Looks like the downy mildew to me. My impatiens were gorgeous till early August (midwest) and then the ones in the ground turned into nothing but little stick bodies. The ones in the pots held out till last week, but they're now showing signs as well.

Well I managed to save them enough that the entire bed wasn't dead. I fed them with miracle grow weekly. watered every other day or less and only watered the ground never from above. they bounced back some so ... but I still go out there and pluck out dead stems

are those PURPLE impatiens? Lovely! I'll have to keep my eyes open, have only seen pink-orange-white ones here.