early March, in dry weather, when temperatures stay below 40°F. Never in fall.

For nutrient timing through the growing season that supports tree health without overstimulating soft late-season growth, see this fruit tree fertilizing guide on mklibrary.com.

For broader fungal disease patterns across the rest of your yard, see our tree fungus guide. If you also grow peaches, apples, or cherries, the peach tree diseases, apple tree diseases, and cherry tree diseases guides cover the shared and species-specific problems on the related fruits. For pruning protocols specific to plum, our plum tree pruning guide covers the open-center training that minimizes disease pressure.

Frequently asked questions

What is the most common plum tree disease?

Black knot is the most common in eastern North America and the most disfiguring. Brown rot is the most economically damaging in humid climates. Plum pockets is the most visually shocking but rarely threatens the tree. Most backyard plums in disease-pressure regions deal with all three at once.

Should I cut down a plum tree with black knot?

No, unless the tree has 20+ galls scattered across the main scaffold limbs. Aggressive winter pruning (cuts 4-6 inches below every gall, with sterilized blades) keeps even heavily infected trees productive for years. The exception is when most of the scaffold is involved and pruning out the disease would leave you with a hatrack rather than a tree.

What’s the difference between European, Japanese, and American plums?

European plums (Prunus domestica: Stanley, Damson, Italian, President) are the cooking and prune plums. Tougher-skinned, later-ripening, more disease-resistant. Japanese plums (Prunus salicina: Santa Rosa, Methley, Shiro) are the large juicy fresh-eating plums. Better flavor, thinner skin, more susceptible to black knot and bacterial spot. American plums (Prunus americana) are the wild species and its cultivars (Toka, Underwood). Hardy to zone 3, smaller fruit, prone to plum pockets. Pick European for disease-pressure regions, Japanese for mild dry-summer climates, American for cold-winter areas with native landscaping.

Will plum pockets kill my tree?

No. The disease attacks fruit and a few shoots but doesn’t harm the underlying tree. Even heavily infected plums (where 80% of fruit becomes bladders) recover fully and produce a normal crop the following year if you skip the dormant spray for a season. The pathogen doesn’t accumulate damage in the trunk or scaffolds.

Can I save a tree with severe black knot if I prune all the galls?

Usually yes, over 2-3 years. First-year pruning gets the obvious mature knots. Second-year pruning gets the small first-year swellings missed on the first pass. By year three, with sterilized blades, dry late-winter pruning, and any nearby wild Prunus removed, most trees stabilize and produce normally. The exception is when galls are concentrated on the central leader or main trunk where pruning would destroy the tree’s structure.

Are plum diseases contagious to other fruit trees?

Most are. Black knot crosses plums, cherries, and chokecherries. Brown rot crosses all stone fruits. Bacterial spot crosses plums, peaches, and nectarines. Cytospora canker (covered in the peach and cherry guides) can move between plum, peach, and apricot. Plum pockets is mostly plum-only. If you grow multiple stone fruits, treat them as one disease system with shared sanitation, spray schedule, and pruning protocol.

When should I spray my plum tree?

Two anchor sprays handle most disease pressure. First: dormant copper at bud swell (late winter, before any green tissue shows) for plum pockets, bacterial canker, and overwintering brown rot inoculum. Second: a fungicide application at full bloom for brown rot, repeated 10-14 days later. For black knot, an early spring fungicide application can supplement winter pruning if your tree is heavily infected. For most backyard plums in most years, the dormant spray plus bloom spray is the entire program.