Hard to tell from the pic, but it MIGHT (and I stress the word might) be the beginnings of rust disease.
It's actually a fungus that feeds off living plants. So if you've ever had fungus and mold conditions (helped by high humidity) where you are growing, that's where it came from.
Get yourself a good organic anti-fungal treatment for relief if that's the case. Also, if that's the case (high humidity levels over 50%) get a dehumidifier and lower the humidity down to 40 - 50%.