Brown Spots on Young Plants - Help!!

What the hell is this?!?

I'm growing four different strains right now and only the Thai X Skunk is effected.

• Thai X Skunk from 420Seeds plants around three weeks old
• DWC, ppm 300 ph 5.6
• only the very lower leaves are effected

These plants will be my first attempt at cloning so I would like them as healthy as possible.

What's wrong with the leaves and what should I do about it?

Seriously?!? I thought this was a good site for people who needed help growing.