To be on the safe side, best to assume that the "root rot" may be phytophthera, which is a common fungal disease that will kill a susceptible plant very quickly, usually in wet conditions. If hoping to replant in the same spot, certain precautions are necessary. There are fungicidal drenches that can be used, but those can cause other problems. Contaminated soil can be dug out and replaced with fresh after a short rest and ensuring good drainage. I wouldn't bother trying to sterilise potting mix if the affected plant was in a pot - just get rid of affected plant and soil (where they won't contaminate anything else), clean the pot and start afresh. This is for shrubs and smaller ornamentals, and based on years of personal experience and advice from professionals. The situation is more serious for crops, fruit trees and other mature trees - professional help would be needed there. Good advice so far, apart from the devils advocate stating "you need to qualify rot", etc - they just wanted a straight answer.