Bonsai Problems & Diagnosis

Work out what is wrong with your bonsai. Browse common problems by symptom and category, each with causes, look-alikes, treatment and prevention.

Start from what you can see

Most bonsai problems are cultural before they are biological: the tree is too wet, too dry, too cold, or too dark, and the pest or fungus that finishes it off arrives second. So start with the conditions, then work through the symptoms below. Each entry covers how to identify it, what causes it, what it is commonly mistaken for, and how to treat and prevent it.

If the tree is a species we cover, its care guide also names the single mistake most likely to kill that particular tree — worth reading before you treat for something exotic.

Cultural & environmental

Insufficient Light (Etiolation): Plants stretch toward light when they don't get enough — a response called etiolation.
Leaf Scorch / Sunburn: Leaf scorch is heat/light/wind damage to foliage: the leaf loses water faster than the roots can replace it, and tissue dries and dies — typically at the tip…
Nutrient Deficiency (Chlorosis): Chlorosis is yellowing from a lack of chlorophyll, usually because the tree is short of nutrients — either because it hasn't been fed, because the soil is ex…
Over-fertilizing (Fertilizer Burn): Fertilizer burn happens when fertilizer salts build up in the small volume of bonsai soil and draw water out of the roots (and from the leaf margins), scorch…
Overwatering: Overwatering is the single most common way bonsai are killed.
Repot / Transplant Shock: Repotting necessarily removes roots, and the tree needs time to regrow a working root system.
Underwatering (Drought Stress): Underwatering is letting the rootball dry out so the tree can't supply water to its foliage.
Winter / Frost Damage: In bonsai, winter damage comes in two forms.
Wire Bite / Scarring: Wire bite is the spiral scarring left when training wire is not removed before the branch grows thick enough to swell around it.

Pest

Aphids: Aphids are small, soft-bodied sap-sucking insects (green, black, brown, or pink) that cluster on the softest, fastest-growing parts of a tree — new shoot tip…
Caterpillars & Leaf-Chewers: A range of larvae and beetles chew bonsai foliage: butterfly/moth caterpillars, sawfly larvae (which can strip pines and other conifers), leaf beetles, and l…
Fungus Gnats: Fungus gnats are small dark flies whose larvae live in the top layer of damp, organic potting soil, feeding on fungi, decaying matter, and occasionally fine…
Mealybugs: Mealybugs are soft, oval, sap-sucking insects coated in white, waxy, cotton-like fluff.
Scale Insects: Scale insects are sap-suckers that live under a protective shell or waxy coating, looking like small bumps glued to bark, stems, and leaf veins.
Spider Mites: Spider mites are tiny sap-sucking arachnids (not insects) that pierce leaf cells and drain them.
Vine Weevil: Vine weevils are a classic 'silent' container pest.
Whitefly: Whiteflies are tiny white sap-sucking insects (like minute moths) that congregate on the undersides of leaves and flutter up in a little white cloud when the…

Fungal disease

Leaf Spot (Fungal): 'Leaf spot' covers many fungi (and some bacteria) that produce discrete spots on foliage.
Needle Cast / Needle Blight (Conifers): Needle cast and needle blight are fungal diseases of conifers that infect needles in warm, wet weather and cause them to discolour and drop, typically a seas…
Powdery Mildew: Powdery mildew is a fungal disease that coats leaves and shoots with a white-to-grey powder.
Rust: Rusts are fungi that produce powdery orange, yellow, or rusty-brown pustules, usually on leaf undersides or needles.
Verticillium Wilt: Verticillium wilt is a soil-borne fungal disease that invades a tree's water-conducting vessels and plugs them, so the tree effectively dies of thirst branch…

Fungal / water-mould disease

Root Rot: Root rot is the destruction of the root system by soil-borne fungi and water-moulds (Phytophthora, Pythium) that thrive in waterlogged, oxygen-poor soil.

Fungal / bacterial disease

Fungal Canker & Dieback: A canker is a localised dead area of bark and underlying wood caused by fungi or bacteria that invade through wounds, pruning cuts, frost cracks, or stress p…

Bacterial disease

Fire Blight: Fire blight is a highly destructive bacterial disease of rose-family pome trees.

Fungal disease (secondary)

Sooty Mold: Sooty mold is a black, soot-like fungal film that grows on the sticky honeydew excreted by sap-sucking insects.