|I've brought my two hibiscus outside now that winter is over. They are green and healthy but have a powdery substance on the trunks. Is this normal?|
|If you can rub the powdery substance off, it's a fungal disease called powdery mildew. It will spread so be careful not to touch any other plants or you'll introduce the disease.|
You can purchase fungicides to eliminate the problem or you can make your own fungicide: Baking Soda Fungicide
Mix 4 teaspoons (about 1 rounded tablespoon) of baking soda and 1 tablespoon of horticultural oil into one gallon of water. Spray lightly on foliage of plants afflicted with black spot, powdery mildew, brown patch and other fungal diseases. Avoid over-using or pouring on the soil.
Best wishes with your garden!