Hi, Dr. Hill,
This plant was photographed on July 26 in a mostly sunny roadside location in the SC fall line region. The upper leaves of each plant have turned white, but I don’t know if this is a kind of powdery mildew or simply part of the plant’s normal leaf growth. The spread of white appears to begin at the petiole, moving toward the leaf apex and only the upper leaves of the plant are affected so far. The tallest plants in the group appeared to be about 4-ft. tall. Thank you.
your plant is the hoary mountain mint, Pycnanthemum incanum. It is one of several species of mountain mint that can be used as an herbal tea. It has a nice aroma and flavor. I usually find it in open woodlands. It is wide-ranging east of the Mississippi River. It does not have a powdery mildew, though it does resemble that – the hairs are a silvery-greyish color on the leaves associated with the flowers.
Steve Hill, Botanist, SCNPS