I have comfrey growing abundantly in my garden. Most has spread of its own accord. Towards the end of the bloom, they all develop a powdery mildew/fungus. Each are in different locations. different soil, air circulation, etc., but all get the mildew. I hate to drop it as mulch or compost it, for fear of spreading the mildew.
Interesting. i've never had mildew on comfrey, even when it's right beside other plants like pumpkins or goji berry that easily mildew.
You mention that its spread of its own accord. Most people only use Bocking 4 or Bocking 14 varieties of comfrey because it doesn't spread by seed, only by root cuttings. Perhaps sterile comfrey (like I've got in my food forest) isn''t prone to mildew.
"The rule of no realm is mine. But all worthy things that are in peril as the world now stands, these are my care. And for my part, I shall not wholly fail in my task if anything that passes through this night can still grow fairer or bear fruit and flower again in days to come. For I too am a steward. Did you not know?" Gandolf