Answer: I apologize if this is the second answer your recieve, there was a computer problem earlier today. Hydrangeas can show this type of symptom if over or under watered, or due to a disease problem such as botrytis or bacterial wilt.
They need soil that is evenly moist yet well drained meaning damp like a wrung out sponge and not saturated/sopping wet or dried out. It is better to water deeply but less often than lightly every day. To know if you need to water, dig into the soil with your finger. If it is still damp, don't water yet. When you do water, apply it slowly and thoroughly so it soaks down to the deepest roots. Apply the water to the soil and avoid wetting the foliage. After watering, wait a few hours and then dig down to see how far it went, it can be surprising.
Using a two to three inch layer of organic mulch over the root area can help keep the soil more evenly moist. This also helps feed the soil as it breaks down slowly over time.
Hydrangeas are potentially subject to a variety of fungal and bacterial diseases, these can indeed spread. In case it is a disease, clean up any fallen leaves, remove affected flowers and leaves, and avoid handling the plant when wet. This fall, do a thorough cleanup off all fallen leaves and old blooms as well.
Since I am not able to diagnose the problem long distance, I would suggest you consult with your local county extension to obtain a specific diagnosis. If it is a disease that requires chemical control, they will have the most up to date information on what to use and how/when to apply it for best results. Since it is a new plant, you might also check with your retailer. I'm sorry you are having trouble with your hydrangea.
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