From the pictures it looks like you are growing them indoors? I don't have any experience with that. But maybe Nancy (Petuniafish) will chime in if that's the case. She is a pro indoor grower. It outdoors tho, and maybe this applies, I would cut off all the old leaves and spray it down with Bayer 3 in 1. Then cut back water and put it in bright shade and work it back into the sun/more frequent water when new leaves form.

Research nutrient deficiencies...I think that's what's wrong with mine...here's a pic I saved..

Spider mites or other sucking insects are a likely cause.