Powdery mildew is a real nuisance when you care for peppermint. Here you need to distinguish between powdery mildew and downy mildew. One occurs in very dry weather, the other – in very humid weather.
Powdery or downy mildew?
Powdery mildew can be recognized by the fact that it only appears on the upper side of the leaves and can be rubbed off with a finger. Downy mildew attacks the undersides of leaves, where it causes spots that show through to the surface.
Natural measures against powdery mildew
Cut off infested shoots
Dispose of shoots in household waste or burn them
Spray plant with diluted milk
Make a decoction of tansy or garlic
Use neem oil
You must spray infested peppermint plants with the decoction every two days.
As a preventive measure, make sure that you do not water the peppermint too much or too little.
Tips & Tricks
Do not plant peppermint too close together and make sure that air can circulate between the leaves.