Im not positive but It looks like a magnesium defiency. I use 1 tablespoon of epsom(magnesium sulfate) salt per gallon of water, or you can make a less concentrated solution and spray it on the leaves and undersides of the leaves. I would also check my ph and make sure its at a stable level.

Now im no expert but I had something very similar happen to me before and all i needed was some epsom salt and a little love.
drudown11 Reviewed by drudown11 on . Brown spots Guys: Need some help/insight (again). I have 7 plants going but 1 of the plants has brown spots on the top leaves. The 3 pics I have attached show top/med/bottom of the plant. Overall the plants look healthy just top part has these brown spots. FYI using 90w (LED) UFO's and I am 2 1/2 weeks into flower and using soil MG on this plant. Appreciate any insight. Thanks :) Rating: 5