How do I tell if this Powdery Mildew still alive?

An indoor grow with small outbreak of PM in a corner. I ran a sulfer burner for 3 days in a row at 2 hours. On the 4th day I'm going through em and find some leaves with PM but I don't know how to tell if it's alive or dead.

After using a sulfer burner, what should the PM look like and act like? Will it change to grey or stay white? Should I be able to brush it off with my finger or blow it off? Will it look very different?

Also, can the same batch of sulfer be used for several days in a row or does it lose it's potency?
 
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