Sounds like mildew on the leaves of the mint but unless I see a photo I am just making a good guess. The cure is to cut back the plants to just one to two inches above ground. New growth will then emerge and you can help it to remain disease free by giving the plant roots water but try not to wet the leaves. You can use the leaves from the top of the plant that do not show any mildew or even freeze the healthy leaves for future use. The fact that the new growth or top one third of the plant looks fine means your plants will outgrow this problem as the summer weather arrives and I see fresh mint in your future. Keep growing, Marianne Binetti