Diseased Fireweed (Chamerion angustifolium)

Observation by Korie Hickel:

There was a fireweed patch on the side of the path leading up to Bird Ridge that had multiple plants with some sort of disease. I saw a few more on the way up, but most of them were concentrated in that patch. The "disease" turned the plant black and grew a white, fuzzy-looking mold centered around the flower but also moving down the stalk.

Jessie Moan with University of Alaska Fairbanks Cooperative Extension writes

This is either sooty mold (as a result of an aphid infestation) or actual aphids themselves. The aphids I tend to see on fireweed are really dark and usually occur in high enough numbers they could be confused with a disease. IT can't enlarge the picture enough to see if it is aphids or sooty mold. Either are common this time of year.

Comments from LEO Editors:

This is the first observation that the LEO Network has received about sooty mold, a collective term for different sac fungi and insects that can affect the health of fireweed (Chamerion angustifolium). Sooty molds don’t infect plants, but grow on surfaces where honeydew deposits accumulate. Honeydew is a sweet, sticky liquid that plant-sucking insects excrete as they ingest large quantities of sap from a plant. Because the insect can’t completely utilize all the nutrients in this large volume of fluid, it assimilates what it needs and excretes the rest as honeydew. Wherever honeydew lands (e.g., leaves, twigs, fruit, yard furniture, concrete, sidewalks, or statuary), sooty molds can become established. LEO Network welcomes others to share their observations about diseases, insects, and black and/or sooty mold in fireweed and other vegetation. Most plants will tolerate a small insect population and light amounts of sooty mold. When sooty molds are present on any surface in the landscape, it indicates there is, or has been, a sucking insect population present in the vicinity. Source: UC IPM Program Moses Tcheripanoff