I've Super Lemon Haze Auto plants growing in my tent and I've never used fungicide before Should I use this product at the beginning of flowering

I have also used this product with positive reviews. Its not a treatment for calcium though. especially if it was a deficiency from lack of it in the grow vs a PH lockout.. The damage will not repair itself. it will just stop progressing. If you gave it more Ca in an available form its all you need to do. Great white is a microbe booster for the roots and soil. It helps the good bacteria and whatnot over populate and out complete the bad ones. Makes roots nice and white.