Question
I used some miracle grow left over from last summer on my plants and they died.  Would letting it sit over winter develop a caustic solution.  The plants turned brown and some are yellow. They look like I put weed killer on them. It occurred to me (after the fact) that perhaps some of the water evaporated and it was too strong.  Although it was in a sealed spray container. Also it has been very hot here.  I have washed them twice with water and applied a disease control spray.  It didn't seem to help.

Answer
You can store fertilizer for a long time as powder or pellets as long as they are stored dry. I would not reuse fertilizer which has been mixed with water (solution) because that could be very concentrated (evaporation as you metion).

Best option now is to water frequently to flush the root zone, but I fear the damage is done.

In future, you can store fertilizer for future years (it will last many years) but do not mix more than you need at a time.