Answer: Based on your description I am not certain what is happening to your hydrangeas. I would suggest you consult with your local Rutgers county extension to obtain a specific diagnosis of the problem(s) and based on knowing that, determine how to proceed. If a chemical control is needed, they will have the most up to date information on what to use and how/when is best to apply it for maximum results. To help them with the diagnosis, they may appreciate an overall photo of the plant to show the pattern of where the problem is, as well as close-ups of the affected leaves or samples enclosed in a clear plastic bag and kept cool so they stay fresh.
In the meantime, pick off the worst affected leaves and clean up any dropped leaves and dispose of them in the trash to try to limit sources of reinfection.
Also, make sure you are watering correctly to help the plant stay as healthy as possible. Your goal in watering is to keep the soil evenly moist like a wrung out sponge, not sopping wet and not dried out. To know if you need to water, dig into the soil with your finger. If it is still damp, do not water yet. When you do water, apply it to the soil surface and water thoroughly and slowly so it soaks down to the deeper roots. After watering, wait a few hours and then dig down to see how far the water soaked in; it can be surprising.
There is no set schedule for watering, it depends on your soil type and on the weather. Using an organic mulch several inches thick over the root area will help reduce watering needs as well as feed the soil gradually as it breaks down over time.
I'm sorry I can't be more specific for you long distance.
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