You have an Mg deficiency, the cause of most brown-spot issues. Take the same advice i gave here;

http://boards.cannabis.com/plant-pro...nts-dying.html

In addition i would flush with a 150-200ppm Cal-Mag solution to combat the deficiency. Check the ppm of your runoff after 1/2gal of the solution. You'll probablly be in the high range and should be around 450-600ppm. It took some years to figure out but i've found that the starter ferts in potting soils/mixes were at high levels (ppm) and caused most of my problems. 1/4 - 1/3 recommended strength should be applied to the soil as well. Pre-flush your soil with 120ppm Cal-Mag to get it down to 400-600ppm. Water with 100ppm Cal-Mag solution (calibrated from zero). Knowing the starting ppm level of your mix takes the guess work out of how much to feed. You just increase the ppm as the plant requries.

Just my experiences. Hope it helps