There are a few reasons that hydrangea leaves can curl like this – you can think about which one is more likely for your plant, but fungus isn’t one of them so you can stop using any fungicide. 1. Most common cause is a sucking insect such as aphids or lacebug feeding under the leaves when the foliage is young. This damages the cells early in the life of the leaf so that as the leaves open they are distorted. Spray underneath of foliage with insecticidal soap if you see tiny insects on the underside of leaves. 2. Herbicide damage. If an herbicide such as weed killer has been applied around the plant or on a lawn surrounding the plant this can cause what you’re seeing. Stop using weed killers on gardens and lawns. This includes pre-emergent products as well as broad-leaf weed killers. Water deeply to flush these out of the soil around hydrangeas. 3. Virus – least likely but possible. Usually with a virus you’ll also see yellowing of foliage not just the curling. There is no cure for a virus, but as we say, this is the least likely cause.