That is really weird. May Night is not known for getting powdery mildew.
Mealy cup salvia is and it does, but it has not ever spread to my May Night. Try some baking soda water or hydrogen peroxide with water. Without any insects I do not think you have to put soap in the water. I would try the baking soda water first. It can help neutralize the soil around it. It is a Catch 22 but powdery mildew likes shade and damp. However when plants have it full sun damages them. Try to find a compromise. I would rinse as much soil off the roots before planting, but May Night will not like it's roots messed with. The nice thing is if it's happy with a good root system you can cut off the majority of leaves give it miracle grow and it will grow like nothing happened. No matter what you do May Night is a baby about transplanting, but rarely dies. May your week be blessed with joy!
