Manganese (Mn) immobile would be my guess.

Red & brown spots on the tops of the leaves and maybe a red streak on the stem or stock as well?

What is the humidity around your plants?

Try making a foil spray with about 5% Epsom salt and give them a good soak when the lights are on. Add some Epsom salt to your water too about 5-ml per gallon works.

If you do have a dry environment that will cause additional transpiration and in turn cause more manganese to be drawn into the foliage.

That's just a guess from what you are saying.
Zandor Reviewed by Zandor on . all my clones have rusty leaves and other problems... ive got 6 clones that were growing fine all along and then yesterday i noticed that MANY leaves on all plants are turning rusty and spotty... some are drying up and dying... i pinched off some of the worst ones, but they seem to be spreading UP the plants and the plants are now growing very slowly and the growth is weak and thin... i use foxfarm growbig just as i always have... nothing has changed outside of it getting a little colder, could that have anything to do with it? Rating: 5