White Spots on Tree Bark: Lichen, Bugs, or Mildew?
White or grayish patches on tree bark are most often lichen — a harmless organism that simply perches on the bark and is not a disease or parasite. The other common causes are cottony white scale or woolly aphids, powdery mildew on green bark and leaves, or deliberately whitewashed trunks. Whether the growth is crusty, cottony, or powdery tells you which one, and lichen itself never needs treatment.
Most likely causes
- Lichen — flat, crusty gray-green-white patches that are completely harmless
- Woolly aphids or scale — raised white cottony bumps on bark and twigs
- Powdery mildew — a fine white coating on leaves and green bark
- Whitewashed trunks — deliberate white paint low on the trunk for sunscald protection
- Fungal crust — a whitewash-like film on shaded, damp bark
Not quite what you're seeing? Answer four quick questions and the clue finder will point you to a closer match.
Compare the possible causes
| Possible cause | Key signs | When it happens | How likely |
|---|---|---|---|
| Lichen | Flat, crusty or leafy gray-green-to-white patches that cling tightly to the bark and flake off when rubbed | Year-round, more noticeable in cool, damp weather and on slow-growing or older trees | Very common |
| Woolly aphids and scale insects | Raised white, cottony or waxy bumps clustered on the bark, twigs, or branch crotches that smear when squished | Spring through fall, when the insects are actively feeding | Common |
| Powdery mildew | A fine, dusty white coating on leaves and green stems or bark that wipes off with a finger | Warm days with cool, humid nights, common in late summer and fall | Common |
| Whitewashed or painted trunks | A deliberate, even coat of white on the lower trunk, often on young or newly planted fruit trees | Applied in late fall or winter and visible year-round | Less common |
| Whitewash-like fungal crust | A thin, chalky white or gray film on shaded, consistently damp bark | Damp, shaded conditions, most often in humid seasons | Rare |
Visual clues to check
- Rub the patch: crusty or leafy growth that flakes off in bits is lichen and is harmless
- Check whether it's raised and cottony — fuzzy white bumps that smear when squished are insects, not lichen
- Wipe a leaf or green stem: a dusty white film that rubs off cleanly is powdery mildew
- Note the location: lichen appears anywhere on the bark, while mildew favors leaves and tender green growth
- Look for a sticky, shiny film or sooty mold below white bumps, which points to scale or woolly aphids
- Ask whether the trunk was painted — a smooth, even white coat low on a young tree is likely sunscald protection
The causes in detail
Lichen
Lichen is by far the most common cause of white patches on bark, and the good news is it's completely harmless. It's a partnership of fungus and algae that simply uses the bark as a perch — it is not a disease and it does not feed on the tree. Lichen tends to show up on slow-growing, older, or thinly leafed trees because there's more exposed bark and light, not because the tree is sick. It's often considered a sign of clean air.
Woolly aphids and scale insects
Unlike flat lichen, these are raised, fuzzy or cottony masses that are actually insects — woolly aphids or soft scale coated in white wax. They pierce the bark to feed and can excrete sticky honeydew, so you may also see a shiny film below them. A light infestation is minor, but heavy clusters on a young or stressed tree can weaken it and are worth managing — see tiny-white-bugs-under-leaves and sticky-residue-on-leaves.
Powdery mildew
Powdery mildew is a fungal disease that leaves a talcum-like white film, usually on leaves and tender green bark rather than mature trunk bark. It wipes away easily, which helps separate it from crusty lichen. It's rarely fatal but can stress a struggling tree; improving air circulation and avoiding overhead watering usually keeps it in check.
Whitewashed or painted trunks
Sometimes the white isn't growth at all — it's diluted white latex paint applied on purpose to protect thin bark from sunscald and winter cracking. It's a common practice on young fruit and ornamental trees. If the white is a smooth, uniform coating low on the trunk, it may simply be paint someone applied for protection.
Whitewash-like fungal crust
On the perpetually damp, shaded side of a trunk you can occasionally get a thin whitish crust of harmless surface fungi or algae. Like lichen, it lives on the bark surface rather than feeding on the tree. It usually reflects a shady, humid microclimate and needs no treatment.
When to worry
- White cottony masses that are spreading across twigs and branches, signaling a growing scale or aphid infestation
- Heavy powdery mildew on a tree that is already struggling, dropping leaves, or newly planted
- Sticky honeydew and black sooty mold building up beneath the white patches
- Dieback, thinning canopy, or oozing near the white areas, which suggests a separate underlying problem
- Lichen itself, by contrast, never requires action — thick lichen usually just reflects a slow-growing or shaded tree
What to do now
- Leave lichen alone — it doesn't harm the tree and is often a sign of clean air; there's nothing to treat
- If the white is raised and cottony, confirm it's insects and manage them culturally, such as a strong water spray or by encouraging natural predators
- For powdery mildew, improve air circulation, prune for light, and avoid wetting the foliage in the evening
- Keep the tree vigorous with proper watering and mulching, since a healthy tree shrugs off minor mildew and light scale
- For a heavy or spreading scale or aphid infestation on a valued tree, have a certified arborist or extension office recommend treatment
What not to do
- Don't scrub, pressure-wash, or scrape lichen off the bark — you'll wound the tree while removing something harmless
- Don't spray fungicide or insecticide on lichen; it's not a fungus attacking the tree and won't respond
- Don't assume every white patch is a disease — most of the time it's benign lichen or protective paint
- Don't paint or coat mature trunks to 'cover' the spots, which can trap moisture and pests
Frequently asked questions
Is white lichen on my tree bark harmful?
No. Lichen is not a disease or a parasite — it simply uses the bark as a surface to grow on and doesn't feed on or damage the tree. It's most common on slow-growing, older, or thinly leafed trees, and it's often taken as a sign of clean air. There's nothing to treat and no reason to remove it.
How do I tell lichen from scale insects on bark?
Lichen is flat, crusty or leafy, and firmly attached, flaking off in bits when you rub it. Scale and woolly aphids are raised, cottony or waxy bumps that smear when squished and may leave a sticky film below. If it's fuzzy and clustered on twigs rather than flat on the bark, it's likely insects.
Why is there a white powder on my tree's leaves and bark?
A fine, dusty white coating that wipes off easily is usually powdery mildew, a fungal disease favored by warm days and humid nights. It mostly affects leaves and tender green bark rather than mature trunk bark, and it's rarely fatal. Better air circulation and avoiding evening overhead watering usually control it.
Should I remove lichen from my trees?
No — removing lichen only risks injuring the bark for no benefit, since it isn't hurting the tree. If a tree with heavy lichen also looks thin or unhealthy, the lichen isn't the cause; it's just more visible on a sparse canopy. Focus on the tree's overall health instead.