It appears to be a nutrition problem. Because you haven't added anything that sounds toxic I'd guess that the pH is off and your plant is suffering from nutrient lockout.
Shovelhandle Reviewed by Shovelhandle on . Eutrophication, nutrient deficiency or spider mites? We decided in June that it could be fun to try to grow some weed. So we got on the net and bought a feminized seed. We planted it in it's definitive pot, as advised by the seed seller. We planted it in a 16" pot, filled with a mix of store bought black earth, potting mix and sheep compost. It's been now about 5 to 6 weeks and the plant is almost 2' high. Unfortunately, about a week ago, we started to see some tiny whitish discolored spots on the leaves, especially the ones at the base. Rating: 5