Dwarf peach trying dying/has died

I have a potted dwarf peach tree that I bought last spring. This year it was just starting to get flower buds (one was just showing pink). I watered its roots with (correctly diluted) natures gift peach leaf curl treatment which can be applied as both a spray or a watering. https://www.amazon.co.uk/NATURES-GIFT-PEACH-LEAF-CURL/dp/B01333H70Q

I went to my tree maybe a week or two later and it seems to be dying/dead. The buds have blackened and the tree smells like rotting fruit. I snapped one of the branches off and it is brown, gungy and smelly inside. Has the tree died? Is this a result of the natures gift? Is this a disease? Is this due to a contaminant in the container I mixed up the natures gift in? (The last thing I used in the container was washing up liquid. Or is this something that the tree can recover from? If it isn't dead any suggestions of how to save it?