Mildew is one of the most stubborn fungal diseases that can affect hydrangea. While powdery mildew is a fair-weather fungus, downy mildew spreads explosively in humid weather conditions.
How can you identify and combat powdery mildew on hydrangeas?
To identify powdery mildew on hydrangeas, look for white to dirty brown coating for powdery mildew and gray/gray-purple coating for downy mildew. Remove diseased parts of the plant and treat the hydrangea with fungicides containing copper (for downy mildew) or products containing sulfur (for powdery mildew).
Identify and distinguish between true and downy mildew
Powdery mildew:
- the upper side of the leaf is always affected
- wipeable, white to dirty brown coating
- Leaves turn brown and dry up
Downy mildew:
- Fungal grass always on the underside of the leaves
- gray or gray-violet coating
- Top of leaf shows yellowish highlights
- Leaf slowly dies off
You can only prevent downy mildew, which often affects hydrangeas. Keep the leaves dry by always watering the hydrangea from below. Algae-based plant strengtheners, which you can obtain from specialist retailers, can effectively prevent infestation.
Fighting mildew
- Remove all diseased plant parts and dispose of them with household waste.
- If you have downy mildew, spray with fungicides containing copper. Make sure that the undersides of the leaves are wetted.
- Powdery mildew is combated with products containing sulfur.
- Treat the hydrangea several times as mildew is very stubborn.
Tips & Tricks
This fungal infestation can also be combated biologically. The lecithin contained in fresh whole milk or whey fights the fungus and can prevent it from spreading further.
