I'm wondering why you sprayed an insecticide. Did you see evidence of bugs? I always say "know thine enemy". It sounds like a fungal problem that you are having. Take a look at these conversations and pics to see if it's the same thing. You can click on the pics of this first one to enlarge. Sorry that some of the pics in the second link don't show.
For a homemade fungal spray, take a look here.
Hope this helps,
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.