Could you add a close up of the hydrangea leaf in the first picture with the red edge, both top side and bottom side? 
Thank you

The one hydrangea seems to be infected with rust. The hibiscus looks to be infected with cercospora leaf spot. Both rust and cercospora leaf spot, a fungal disease. Cercospora is caused by the pathogen Cercospora. Rust uses a secondary host of hemlock. Both can be a problem when plants are watered overhead and the leaves get wet, or in seasons with frequent rain.

To manage cercospora leaf spot, always water at the base of the plant and keep leaves dry. Provide one inch of water per week to help alleviate the stress on the hydrangea. Clean up fallen leaves and dispose of them. If there are just a few leaves with spots, clip them off to keep the healthy leaves from being exposed. Clean your tools with a bleach solution to keep the fungus from spreading to other plants. Place clean mulch around the base of the plant to prevent soil from splashing onto the plant during rainfall. Keep mulch pulled back from around the plant stems so it doesn’t touch the plant stems.

Rust is an obligate parasite, so it needs living tissue to grow. The pathogen needs two hosts to complete its life cycle. Thinning the inside of the hydrangea bush, cleaning rust infected leaves and fallen plant debris around both hemlock and hydrangea may help to manage this disease. Many products can be used to disinfect cutting tools.

Using a fungicide spray according to label directions can protect fresh leaves. This is a recommendation from Extension:

“Products containing chlorothalonil, myclobutanil or thiophanate-methyl are most effective when applied prior to or at the first sign of leaf spots. These fungicides work best to protect newly developing leaves, but they will not protect new growth that emerges after the application has been made. Consequently, to keep plants relatively free of the disease, multiple applications (usually every 10 to 14 days) may be necessary.

Remember with all pesticides, always read, and follow the label directions. Complete coverage of the leaves with the fungicide in conjunction with proper timing of applications is important for successful leaf spot management.”

More details here: Hydrangea Diseases (psu.edu)

Hydrangea: Diseases and Pests (uconn.edu)

Thank you for your question and have a great gardening summer.

Make sue you clean up all the leaves this fall and dispose of the leaves in the trash, do not compost the leaves. Also replace any mulch on the ground around the infected plants. This may help to control the infection.