Why marigolds plants died easily?

I have marigolds and vincas in my garden. The vincas are excellent, but some of the marigolds are dying, they look sad, do you why? I have put plant vitamin on them and they still dying.

Why marigolds plants died easily?
I have always had this problem with marigolds until this year. My roses got black spot so I purchased Ortho spray for fungus and bugs. I figured if it was good for my roses why not spray all of my flowers with it. I have the most beautiful marigolds I have ever had!
Reply:Marigolds are a favorite breeding plant of red spider mites. Take a piece of white paper and put underneath one of the plants. Then soundly "thump" the plant. If you get tiny reddish specks on the paper (like pepper) you have spider mites.
Reply:In too sunny of a spot; they need shade w/ little bits of sun. Underwatering, they need to be moist at all times.