Hi there DMuniz and welcome. We really need to know what part of the world you are in, to be able to help you. Please fill in your profile with your city/state/country - your profile is the little person icon in the blue sidebar at the left of the home page above the Search box. The weather in your area may have something to do with the plant's issues.
I'm not sure what plant you're calling a "purple potato bush tree" so could you please post a picture of both the healthy one and the one that's having problems? We can then ID the plant and also suggest some ideas as to the problem.
First thought off the top of my head is that Miracle Gro Plant Food might be much too strong to suddenly use on an established yard planting. Often it's better to do too little than too much especially when it comes to fertilizer. Are you talking about the soluble blue stuff that you dissolve in a watering can and water the plant with?
