Hello, janellw. Can you post pictures of the black leaves on the shrub and a close up of the leaves (front and back)? Then post another picture of the brown spots on the stems?

Root rot can produce brown spots on the stems although more obvious symptoms would include continuous wilting regardless of how much water you provide.

Excessive watering and-or soil that drains poorly can help develop this problem.

There are other types of fungal infections -in addition to root rot- that could cause black-ish leaves such as anthracnose. It could also be a case of powdery mildew where the leaves do not turn exactly black but maybe dark gray or purple-ish sometimes. The pictures may help us see what you see.

When you have fungal issues, the best thing you can do is make the environment difficult for the fungi to continue spreading. Sometimes Mother Nature does that when high summer temperatures and low humidity arrive. Other times, you need to control soil moisture better.

The best way to proceed is to use the finger method so you know when it is ok to water. On a daily basis, insert a finger into the soil to a depth of 4" every morning. If the soil feels dry or almost dry then add 1 gallon of water. If the soil feels moist, do not water. If the soil feels wet, determine why and take appropriate steps to prevent that if necessary. Do this every morning for two weeks. Every time that you water, make a note on a wall calendar. After two weeks, revisit the information on the wall calendar and determine how often you had to water. Say, every 2 days? every 3 days? Etc. Then set your sprinkler to give the hydrangea 1 gallon of water on that same frequency. If the temperatures change 10-15 degrees and stay there, recheck for two more weeks using the finger method.

Good sanitation techniques can also help. These are things like never watering the leaves but watering the soil only. Water the plants early in the morning. Discard in the trash all plant debris under the hydrangeas. Because this is an ongoing multi-year problem, replace the mulch with new mulch (3-4") and -in the fall- throw the dried out blooms and leaves in the trash. Allow some separation between shrubs to improve air flow. In large plants, open up areas (by pruning) so sun and air get deeper into the shrub.

See the link below for additional information on hydrangea diseases. If you cannot post pictures, feel free to take leaf samples to a plant nursery or Agric Extension Service for analysis (place the leaves in a sealed transparent bag).

Does that help you?
Luis

Here is a link that might be useful: Hydrangea Diseases