About 3 weeks back started feeding the plant with miracle gro plant food(20 drops in 1 quart of water) with every watering. The leaves started turning brown/black and dropping. The plant has lost almost 50% of its leaves.
I do'nt know if there was something wrong with the plant food, or if the plant has a disease.
Does anyone have any suggestions as to how I might improve its health?
You mention that you have been watering your rubber tree with Miracle Grow everytime you water and it's been 3 weeks. Hope you aren't watering too often. These plants don't like a lot of water. You should water when the top 2" of soil is dry. Let the water run through and don't let the plant sit in water for more than an hour or so.
This isn't the best time of year to fertilize if you are in the nothern hemisphere. Wait until spring. You might also want to use something more organic like fish emulsion as this is a rather slow growing plant and the Miracle Grow is like steriods.