That looks like my last girl. Brought a plant in from outside during summer (potter roses) and brought mites in too. I thought it was a serious pH problem when it started. At about 2 wks of flower my soil pH tested high +7 with a 6.4pH H2O flush. Waisted a lot of time doing pH adjustments which were only part of the problem. 4 wks is when i took the photo of the leafs. I didn't see them until i used a micro photo shot at 9 wks. She was mostly dead but she still smoked fairly well. CHECK CHECK and CHECK for mites at the fatest stem area of the under leaf.

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