I only plant tomatoes and a couple of years ago a type of blight attacked my plants causing leaves to turn yellow with black spots and work up the plants eventually turning leaves to fall. Went to garden center and bought a concentrate to spray but was told to move garden. I moved around corner to side of shed (previous plot in back of shed). It was not far enough away. The garden center told me the blight stays in the ground. I wondered if building a raised garden with all new dirt on top would help. Do not really have another spot to make another garden.
Raised Garden as Prevention of Disease in Soil
Certified GKH Gardening Expert
As long as you have a barrier between the ground, and your new soil then it should be much better. You might apply a thin layer of wettable sulfur to the bottom to kill off any disease starting.
I do believe a raised bed will be much better for your situation.