Magnesium

All you need to know, RHS advice here, about half way down:

http://rhs.org.uk/advice/profile?pid=456

To be effective magnesium must be applied as a foliar feed. The problem is not usually shortage of magnesium in the soil. Tomatoes are fed with high potash fertiliser and there is an interaction between potash and magnesium which prevents magnesium being taken in by the plant.

A tablespoon of Epson salts (magnesium sulphate) in a gallon of water is what I use. Any leaves that have turned yellow or gone brown are probably beyond saving but the rest of the plant should green up within a few days.