Below are some photographs of strange adaxial growths on a patch of Asclepias syriaca at a preserve near me. There were perhaps 20 individuals that had variations on the growths, all on their the upper leaves. The milkweeds in the rest of the field all had normal leaves. Anyone seen this before?

From the few publications I’ve found, enations can be caused by viruses (Geminiviridae, Luteoviridae, etc.), and thus could be transmitted to nearby individuals via insect vectors or by the connected roots (milkweeds are clonal). Could be a genetic mutation, too, I suppose — could be spread by seed or via clonal growth.

Photographed at Hildacy Farm Preserve in Media, PA.