Barney's farm show

My 5th week of flowering went like this... Everything was normal beginning of week. On the 3rd day I had small rusty spots on one of my Grape Ape's leaves. The next day it has spread on Runz muffin. I end up with the conclusion it could be nutrients lockout. It isn't a big issue at all. Will treat them with CalMag and hope this will fix the problem. However this issue, I like my ladies and their aroma, look and blooming grow!