Powdery mildew - regardless of whether it is real or false - is very rare in indoor plants and therefore mainly affects yuccas that are cultivated outside in the summer or that remain in the garden all year round. However, a white, flour-like coating can also be an indication of the gall mites that are quite common in indoor yuccas.
Powdery mildew can be combated with whole milk
Combat powdery mildew effectively
If it is actually powdery mildew, you can treat the leaves of the affected yucca with a mixture of whole milk and water. Mix one part fresh whole milk with 10 parts water. It is essential to use fresh milk, as only this contains the necessary microorganisms that are effective against the fungus.
Recognize and combat gall mites
If, on the other hand, it is a question of gall mites - which, by the way, do not leave any galls on yuccas! - then you can fight them effectively with dishwater. To do this, mix water and a few drops of washing-up liquid (e.g. Pril, Fit…) and wipe the leaves of the affected plant with it for several days in a row.
tips
In the case of powdery mildew, untreated milk is best: you may even have a local dairy farmer who will sell you untreated milk. supermarket milk is. even if it says "fresh milk", it's always pasteurized.