The powdery mildew is a living organism (fungus) showing as a seemingly harmless powdery substance that manifests on your plant without warning to eat it up and reproduce more of itself on the plant causing it to curl upwards before completely dying out. It is an easily spreading infection that can move from your leaves to the buds and escape the notice of even the most careful growers at any point. It can affect all plants but quite commonly attacks cannabis especially those grown indoors.