Any changes with the plant? The damage too her doesn’t look like root rot. I lost three cycles earlier this year too that and it looks nothing like you have. It could be a nutrient imbalance as said above. But it could be heat stress. EC is the electrical current in the nutrient mix, in other words the strength of the nutrient solution. Your aiming for for 1-1.1 maximum with that, but some people do go above this. I fill my buckets up with a 1.1 strength mix and it isn’t unusual for me to stray up to 1.5. Just need to Know your plants and use quality nutrients