Problem with a Plant - need some advice

I have 3 Atomic NL plants that are 45 days old and 35" tall. I have had them on 12-12 light for the past week, after having them on 18-6 since they sprouted. I put them out during the day and if any additional light is needed I have them under floros. This morning one of the plants had brownblack spots on the older fan leaves. There were also a few rust colored spots. The worst is on the lower leaves and the leaves that are higher on the plant don't seem to have any spots. Does any one know what this is, what causes it and what I should do about it. The spots just seemed to appear over night. The other plants look fine and are beginning to flower. Also, how can one tell when the plants are in full flowering mode? Thanks for any help anyone can give me. Airace

have u been using fertilizer?

airace try http://www.growadvice.com if thats any help

sounds like you have over fertised