I often get leaves like this on my plants after several weeks into flower. I must echo the experts here, get a handle on your pH. It is more than just the pH going in. You might want to add pulverized lime to your MGrow soil to help buffer pH issues. 2TBSP/Gallon.

Ignore me if Rusty says otherwise
FatSean Reviewed by FatSean on . Leaves Curling and a Bit Leathery My first indoor grow and the 4 week old plant's leaves don't look good. The first few sets of leaves were turning yellow and brown. As soon as I had more leaves I removed the first ones. I notices a few gnats so sprayed the remaining leaves and soil once with SchultzGarden Safe/ Fruit & Vegetable InsectKiller. Now the yellow/brown symptoms are gone but the leaves now are curling and look a bit leathery. Here is my questionnaire and I will add more photos in another post. I have been lurking on Rating: 5