Sadly, powdery mildew is difficult to control, not least because, apparently there are several types (species) of it. It may have developed at the source鈥攖he (likely) greenhouse in which it was grown (forced, possibly) and prepped for sale. One source suggests that while in a greenhouse, mint plants are constantly 鈥渨ashed鈥 with overhead irrigation. Once that ceases for more than a few days, the humidity of that environment opens the risk of mildew. (It can not germinate when plants are wet.) Why are you only seeing it now, you might wonder? Well, it can take up to a week for it to develop into a visible colony. You can try removing the infected plant parts. Increase air circ around the plant. However, we must advise that survival rate is low, success with mint indoors will be a challenge. Your best bet is to grow it outdoors in the ground or in a pot, in season. It is a perennial, but like most perennials, it goes dormant鈥攁nd it will go dormant; you can not fool Mother Nature. We wish we had better news…
Sadly, powdery mildew is difficult to control, not least because, apparently there are several types (species) of it. It may have developed at the source鈥攖he (likely) greenhouse in which it was grown (forced, possibly) and prepped for sale. One source suggests that while in a greenhouse, mint plants are constantly 鈥渨ashed鈥 with overhead irrigation. Once that ceases for more than a few days, the humidity of that environment opens the risk of mildew. (It can not germinate when plants are wet.) Why are you only seeing it now, you might wonder? Well, it can take up to a week for it to develop into a visible colony. You can try removing the infected plant parts. Increase air circ around the plant. However, we must advise that survival rate is low, success with mint indoors will be a challenge. Your best bet is to grow it outdoors in the ground or in a pot, in season. It is a perennial, but like most perennials, it goes dormant鈥攁nd it will go dormant; you can not fool Mother Nature. We wish we had better news…