I recently discovered a mold/fungus disease growing on some plants. From searching google, it seems to be something called "sooty mold." I went to Home Depot and got an insect, disease and mite control spray. It doesn't seem to have helped. I'm a little worried as it appears to be spreading. Any ideas on what I can use to stop this and get it off my plants?
Sooty mold is typically a result of honeydew-secreting insects like aphids and scale. So if you want to fix the problem in the long term, you will need to find the insects and take care of them rather than worrying about the mold. The mold grows on the honeydew, it's not actually attacking the plant (although it looks ugly, and if too much of the leaves get covered up in it, it can cause them to not be able to photosynthesize properly). I don't see aphids or scale in your pictures...is this plant underneath a tree or some larger shrub? I've seen cases where there was a tree that had aphids and the honeydew dripped down onto the plants underneath and they developed sooty mold as a result even though the bugs weren't on them.
There seems to be no end to the problems for a wanna be gardener. This black stuff is on EVERY plant under my big ol' maple tree, which has lots of it on lower leaves. I am fearful of it killing many plants. I have such a large amount of shrubs, Japanese maples, roses, azaleas, hawthorn, and many more covered with this stuff. I understand what it is, but at this point, a point where most leaves are black, what can I do?
Try to find the actual pest.
Scales and aphids are the most prolific producers of honeydew. If you can get these under control there will be a lot less sooty mold on the plants underneath.
You can try washing off the leaves before they get very much sooty mold. Since honeydew is mostly sugar water, it dissolves very easily in water with just a tiny bit of soap. I wash honeydew off my truck often, since it is parked under a tree with both aphids and scale.
You can try dish washing soap, 1 tablespoon per gallon, and spray the plants. Then spray them with a stronger blast from a hose. The soap starts loosening the sugar water, the blast from the hose washes a lot of it away.
You can use insecticidal soaps on the plants with the aphids and scale. For scale you need to ID it so you time the spray right. Only the juveniles are susceptible to most pesticides.