You can spray your plants with a mixture of milk and water. The mixture can be anywhere from 1 part milk to 10 parts water, up to half milk and half water. Any kind of milk will work, even reconstituted powdered dry milk.
IIRC, it's the lactic acid in the milk that does the trick, but I could be remembering that incorrectly. I can't find the link right now to the study about it.
Apply the spray thoroughly, until it drips/runs off the plants. Be careful to get both sides of the leaves, and spray the stems from all directions.
As far as I know, powdery mildew is not a soil borne disease. It's a fungus and the spores are spread by the wind.
if you could post some pics it might help identify the problem. Some insect infestations can be mistaken for powdery mildew. [url=https://www.helpfulgardener.com/forum/viewtopic.php?t=3724]How to Post Pictures & Photos[/url]
Good luck to you!
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams