Fungal Diseases in Bonsai: Identification, Treatment, and Prevention
Fungal diseases are the most common category of disease affecting bonsai, and they are closely tied to environmental conditions. Most fungal pathogens need moisture, warmth, and limited airflow to establish and spread. Understanding these connections helps you both treat problems when they arise and prevent them from developing in the first place.
Powdery Mildew
Powdery mildew is one of the most recognisable fungal diseases in bonsai. It appears as a white or pale grey powdery coating on leaves, young stems, and developing buds. Despite its dramatic appearance, it rarely kills a tree outright, but it weakens the affected tissue, reduces photosynthesis, and left unchecked can cause significant leaf distortion and early drop.
It is particularly common on Japanese maples, Chinese elms, and roses in late summer and early autumn when days are warm and nights are cool and humid. Poor airflow through the canopy is a significant contributing factor.
Improve airflow by opening up dense areas of the canopy through careful pruning. Avoid wetting foliage in the evening and treat affected trees with a sulphur-based spray or a diluted solution of bicarbonate of soda with a small amount of dish soap. Several effective organic treatments are available and work well with consistent application.
Botrytis (Grey Mould)
Botrytis cinerea, commonly called grey mould, produces a distinctive grey, fluffy coating on affected tissue and typically attacks flowers, soft new growth, and already weakened or dead plant material. It thrives in cool, damp, stagnant conditions and is most common in winter or during extended wet periods.
Remove all affected tissue promptly and dispose of it away from your other trees. Improve ventilation around the tree and reduce humidity where possible. A copper-based fungicide applied to affected areas and the surrounding tissue helps prevent spread.
Anthracnose and Leaf Spot Diseases
Anthracnose and related leaf spot diseases produce irregular dark spots or blotches on leaves, sometimes with yellow halos, and in severe cases cause significant leaf drop. They spread through water splash, which makes overhead watering a significant risk factor. Wet springs and summers with frequent rain are when these diseases are most active.
Remove and dispose of affected leaves, avoid wetting foliage when watering, and treat with a copper-based fungicide if the problem is widespread. Improved airflow and a sheltered position during very wet periods reduces the risk of recurrence.
Rust
Rust diseases produce distinctive orange, yellow, or brown pustules on the undersides of leaves. They are less common in bonsai than the diseases above but can affect certain species including junipers, hawthorns, and roses. Some rust species require two different host plants to complete their life cycle, so removing one host can break the cycle.
Remove affected leaves and treat with a fungicide specifically labelled for rust. Avoid overhead watering and ensure good airflow around the tree.
General Prevention Principles
- Maintain open branch structure to maximise airflow through the canopy
- Water at the base of the tree rather than over the foliage
- Remove fallen leaves and debris from the soil surface regularly
- Clean tools between trees to avoid transferring fungal spores
- Seal pruning wounds, particularly on maples and other susceptible species
Finally
Fungal diseases respond best to early intervention and improved growing conditions. A tree kept in good health, with appropriate airflow and careful watering, will resist most common fungal problems far more effectively than one that is stressed or growing in cramped, damp conditions.
