Leaf and stem rot

Foliage on hydrangea is being eaten

Question from Olga

Foliage on hydrangea is being eaten very fast. Leaves up to the rod. Can't see anything on the leaf except smth like a fluff and once a dark blue insect. Used Ultimate Bug Killer but to no avail. Plan is in the shade. The same damage but to a much smaller extend is on the rose that is on the sun. What might it be? And how to cure?
Many thanks