It sounds like overwatering combined with fertilizer while stressed. Do not water until the ground is dry down 2"(check with a stick or your finger) and don't ferilize stressed plants. I can't tell if they have a disease without a picture, but others on here are better at diagnosing from description than I am.

The secret to growing snapdragons, is snap off the dead heads, once they have bloomed. If you don't do this, then the stem will die back. They are so easy to snap off. It took me years to figure this out. Barbra

Snapdragons prefer cooler weather and it has been hotter than normal this year. The heat will definitely slow them down in the summer. Pansies will completely die out in summer and will need to be replaced in the fall.