Hey Growmie, seeing as your in flower, and those leaves are low, I would guess it’s a potassium deficiency. The oldest fan leaves begin to take on a strong yellow hue on the edges and tips, and/or these start to get “burned,” showing spots, and eventually dying. Google potassium deficiency in cannabis and you’ll see similar pictures to what’s going on with your plant, Good Luck Growmie 🇨🇦❤️🌱😎💨