Looks like a fungal infection..but it's hard to say..
Don't over-react and start watering with strange concoctions..
It may simply be too much water..bad PH numbers..or any number of things..
The link below leads you to a visual chart..have a gander..see if anything jives up..

http://www.overgrow.com/growfaq/97
And good luck!
Johnny Hempseed Reviewed by Johnny Hempseed on . Possible Nutrient Deficency? not sure whats wrong. just got transplanted. 18" Clones. under flouros. Rating: 5