Identify, understand, act — without overreacting

On a Japanese maple, what people often call a “disease” online is very often a reaction to the environment: drying wind, harsh sun, compact substrate, excess cold water. This page helps you distinguish:

what is cosmetic,
what comes from a cultivation adjustment,
and what deserves real vigilance (rare, but important).

Before concluding: the quick 2-minute triage

1) Container or in the ground?

In a pot, the cause is often the substrate / water management duo.

2) One side only, or the whole tree?

Dieback on a branch or on one side points more toward a vascular/wood issue than a simple overall stress.

3) Leaves affected but healthy wood?

Often a foliar issue (powdery mildew, anthracnose…): impressive, rarely serious.

4) Cold, wet substrate that never dries?

Think roots: Japanese maples hate suffocation, especially in winter.

The most frequent problems (not necessarily diseases)

Leaf scorch and drying

Appearance: browned edges, dry tips, “scorched” leaves, often after dry wind / heat / strong sun.
Meaning: water stress / drying out (not an infection).
What to do: protect from wind, aim for gentler light, stabilize watering, check drainage.

By far the most common case, especially on finely cut foliage.

Roots and root collar: where real trouble starts

Root rots (often linked to excess cold water)

Appearance: diffuse decline, weak growth, less vigorous foliage, sometimes early autumn colour, heavy/compact rootball.
Typical context: soil that stays wet in winter, pot that drains poorly, saucer with standing water.
What to do: prioritize drainage + aeration (soil or substrate). Avoid water sitting at the collar.

A maple that leafs out normally in spring and then suddenly starts to wilt often points to root rot that developed during winter.

For maples, excess winter water is far more dangerous than “cold”.

Wood and sap flow: cases to watch closely

Verticillium wilt (soil-borne dieback)

Appearance: wilting and drying of a branch, sometimes one side of the tree, potentially progressing.
Good to know: there is no simple “miracle cure”. The goal is to reduce stress and manage the tree cleanly.
What to do: remove affected wood back to healthy wood (clean tool), avoid stress (drought / excess water), keep fertilization moderate.
When to worry: if dieback progresses quickly or returns despite good conditions.

Black patches on the trunk: sorting superficial vs canker

This is a frequent question. There are three main scenarios:

1) Superficial deposit (sooty mould) — mostly cosmetic
A black “soot” film sometimes linked to aphids/scale insects (honeydew). If it comes off with light rubbing on a small test area, it’s often superficial.

2) Lichens / algae — harmless
Dry grey/green/blackish patches “sitting” on the bark: common, non-pathogenic.

3) Lesion / canker (bacterial issues such as Pseudomonas, or other cankers) — to monitor
A darker area, sometimes sunken, cracked, slowly expanding. Sometimes accompanied by twigs that blacken/die back.
What to do (without excess):

avoid injuries (rubbing, tight ties, tool impacts),
keep the trunk dry (avoid watering over the bark),
if the patch does not go all the way around the trunk/branch, apply Bordeaux mixture with a brush (a “crepe batter” consistency),
if the lesion is on a branch: remove the affected part in dry weather, with a disinfected tool,
if the lesion is on the trunk (or seems to “girdle”): it’s better to diagnose before cutting.

If it’s wet black and sticky/oozing, be more cautious: chronic moisture is often an aggravating factor (review drainage).

Foliar diseases (often spectacular, rarely serious)

Anthracnose (irregular spotting/browning)

Appearance: spots, marked edges, sometimes along veins, often after a wet spring.
What to do: collect fallen leaves, lightly aerate the canopy (no heavy pruning), avoid watering the foliage.

On a vigorous tree, it is generally mostly cosmetic.

Powdery mildew (white/grey coating on leaves)

Appearance: powdery deposit, dull leaves, sometimes slightly distorted.
Context: lack of air flow, cool nights / mild days, slightly stressed plant; some cultivars (red leaves) are more sensitive than others.
What to do: improve aeration, water at the base (not on leaves), remove heavily affected leaves if needed.
Treatment: only if the attack persists, using an approved “powdery mildew” product for ornamentals, and staying moderate.

Pests (often straightforward to manage)

Aphids: mostly annoying (honeydew → sooty mould).
Scale insects: watch if the tree weakens.
Mites / galls: rare outdoors; they can become problematic under greenhouse conditions.

In most cases, a healthy plant (no excess water, no drought, no excess nitrogen) resists much better.

When to contact us (or request a diagnosis)

dieback of an entire branch or one side,
overall decline + cold, wet substrate/soil that stays permanently damp,
a black trunk patch that progresses, becomes sunken, cracks, or seems to “girdle”,
a potted tree that can’t stabilize despite correct watering.

For a useful diagnosis, send:

2 photos of the whole tree (front + side),
2 photos of leaves (top/bottom),
1 photo of trunk/collar,
1 photo of the pot/substrate (or surrounding soil).

FAQ

Leaves are turning brown: is it a disease?
Most often no. It’s frequently stress (dry wind/sun/irregular watering), especially on fine foliage.

How do I recognize verticillium wilt?
Localized dieback (one branch or one side), wilting then drying, sometimes recurring.

Black spots on the trunk: is it serious?
Not necessarily: it can be superficial (sooty mould), lichens, or a canker. Be more concerned if the area is sunken, cracked, wet/oozing, or progressing.

Powdery mildew on Japanese maple: what should I do?
Improve aeration, avoid watering the foliage, remove heavily affected leaves. Treat only if the attack is significant—especially if it happens early in the season.

Are leaf spots (anthracnose) dangerous?
Often no: it’s mainly cosmetic. Hygiene (fallen leaves) and aeration are usually enough.

Diseases and problems in Japanese maples