Bishop’s Weed Leaf Blight
Why do the leaves of my Snow on the Mountain (Bishop's Weed) turn "rusty" in certain garden areas? I live in the Midwest (Wisconsin) and this happens every year. Formerly attractive borders look lousy.
This is quite common for Bishop's weed in mid-summer. The more sun a plant gets, the more likely it will have browned edges. Keeping the soil moist will help but the truth is, the plant doesn't like hot weather. If the browning is extensive, consider using hedge shears to trim off unsightly foliage. Nicer foliage returns with cooler weather.