What are your temperatures, are you spraying them with anything, and what type of light is it? Sometimes my pistils brown too, when they get a bit too close to the light. With some strains, the pistils will brown several times through the grow, but yours looks like it is environmentally induced.
Hey jojo677 I am running a 1000w HPS on time my temp is about 75-80 off time about 68-72! I haven't sprayed in 2 weeks! My RH is 50% I recently started co2 enrichment and it's the buds that are near the rain tube. In 1080cuft I run 3 on the regulator for 15 minutes every 3 hours could this be a factor? They were pure white before I put this system in!