Apple Tree Diseases: How to Identify and Treat the Big Ones
Apple trees attract a roster of diseases that nothing else in the yard has to deal with. Most of them are manageable if you catch them early. One of them, fire blight, can kill a healthy tree in a single growing season if you ignore it.
The other big problems (apple scab, cedar-apple rust, sooty blotch, powdery mildew, black rot) range from cosmetic to harvest-ruining but rarely threaten the tree itself. Here’s how to tell which one you’re looking at, what to do about it, and which problems you can let the tree handle on its own.
How to tell which disease you’re looking at
Quick visual triage. Match the most obvious symptom to the likely culprit, then jump to that section.
- Olive-green velvety spots on leaves and fruit in late spring: apple scab
- Blackened shoot tips curled into a shepherd’s crook, sticky tan ooze on bark: fire blight
- Bright yellow-orange leaf spots with tiny finger-like growths on the underside: cedar-apple rust
- Smudgy dark blotches and pinpoint black dots on fruit skin in late summer: sooty blotch and flyspeck
- White powdery coating on new shoots and curled young leaves: powdery mildew
- Concentric brown rings on fruit, purple-bordered “frogeye” leaf spots, sunken branch cankers: black rot
Almost every disease on this list takes off during wet weather in spring and early summer. If you live somewhere with humid springs (anywhere east of the Sierra, plus coastal California from Eureka to Monterey), you’ll see at least one of these every year.
Apple scab
Apple scab (Venturia inaequalis) is the most common apple disease in North America and the one that turns up first every spring. The fungus overwinters in fallen leaves, then releases spores during the first wet weather after bud break. Spores landing on wet green tissue infect within 6 to 28 hours, depending on temperature, according to Penn State Extension.
What it looks like
Fuzzy yellow-green spots on the upper leaf surface, about a quarter inch across, showing up by late April or May. As the spots mature they turn dark olive, then nearly black, with a velvety texture. Heavy infections cause leaves to twist, yellow, and drop early. On the fruit, scab starts as the same olive lesion and develops into black corky cracked patches by midsummer. The flesh underneath is usually still edible, you just cut around the damage.
What to do about it
Sanitation is half the battle. Rake every fallen leaf in autumn and bag them for trash or hot-compost them. The fungus needs that leaf litter to overwinter, and removing it cuts the next year’s spore load by 80% or more. Shredding fallen leaves with a mower in fall speeds the breakdown if you’d rather not rake.
For active infections, a copper fungicide applied at green tip (when the first quarter inch of leaf shows) and again at pink (just before bloom) protects new tissue during the highest-risk window. I keep Bonide Copper Fungicide concentrate in the shed for exactly this. Mix per label and hit the tree before a rain event, not after. A pint of concentrate runs $15 and covers a backyard tree for two seasons.
If you’re planting new and live somewhere humid, pick scab-resistant cultivars from the start. Liberty, Enterprise, and Pristine all carry resistance genes and need little to no spraying. Granny Smith, Gala, and Honeycrisp do not.
How to keep it from coming back
- Rake or shred fallen leaves every fall
- Prune the canopy open in winter so leaves dry faster after rain
- Avoid overhead watering. Drip or basin irrigate at the root zone instead
Fire blight
Fire blight is the disease that kills apple trees. It’s bacterial (Erwinia amylovora), not fungal, so fungicide does nothing. It’s also the only common apple problem that can take a mature tree from healthy to dead in one growing season. If you grow apples or pears in your yard, learn the symptom and act the day you see it.
What it looks like
Look at the tips of new shoots in May and June. Blackened, oily-looking dead tissue with the tip curled into a tight “shepherd’s crook” is the signature. The dead tissue often drips a sticky tan exudate that hardens into beads on the bark. Blossoms wilt and turn black on the cluster. Whole branches can die back within weeks.
Why it happens
Fire blight bacteria overwinter in cankers on the tree and ooze out during warm, wet weather. Bees and other pollinators carry the bacteria to open blossoms during bloom. UC IPM notes that infections take off when bloom temperatures hit roughly 60°F with humidity above 80%, which is exactly what a wet NorCal spring delivers.
Cultivar matters. Gala and Fuji are devastated in bad years. Granny Smith holds up better. Among rootstocks, M9 and M26 are highly susceptible: a fire blight infection that reaches the rootstock kills the entire tree regardless of which variety is grafted on top.
What to do about it
This is the only disease in this guide where the response needs to happen the day you spot it.
- Prune out infected branches at least 12 inches below the last visible symptom, ideally 24 inches if you can spare the wood. Cut into clean, healthy wood.
- Sterilize your pruners between every cut. Wipe the blades with 70% rubbing alcohol, or dunk them in a 10% bleach solution (1 part bleach to 9 parts water). Skip this step and you’ll spread bacteria to every branch you touch. I use Felco F2 Bypass Pruners for fire blight work because the blade disassembles for cleaning, and a spray bottle of alcohol lives in my pocket during the prune.
- Burn or bag the prunings. Don’t compost them. The bacteria survive in dead wood.
- Avoid heavy nitrogen fertilization. Excess nitrogen produces the lush, succulent new growth that fire blight bacteria target. When you fertilize your apple tree, use a balanced formula and don’t overdo it. For a deeper look at nutrient timing across the season, this fruit tree fertilizing guide breaks down nitrogen, potassium, and phosphorus by growth stage.
For trees with a history of fire blight, a copper spray at silver tip (early bud break, when the first scales separate) reduces overwintering bacteria. Streptomycin sprays during bloom work but are restricted in many states for home garden use.
How to keep it from coming back
The single best prevention is dormant-season inspection. Walk the tree every January with a pair of pruners and look for sunken, cracked, dark cankers on the branches. Those are the launching pads for next spring’s outbreak. Cut them out before bud break. Combine that with conservative fertilization and resistant cultivars and most yards stay fire-blight-free.
For more on dormant pruning technique, see our apple tree pruning guide. It covers cut placement, sterilization, and the difference between maintenance pruning and disease pruning.
Cedar-apple rust
This one looks worse than it is. Cedar-apple rust (Gymnosporangium juniperi-virginianae) requires two completely different hosts to complete its life cycle: an apple or crabapple in the rose family, and a juniper or eastern red cedar. The fungus alternates between them, overwintering on the juniper as a brown woody gall and moving to apples in spring.
What it looks like
On the apple side, bright yellow-orange spots on the upper leaf surface with a red or rust-colored border. Flip the leaf over and you’ll see tiny tubular fingers (less than a tenth of an inch long) projecting downward. These are the fungal fruiting structures shedding spores. Heavily infected leaves drop early but the tree survives without intervention.
On the juniper side, golf-ball-sized brown galls grow on the branches. After warm spring rains, those galls produce neon-orange, gelatinous “tentacles” that dangle from the gall and release spores back toward apple trees up to a mile away.
What to do about it
The University of Minnesota Extension recommends keeping eastern red cedar and juniper several hundred yards from apple trees. In suburban yards that’s almost never possible, so prevention falls to cultivar selection and gall removal.
- Plant resistant cultivars. Liberty, Freedom, Redfree, and Enterprise resist rust as well as scab. Honeycrisp and Jonagold are highly susceptible.
- Remove visible juniper galls in winter before the orange tentacles form. Even taking out the closest infected juniper in your yard cuts local spore pressure.
- Skip the fungicide on mature trees. Rust rarely justifies the expense or labor. It looks dramatic but most trees tolerate it without losing structure or vigor.
Sooty blotch and flyspeck
These are two fungal complexes that show up together so often the extension services treat them as one problem. Sooty blotch produces smudgy dark gray-green patches on the fruit skin. Flyspeck produces small, shiny, black dots in tight clusters that look like the bottom of a screen door. Both develop in late summer on humid, shaded fruit.
What it looks like
Both blemishes are skin-deep. The apple under the discoloration is fine. Peel it or wipe the surface with a damp cloth and the fruit eats normally. You’ll notice them most on apples that hung in the inner canopy of a dense tree where air didn’t circulate.
What to do about it
If you grow apples to eat, ignore sooty blotch and flyspeck. If you grow apples for the county fair, you need a summer fungicide schedule every 14 days from petal fall through harvest, plus an open canopy.
The least expensive fix is pruning. Open the canopy in winter so summer air moves through the branches and dries the fruit surface. Most cases disappear after one good restoration prune. Our apple tree pruning guide covers the modified central leader and open-center cuts that get air into the tree.
Powdery mildew
The pathogen is Podosphaera leucotricha, a different species from the powdery mildew that hits maples and crepe myrtles, but the symptom looks the same. White, felty coating on new shoots in May. Distorted, curled young leaves.
What it looks like
The youngest leaves at the tips of new shoots come out smaller than normal, edges curled, dusted with what looks like white flour. Severe infections turn the affected shoots gray and lifeless. Unlike scab, mildew thrives in dry weather with cool nights and warm days, so it tends to hit during exactly the years apple scab takes a break.
What to do about it
A horticultural oil or neem oil spray at bud break and again at pink suppresses early infections. For ongoing problems, prune out the worst-infected shoots in winter. The fungus overwinters inside dormant buds, and removing those buds removes next year’s outbreak.
Resistant cultivars include Liberty, Enterprise, and most modern disease-resistant releases. Jonathan and Rome Beauty are particularly susceptible.
Black rot
Black rot (Diplodia seriata, formerly classified as Botryosphaeria obtusa) shows up in three different forms on the same tree, which makes it confusing the first time you encounter it.
What it looks like
- Frogeye leaf spot: round leaf spots with a purple or reddish border and a tan center. Looks like the eye of a frog. That’s where the name came from.
- Fruit rot: large brown rotten areas on the apple, often with concentric rings. The fruit usually stays attached and mummifies on the branch instead of falling.
- Branch canker: sunken, dark patches on limbs that may girdle the branch and cause dieback above the canker.
Why it happens
Black rot lives in dead and dying wood. Cankers from old fire blight infections, broken branches, sunburned bark, any wound becomes an entry point. Once it’s in the tree, it produces spores that infect leaves and fruit during wet weather. The University of Minnesota Extension confirms that the disease persists in mummified fruit on the branches over winter, which is why those papery dried apples you see hanging in old orchards in February are a real problem.
What to do about it
- Remove mummified fruit from the branches every winter. Bag and trash them.
- Prune out cankered branches back to clean wood. I use Felco F31 Anvil Pruners for the dead wood. The anvil blade cuts dry branches that bypass pruners would just crush.
- Maintain tree vigor with steady water and proper fertilization. Stressed trees attract black rot. Vigorous trees wall off infections.
- Skip the fungicide. Sanitation does the work fungicide can’t.
When to call an arborist
Three situations warrant a paid visit. First, fire blight that has reached the trunk or central leader and you’re not confident about cutting back to clean wood. Second, more than 30% of the canopy involved in any disease, where you need to decide whether to treat or remove. Third, any infection on a mature tree (15+ years old, the kind you can’t replace without waiting a decade) where the cost of getting the diagnosis wrong is high.
An ISA-certified arborist consultation runs $75-200 in most California markets and is far less expensive than losing a mature tree or doing the wrong treatment. Find one through the Trees Are Good arborist locator or read our guide to what an arborist does before you call.
Prevention checklist
Most apple disease problems trace back to four habits. Get these right and you’ll skip 80% of the trouble.
- Sanitation in fall. Rake leaves, pick up dropped fruit, remove mummified fruit from the branches. This single habit prevents apple scab, black rot, and sooty blotch all at once.
- Open canopy. Prune in winter so the interior of the tree sees light and feels air movement. Wet leaves and trapped fruit are how fungi thrive.
- Resistant cultivars. Planting new? Liberty, Enterprise, and Pristine give you a tree that needs almost no spraying. Honeycrisp and Gala give you a tree that needs constant attention.
- Conservative nitrogen. Apple trees do not need annual heavy feeding. For nutrient timing through the growing season, the seasonal fertilizing guide on mklibrary.com lays out a schedule that minimizes the lush growth fire blight chases.
For broader fungal diagnosis across all the trees in your yard, see our tree fungus guide. Apple is one species in a much larger pattern of fall sanitation and spring spore management. If your apple tree dies and you’re looking at tree removal costs, the same arborist who diagnoses the disease usually quotes the removal.
Frequently asked questions
What is the most common apple tree disease?
Apple scab is the most common, by a wide margin. Olive-green velvety spots on leaves in spring, black corky patches on the fruit by summer. It’s not deadly but it ruins the harvest. Fire blight is the most dangerous and the one to act on immediately.
Can I save a tree with fire blight?
Sometimes. If you catch it early (small shoot tips affected, not yet into major branches), aggressive pruning 12 inches below visible symptoms with sterilized tools usually controls it. If it reaches the trunk or rootstock, the tree is finished. Cultivar matters too: Granny Smith often pulls through, Gala often doesn’t.
Are apple tree diseases contagious to other trees?
Some are. Fire blight spreads readily to pears, hawthorns, and ornamental crabapples, which are all in the Rosaceae family. Cedar-apple rust moves between apples and junipers. Apple scab is mostly apple-specific. If you have pears nearby, treat them as a single fire blight risk and prune them on the same schedule.
When should I spray my apple tree?
Apple scab: copper at green tip (first quarter inch of leaf showing) and again at pink, just before bloom. Fire blight: copper at silver tip on trees with a prior infection history. Powdery mildew: horticultural oil at bud break. One or two well-timed sprays in March and April do more for a backyard tree than a full season schedule applied at the wrong moments.
Will apple scab kill my tree?
No, almost never. Even severe scab infections defoliate the tree but it leafs back out the following spring. The damage is to the fruit harvest, not the tree itself. Multiple years of heavy scab can weaken the tree enough to make it vulnerable to other problems, so it’s worth controlling, but it isn’t an emergency.