[Dutch]

My guess and this is only a guess is that you have house plants. Many times these plants can be carriers of plant diseases and show no sign of the disease. The same can be said about the soil the house plants are grown in and the insects they host. In a forced air heating system, mites which are almost microscopic, can get dispersed throughout the entire house. These mites can carry with them fungal and bacterial diseases.

Another guess is that if you look at the bottom roots yours are not white. They are likely tan. If they are tan this is a fungal disease caused by wet roots. Same as would happens to you if your feet were wet like that. There is damp-off disease at the soil line too. Darrel (Fusion_power) has stated dozens of times don't water your plants until you see them wilt. They will tell you (wilt/bow) when they need water.

I think the leave disease is just a small piece of the puzzle and is probably a secondary problem. I do see small black spots in some of the pictures that might an insect and could be a carrier of whatever is infecting the leaves.

In closing I hope the potting soil you are using is not the moisture control stuff. The Miracle-Gro in the yellow/green bag in the right stuff. The Miracle-Gro in the blue/green bag is the moisture control stuff and does work well for growing vegetable seedlings because it doesn't cycle correctly from wet to dry.
Dutch