I'm growing water spinach (Ipomoea aquatica) in a pot in my backyard. This plant, and it seems a lot of my plants in the yard, grow well for a while, then seem to succumb to some sort of illness. My backyard is beside a lake. I assume that fungi are an issue due to the proximity to the lake and my general location (Thailand). Attached is a photo of the water spinach leaves, the white stuff is on the underside of the leaves. The plant begins yellowing when the white stuff appears. It looks like something is also eating the leaves, but, this hasn't been a major issue.
The other leaf is from another plant in the yard, I don't know the plant type. It also has a similar white substance on the leaves, but accompanied by some type of dark-colored parasite or other things.
Any ideas what diseases these are and what I can do. Since I'm going to eat the spinach, I don't want to use any chemical-type treatments.