It's very hard to tell what it could be without a microscopic analysis. Have you called your County Extension service? It's possible they have a microscope at their office and could at least identify your leaf spotting disease. I wouldn't assume it is fungus, necessarily. It seems more likely to be insect-borne than fungal just because it's affecting everything, not just some plants.
Try a couple of insect abatement solutions like a mild soapy water spray on a plant or two, to see if it makes a difference. My favorite insecticide is a spray called Capt. Jack's Dead Bug Brew available at Home Depot or Lowe's and it is certified for use even on organic vegetables, so non- toxic to animals and people.A baking soda solution of 1/2tsp. per quart of water sprayed on foliage can prevent the spread of fungus, too. Try some different things in different corners of the yard, see what works! Document where you put what, though, so you don't forget.
If you're up for a 'shotgun' solution, Bayer's 3-in-1 systemic fungicide/miticide/insecticide is a good product that I've used (in desperate times) on my orchids. I absolutely do NOT advocate "nuking" your whole yard because you will poison beneficial insects as well as bad ones, never a good thing. Try the benign methods first, that's the responsible way to proceed.
The fact that there were several dead trees you had to remove speaks volumes, too. The garden seems to have been neglected for quite a while - was the house owned by older people, maybe? Anyway, just some vigorous new growth might prove to be more resistant to this plague. Give the plants you want to save some pelleted, time-release fertilizer and see how they come along. Plants that grow weakly are much more disease-prone than strong, healthy plants. You can even make a little 'sachet' or two out of newspaper with the pelleted fert inside, and push them down into the soil near the pond plants. (you don't want to put fert straight into the water, as it will grow algae)
