just a thought, might possibly be a calcium deficiency. the following is from "Marijuana Garden Saver, Handbook for healthy plants"

Calcium deficiency sometimes occurs in soil-less growing mediums that have not been supplemented with lime.
Distilled and reverse osmosis water, as well as some tap water, lack significant amounts of dissolved calcium.

One symptom the book lists is, Necrotic blotches of tan, dried tissue. Another is stunted plant growth and makes the leaves turn dark green. I noticed your leaves dont look dark green, but i also noticed what appeared to be necrotic blotches of tan. (Plus you mention the soil-less mix). Good Luck
shmedy Reviewed by shmedy on . Need help Hi everyone, Need help. Some redish rust color spots have appeared on a few leaves of two of my plants during flowering. The spots aren't ,really, going crazy but definitly spreading when they show on a leaf? Checked k-def? and trying to apply a mild n-p-k but am unsure if the spots and yellowing aren't normal for some strains during flowering? My medium is Sh. peat 75%-- 25% mixed soil. First time using this type of medium. total ph for that mix is around 5.0 I believe I feed one with BC Rating: 5