Artillery Fungus (Sphaerobulus stellatus)

Homeowners are occasionally puzzled by the appearance of small black spots spattered on cars, the leaves of ornamental plants, siding, downspouts, soffits and windows of their homes. These spots are often mistakenly identified as tar, scale insects, or insect excrement. The dark spots are 1-2 mm in diameter and slightly raised to globular.

About five hours after opening, the glebal mass is ejected into the air. This discharge mechanism produces a very small amount of energy—roughly 1/10,000 hp—and can launch the mass up to 20 feet. Upon contact, the sticky coating adheres to any surface it touches. The fruiting body is strongly phototropic, so the glebal mass is generally propelled toward the strongest source of light. Outdoors, this is usually the sun or highly reflective surfaces, such as glass or light-colored walls. Some homeowners have noted that lighter-colored cars parked next to darker vehicles often show more of the dark spots.

This fungus grows on dung and well-rotted wood, such as the wood chips used in foundation bed mulches. It prefers open areas with little shade and adequate moisture. The optimal temperature for producing fruiting bodies is 50–68 °F. Fruiting bodies are generally not produced above 78 °F, so the problem tends to be most noticeable in spring and fall.

Fungicides have not been evaluated for controlling this fungus. Adding fresh wood chips each year may reduce the problem if old mulch is fully covered. Scraping glebal masses from windows or walls could potentially reintroduce spores to the beds, as spores within the glebal mass have been reported to remain viable for up to 11 years.

The outer coating of the spots is brown, darkening with age. When opened, the center appears off-white, granular, and gummy.

(Modified from Ann Hazelrigg, Plant Diagnostic Clinic Coordinator, Plant and Soil Science Department, University of Vermont)

*The black spots are masses of mature spores expelled from fruiting bodies of the fungus. This fungus is a relative of the bird's nest fungus and is commonly called artillery fungus by mycologists. The spherical fruiting body is approximately 2mm in diameter and produces spores internally. When mature, the fruiting body splits open forming a cup-like structure. The round mass of spores, known as the glebal mass, is about 1mm in diameter and rests in liquid at the bottom of the cup.*​