Brown spots

I had the same issue with my LSD-25 plants this time around too. Stems and stalk turned purple, spots on the leaves. I think the purple issue was from a new stronger LED light and the spots from under feeding. The buds are looking good at 55 days but my plants look ragged and abused. I raised the light, increased Cal-Mag and new growth came in green. The older leaves still have spots. Thanks for sharing your photos, you have helped me as a novice grower.

https://dutch-passion.com/en/blog/a-visual-guide-to-cannabis-deficiencies-n987 First thought P-K, Looks a lot like Manganese deficiency as well, although that seems to be less likely compared to a P-K deficiency. Good luck.

Hey buddy, it really looks like a calcium deficiency. Your cannabis plant may show signs of calcium deficiency if the pH at the roots is too low. This happens because when the pH in your root zone is too acidic, your cannabis can't properly absorb calcium through its roots. So, the first step is to make sure you have the correct pH for your growing medium. Calcium and magnesium deficiencies often appear together in cannabis. Many growers decide to buy some kind of Calcium-Magnesium supplement (often called Cal-Mag) for their grow room, in case this common deficiency shows up. Good luck with your grow and hugs!

I was thinking maybe potassium deficiency? Never personally experienced it but looks like what I’ve seen in books for K deficiency. Or consider fungal or some disease

Id guess its either calcium deficiency or its a fungal infection.