I'm new to gardening and planted a lot this year! Many plants are doing really well and it has been rewarding to watch everything thriving. However, three smallish endless summer hydrangea I planted on the side of my house, this spring, sort of out of my attention, got what I think is black spot. I probably under and over watered them a couple times, maybe stressing the plants -( it is REALLY hot right now here). I treated them about a week ago by spraying with Bayer Advanced Disease Control for roses, flowers and shrubs. I thought this would fix things. My husband used a fungicide on the grass, which may have gotten on the plants (but the bag of fungicide doesn't indicate that it is hurtful to flowering shrubs). Anyway, I just took a look at the plants and the leaves - perfectly good green leaves - are just falling off the plant. I ended up cutting one of the plants almost completely back, so now there are only some ugly twigs where once I had a plant. Ugh. What should I do now? Pull it up and start over? The other plants don't have the same degree of leaf drop, but it is happening to all of them!
What made you think it had blackspot? I think Blackspot, Diplocarpon rosae, is a fungal disease mainly of rose bushes. But hydrangeas have their own leaf spot fungal diseases they are prone to. Cercospora leaf spot is one of them: https://apps.caes.uga.edu/urbanag/home&g ... oryid=2773
but there are others. Luis is right that pictures might help.
Also, do clean up the fallen leaves and get rid of them. If it is a fungal disease, those leaves can keep spreading it.