If you’ve ever noticed small, dark brown or black pinhead-sized specks stuck to your vinyl siding, stucco, or even your car — and tried to wipe them off only to find they’re practically fused to the surface — you’ve likely encountered artillery fungus. Also known by its scientific name Sphaerobolus stellatus, artillery fungus is a wood-decay organism that lives in mulch and organic matter. It’s one of the most frustrating exterior problems a homeowner can face because it is exceptionally difficult to remove once it has adhered to a surface, and the window for effective treatment is narrow.
The name “artillery fungus” comes from the organism’s remarkable and destructive reproductive mechanism. When the fungus matures inside a mulch bed, it develops a tiny cup-shaped fruiting body filled with a spore mass. The cup is phototropic — meaning it orients itself toward the brightest light source in its environment, which is almost always the reflected light from a light-colored house wall or a parked vehicle. When conditions are right, it builds up osmotic pressure inside the cup and literally fires its spore mass into the air at speeds that can project it up to 20 feet. The spore mass hits the target surface with enough force to adhere on contact, and then the sticky, tar-like coating hardens within hours. By the time most homeowners notice the spots, they have already bonded to the siding surface at a molecular level.