Answers
Looks like powdery mildew. If there are only a handful of leaves infected, remove them. Don't compost them, put them in trash. It looks like the plant is indoors, you can try moving it outdoors to maximize the air circulating around the plant. As mint is so easy to grow, I'd recommend some new plants if the infection is severe. Plant outdoors or keep a pot outside if possible.
Posted
by: Lindsay McMenemy
(4 points)
Posted: May 14, 2015
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I agree that it looks like powdery mildew. Powdery mildew can spread quickly, so remove the leaves so that the white spores do not come in contact with other leaves. Try to water so that the runoff from the infected leaves does not touch the healthy leaves. Remove the infected areas as soon as possible. almost 9 years ago.