Fusarium is a type of fungi that can cause root rot. It can infect Cannabis but you are more likely to see pythium before you see fusarium. Fusarium causes the roots to look reddish, instead of the typical browning look that pythium causes. From experience I can tell you that pathogenic Fusarium kills quick.
With that said, I would agree with Mutt, it is looking to me more like nute burn or a PH imbalance