Powdery mildew is easy to identify. It looks like white powder.
Powdery mildew on gerbera
An uninfected leaf
Powdery mildew is a fungal disease. Wet or humid conditions are conducive to powdery mildew infections. Treat it like you would other garden maladies. Soaps and oils can be effective, and fungicides can be called on if they list powdery mildew among their targets. Untreated, powdery mildew can damage leaves. Left unchecked, it can dramatically weaken bonsai.
Damage caused by powdery mildew
Sometimes the infections can look like white droplets.
Powdery mildew on calendula
Powdery mildew is less likely to strike conifers than it is broadleaf varieties. Japanese maples are quite susceptible, as are other deciduous varieties like stewartia or ume. If you have trouble with powdery mildew, moving the affected bonsai to a place that gets more light can help, as can ensuring the foliage doesn’t stay too wet. Systemic fungicides can help with difficult cases.
Interested in trying a “green” approach to managing powdery mildew? Try milk. I don’t have direct experience with this approach, but a 1:10 mixture of milk and water can be pretty effective. If you’ve tried this, do let us know how it worked!
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