About 3 weeks ago I noticed curling on the leaves of one of my hydrangea plants. Now many of the leaves of the new growth are curled and brown. The flowers are also brown and appear to be dying. These are established plants. What might be causing this problem? Thanks.
I see a couple problems in your pictures. The small round spots with tan centers on leaves are likely cercospora fungus. The curling , browning leaves and flowers are likely from gray mold fungus called botrytis blight or anthracnose. These problems occur especially during wet weather. Keep leaves dry during watering, and keep dead leaves and flowers clipped off and thrown away. There are some fungicides you can use, too.
Look at the bottom of the distorted leaves for small insects called aphids. Their feeding can distort the leaves, too. Aphids can be controlled by spraying both sides of leaves with insecticidal soap (a garden product, not dish soap) or a few other garden products. Here is detailed info on aphids- https://www.extension.umn.edu/garden/insects/find/aphids-on-deciduous-trees-and-shrubs/
Here is an article with leaf disease details, recommended actions and fungicides. Some fungicides are only for commercial growers, but you can find home garden products containing chlorothalonil or mancozeb and some others-
Herbicide injury (from drift, especially on hot days) and several viruses can affect hydrangea and distort the leaves. If none of the above actions helps, and your new growth continues to have problems, you can submit samples to MSU Diagnostic lab for an exact I.D. Here is their website: https://pestid.msu.edu
As you can see there are many possibilities. So, a little detective work is in order! Thank you for using our service.