Most bonsai problems start with stress: too much water, too little light or poor soil. A stressed tree is far more likely to be attacked by fungus or insects. Below are the problems growers see most often, how to recognize them, and what to do.
Root rot
Signs: leaves yellow or wilt even though the soil is wet, soil smells sour, roots are dark and mushy instead of firm and pale.
What to do: remove the tree from the pot, cut away soft roots with clean shears, and repot into fresh, free-draining bonsai soil. Water less often and make sure the pot drains freely.
Powdery mildew and other fungal problems
Signs: a white, dusty coating on leaves, or brown and black spots that spread.
What to do: remove affected leaves, improve air flow around the tree, water the soil rather than the foliage, and use a fungicide labeled for the plant if the problem continues. Full guide: powdery mildew on bonsai trees.
Spider mites
Signs: tiny pale speckles on leaves, fine webbing, foliage turning dull or bronze. Junipers and indoor trees in dry air are common targets.
What to do: rinse the foliage with a strong spray of water, raise humidity, and treat with insecticidal soap or horticultural oil, repeating as the label directs.
Scale insects
Signs: small brown or white bumps on stems and the undersides of leaves, sticky residue on leaves.
What to do: scrape them off with a soft brush or cotton swab dipped in rubbing alcohol, then treat with horticultural oil. Check again every week for a month.
Aphids
Signs: clusters of small green, black or white insects on new shoots, curled leaves, sticky honeydew.
What to do: wash them off with water and use insecticidal soap if they return.
Problems that look like disease but are not
- Leaf drop after moving the tree: usually shock from a change in light. Keep it in one bright spot and it will recover.
- Brown juniper foliage: often caused by keeping a juniper indoors or letting it dry out. See why Japanese juniper bonsai turn brown.
- Yellow ficus leaves: usually overwatering or low light. See ficus bonsai leaves turning yellow.
- Wilting: can be either too dry or too wet. See why bonsai leaves wilt.
How to prevent bonsai diseases
- Use free-draining bonsai soil and a pot with drainage holes.
- Water only when the top of the soil is dry.
- Give the tree plenty of light and fresh air.
- Clean your tools between trees.
- Inspect the leaves, both sides, every time you water.
- Keep new trees apart from your collection for a couple of weeks.
More guides: bonsai pests and diseases and juniper bonsai pests and diseases.