Powdery Mildew (PM) is a quite common fungal disease that affects indoor and outdoor gardeners of every plant type, including cannabis. PM can hit your plants at any stage of growth. Even seedlings and clones can get PM. In fact, clones with PM are a common occurrence because when mother plants are infected with PM, their clones are infected too. If you are bringing new clones into your grow, always screen them rigorously and do not accept any clones with PM.
Visually, you will first notice powdery mildew by the white residue spots on the leaves or branches or stems of your plant. They can be spotted as tiny slivers or large swaths. Although powdery mildew is likely to appear on the top sides of leaves, these spores can develop all over stems, flowers and the undersides of leaves as well. If you look at the white residue closely, you will notice they appear slightly raised and fuzzy.