Powder Mildew

Looks like you got some other issues with the leafs, potassium could be the cause on the edges yellowing. WPM is a cunt of thing to deal with if that is indeed what it is, confrim its not just some minerals dried up on it. Some relief can happen with spraying the leaves down with horticultural oil. helps prevent the spores from germinating the oil layer makes it harder. Other things that can help is adding in Si aka sillica to the grow. This can help make the plant more robust to the spores and makes it harder for them to penetrate the leaves. One thing you want to do is not get the leaves wet. WPM spores germinate when its wet and damp, and the fungus its self releases lots of spores when its dry and hot. so catch 22. Do not reuse the soil unless its been sterilized, you need to sterilize all the pots and tent its grown in. one user who has had extensive WPM issues have found a strong link to WPM and dew point. So if your hitting the dew point for the RH/temps in your growing environment your likely going to have WPM. Even if you apply all the preventatives listed above. Others have said to spray milk on them. Don't.... a better solution is to make a lactobacteria serium from the milk and use that. Its all about changing the PH of the leaves surface to help prevent the spores germination and causing an issue in the first place or out competing for real estate by layering it in bacteria that won't harm the plant. Other options include potassium carbonate. shown to have about a 30% effectiveness on WPM, reapplication is needed, especially after a rain.