I only realize now that you were probably the person to ask.

For all I know now, it is probably spot leaf disease. That explains why it is still around (killed my entire garden last year) ánd why it affects all kinds of plants.

I'll need to do some research on the method you mentioned and maybe use a dictionary to look up some words, to make sure I understand correctly. Like 'gauge'... I know the word from buying ear piercings, but I'm not sure whether a 'gauge copper wire' is a hollow one or not.

Never read about that method before. I've tried about everything I knew: neem oil, garlic spray, nettle spray, neem oil, garden soap.

The last trick seems to be slowing it down. For now, at least. I added quite some lava rock to the soil, and sprayed it all over the plants, covering them entirely with a thin layer of it.
I already noticed some good results on the bush that has (peach) leaf curl disease, and so far, only a few of the plants show signs of infection. Removed all the infected leaves, of course.

I'll try to find more about the copper and peroxide and give it a try.
Thanks for the tip! <3