In this forum about 1/2 way down page is a thread called Zinc Tox/nute Burn...one of the posts in that thread (by Dutch lover) has a link to plant problems that is excellent. I tried to copy link but I don't think its going to work because I suck at that kind of thing.

International Cannagraphic Magazine Forums - The Complete guide to Sick Plants,pH, and Pest troubles!

Hope it helps.