Plant disease ?

	Some of my plants (Hygrophila difformis and Hygrophila stricta)
 start developing the following disease. The plant leaves are still green but
there are quite a few small yellow dots with black centers on them. 
After a while, the black dot gets larger and leaves a hole in the middle.
Any idea what kind of disease is this ? All of the other plants in the tank
(Ludwigia, water sprite, bacopa, anubias, riccia fluitans, rotala rotundifolia)
are doing great.

I used 3x15w (2 vita-lites, 1 tritons) on the 20 gal tank. For fertilizer,
I used Kent FW Plant Supplement and Kent Trace Element (both one cap per week).

Thanks for your help.