looks like mildew, I can see the black spores in the middle left of the first one. you may also have insects (looks like aphids) that may have weakened the plant or spread the disease. get some fungicide (ive used cinnamon with success) but id really recommend milk. its weird, but spraying milk on the leaves has worked like a charm for me in the past. rake up any dead material on the ground and remove the infected leaves, then spray the milk (you can dilute it if you want, but the more the better). might want to throw in some insecticidal soap (or just regular dish soap in the milk) just to make sure you get any bugs that might be there. ive had this on my brugmansias and its annoying, but easy to get rid of if you keep at it. good luck :)

if using dish soap, do NOT use any with the word "ultra" in the name. That means there's some ingredient in the soap that strips the waxy coating off plants, doing a lot of damage.

Thanks for the suggestions/advice. Turns out it's actually "rugose spiralling whitefly". It's pretty ugly but so far the plants don't seem to care about it very much, they're still otherwise healthy-looking and productive. Keeping an eye on it though!