HELP, crispy brown leaves??

[stevendre]

can i have your opinions on a problem i have, my plants leaves are turning crispy brown, and more or less crumble in your hands when touched. i know the easiest way for me to show would be a picture, i tried this but you carnt see very well with the picture.
they actualy look like burnt off the lights but lights are a ft from the buds, so there well clear, plants are three weeks from harvest, buds are still white, so there not browning off yet, plants are in 15litre pots of coco, any advice would be apreciated, cheers

i have managed to get pictures of my problem, BUT to large to add on here, dont know how to resize them, but have somehow managed to add as my aviator, so any comments?.