I?ve found many of my mint and oregano plants are prone to powdery mildew, which appears as white splotches. It spreads with the wind, so it?s better to remove any infected parts. It?s caused by damp leaves or overhead watering. My oldest (and favorite) plants are located just under a gutter. They love the high amounts of water, but also get powdery mildew soon thereafter. Hope this helped
Beelank