As drastic as these symptoms appear to be, they are not a serious threat to long-term plant health, and are fairly typical for hydrangeas this late in the growing season. Several types of leaf-spotting infections (some fungal, some bacterial) occur on hydrangea, especially in years when the weather is wet or on plants that are frequently irrigated. (Wet foliage tends to be more vulnerable to infection by disease spores.)

No treatment is needed, nor would a fungicide be effective at this stage. You can collect and dispose of the fallen hydrangea leaves as we move into autumn to try to reduce some of the spore presence for next year, though it's not a foolproof measure to eradicate the disease. Since the weather varies from one year to the next, so can the severity of disease, so this won't necessarily be as extensive in future years.

The plant that appears to be a Mulberry tree sapling in the middle of the hydrangea cluster should be removed if you didn't deliberately plant it. Its added shade will reduce the hydrangea bloom abundance as it matures, and it also may decrease the air circulation over the plants, which is needed to help keep the foliage drier and more disease-free. Mulberry is a weedy and invasive tree (it's highly unlikely it's our native species, because they are fairly rare) and grows rapidly, so they are easier to remove while young. If it can't easily be dug out, it will need to be cut down to ground level repeatedly so it exhausts its root stores and stops regrowing (eventually).

Miri

Hello Michael,

The main risk with over-watering is over-saturating roots so that they become oxygen-deprived, but this usually leads to wilting and stem dieback due to root death, so probably isn't happening here (at least not yet). However, watering methods that wet the foliage (like sprinklers or hoses aimed at the tops of the plants) do increase the risk for foliage infections, so either only watering the soil around the base of the plants or watering early enough in the day so the foliage can dry by nightfall are the best approaches to minimize this risk. (You can't do anything about rain or heavy dew of course, but by managing how you water, you can lessen the opportunities for infection.)

To determine when watering would benefit the plants, feel the soil around six inches deep near their roots and water thoroughly only when it's becoming relatively dry to the touch at that depth. As a very rough guideline, this might mean soaking the planting about once a week in hot, dry weather, but a lot depends on soil type, drainage, and other factors. It's not unusual for Hydrangea to wilt during hot days, even in shade and with sufficient soil moisture, so if or when this happens in the future you can check the soil before assuming the plants need water; often, they recover by evening or the next morning on their own and don't need water.

Miri

Hello Michael.
No need to strip the remaining leaves. Those that are spotty but still have green tissue are still helping the plant to photosynthesize and store food.
Once the frosts of autumn make leaves die and fall, that is the time to clean up and around the plants, raking out as many of the leaves as you can and throwing them in the trash. A fresh layer of mulch is also helpful.

Here is our Hydrangea page that you may enjoy reading:
https://extension.umd.edu/resource/hydrangea-identify-and-manage-problems


Christine