Good luck with the new soil, that has to be the problem, unless there is something in the air, lol. I have been battling sooty mold, fungus, and lichens on my plants.
Yesterday I repotted one of my favorite plants, a double Rose of Sharon. The old soil had broken down and stayed wet. I took half of it out and mixed Miracle Grow with the old. I hope I did not screw up, after reading your experience, I should have used new soil. I will keep an eye on it.
Ah thanks.. yeah hard to say the problem for sure.
Hope your rose of Sharon does okay with it's transplant