What Diseases And Problems Are Hydrangeas Prone To?

It probably is a caterpillar or a cricket. If you find caterpillar worms on the leaves, you can get rid of them by spraying or dusting with Bacillus Thruingensis. If you find no worms, it might probably be because of snails and slugs so you can use the same to treat the ground under the leaves.

The leaves are my hydrangea are quite yellow - what should I do?

Hydrangeas are affected by diseases and insects under certain growing conditions though Hydrangea species being found in the landscape areas are relatively pest free. Powdery mildew is the major disease found in bigleaf Hydrangea as this is a common disease of plants being grown in shade and under high humit conditions. In this disease leaves of a plant are covered with a light gray colored powdery-looking substance.
In late summer and early if leaves develop brown to gray lesions surrounded by pruple holes then it's a sign or indication that leaves are being attacked by several fungal leaf organisms. Though plants are rarely killed by this disease but they look very unattractive due to severe infestation.
Root rots normally attack Oakleaf Hydrangeas. Armillaria is the most common root rot. The plants noramlly die of this root rot. The plants will not recover when watered if look wilted.
Rust is the common disease of Smooth Hydrangea. In this disease small organge spots appear on the back side of leaves. These spots release orange dust if back sides of leaves are rubbed. These orange dust contains the spores of the fungus. This disease rarely kills plant and usually attack plants near the end of growing season.

Tdrangea has a web coveing the beautiful blooms and little white powdery substance on top and under the leavs  I have cut off the blooms and now it looks like hell  I have sprayed it twice with dish detergent and water  what happened  b

White bug on underside of leafs and on stems