Rusty veins, wont wipe off

I've noticed this rusty area in one of the top colas leaves. It's only on this one top, the other parts of same plant don't seem to be affected.
I am Growing in soil, ff trio plus cal mag. Ph around 6. Last feeding i introduced beastie blooms but I think this started even before. Can you help me diagnose this problem? The plants are about 6 weeks into flower, sativa dominate.

Could be light stress if its only affecting the top of the tallest cola. What kind of lights, and at what distance? Whats the lights on temps?

160w led , distance from top of bud to light is about- 1 ft or more, it does seem to be hotter but averaging about 75 during the day, cool at night but not below 60..hmm

That particular leaf feels very dry too

The light stress could be due to the intensity of the light at that height, not the heat. I have found that plants tolerate more intense LED light under warmer temps, like mid 80's. I would suggest trying to get ur temps up to the 80's and/or increasing the distance from the plant. These are my humble noob suggestions derived from my actual experiences.

Thank you so much