I’ve been growing medical marijuana for some time. I have three young plants from clones about 10-12 weeks old—outdoors in Southern California. They grow in pots in full sun. All three have begun to show the type of leaf seen here. There’s no sign of insects, but some of the middle leaves on all three plans are turning brown as shown in the photo. Two or three leaves at least on each plant exhibit this. Nice looking buds are forming but not ready for harvest yet. They have a strong MJ odor. Why the discoloration? I’ve grown before and have not seen this. — White11
It looks to me like you have a Phosphorous (P) or Potassium (K) deficiency. These types of deficiencies start in the older leaves and affect the margins between the veins of your leaves first. As it gets worse, brown rust spots appear and grow from the outside of the leaves inwards. Leaves and stems often turn red or purple and then progress to falling off the plants eventually. These types of deficiencies drastically slow down plant growth and can diminish harvests significantly.
There may be phosphorous and potassium in your soil but it’s unavailable to the plant roots due to salinity (too much fertilizer salts in your mix) or pH fluctuations. Before adding any fertilizer containing potassium, first flush your medium with plenty of plain pH balanced water. Then slowly introduce some mild NPK balanced fertilizer and be patient; new growth won’t show signs of healthy behavior for several days.