Caitlin, if I were you....I would alter my "battle plan":
1) Take some pictures, and post them here
2) contact my County Extension Agent and get both soil, AND plant samples sent off for testing
3) correct any 'obvious problems' (too deep, too wet, too shady) while waiting for the Agent's report on the test results.
4)once the results are back, act accordingly as to treating the type of rot you have, and modifying your soil (if needed0 following the University recommendations
just my two cent's worth!!
I would also start "pulling plants" that are the 'worst affected", remove the rotted areas, dip in 10% bleach water (make one gallon of solution, and then add I teaspoon of Daconil Fungicide to the bleach mixture), dry overnight, and plant in any "sterile, soilless, potting mix". Make a solution of tw0, unbuffered aspirin tablets in one gallon of water, water the pots well with this solution, and place in a 'semi shaded " area........and cross your fingers
!
When you get the soil / plant results, modify what you need to, and replant those potted ones when new growth has started.
