The organism commonly known as artillery fungus, Sphaerobolus stellatus, is a widespread nuisance primarily recognized for the cosmetic damage it inflicts on nearby property. This tiny fungus is a common problem associated with garden landscaping where wood mulch is heavily utilized. It derives its name from its unique method of spore dispersal, which involves forcibly launching masses of spores onto surrounding surfaces. While the fungus itself is harmless to human health, its projectile spores are difficult to remove once they adhere to surfaces like house siding or vehicles.
Appearance of the Fungus Itself
The actual body of the artillery fungus is a small, cup-shaped structure that is usually overlooked by homeowners. These tiny fruiting bodies typically measure between a pinhead and about one-tenth of an inch in diameter. They often appear yellowish, brownish, or sometimes a pale orange color as they develop within the decaying organic matter of the mulch bed. The common name Sphaerobolus stellatus refers to the star-like appearance of the fruiting body after it has opened to prepare for spore launch. Because the fungus grows directly within decaying organic material, people rarely notice the organism until its spots appear on nearby objects.
Identifying the Black Spots
The most apparent sign of artillery fungus is the small, highly adhesive black spots that appear on light-colored surfaces. These spots are not the fungus itself but the entire spore mass, called a peridiole, which is launched with significant hydraulic force. Each peridiole is perfectly round and measures only one to two millimeters in diameter, often leading homeowners to mistake them for specks of tar or small insect excrement. The dark color is attributed to the densely packed spores, making the spots highly visible against white siding or car paint.
The fungus exhibits a strong phototropic response, meaning it preferentially aims its launch toward bright surfaces, maximizing the chance of spores escaping the dark mulch bed. The difficulty in removal stems from the strong, tar-like adhesive substance that coats the peridiole upon impact. This sticky material quickly cures and bonds tightly to the surface, making it highly resistant to simple washing. Once cured, attempts to scrape them off often result in permanent scratching or pitting of the underlying surface finish.
Preferred Growing Environments
Artillery fungus is a saprophyte, obtaining its nutrients by decomposing dead organic matter, and it thrives primarily in certain types of wood mulch. It shows a strong preference for shredded hardwood mulch and older, aged mulch beds that have been in place for several seasons. These materials, particularly those rich in lignin and cellulose, provide the necessary food source as they reach an advanced stage of decomposition.
The fungus requires specific environmental conditions to flourish. It prefers areas that are consistently cool, shaded, and have high moisture levels, typical of deep, undisturbed mulch layers. Once mature, the cup-like fruiting body can invert suddenly, using internal pressure to propel the sticky peridiole up to 20 feet horizontally and sometimes 10 feet vertically onto surrounding structures.
Cleaning the Spore Damage
Removing the dark, cured spore spots requires patience and the correct approach, as aggressive scraping is generally ineffective and can scratch paint or siding. For hard, non-porous surfaces like vinyl siding or vehicle finishes, specialized enzyme-based cleaners designed to break down organic matter may offer some success. These cleaners need sufficient dwell time, sometimes hours, to soften the adhesive before a gentle scrub with a soft cloth is attempted.
On porous surfaces like wooden decks or fences, very light sanding with fine-grit sandpaper can sometimes remove the spots without causing significant damage to the wood grain. Power washing can sometimes dislodge newer spots, but caution must be used to avoid damaging the underlying material, especially when using high-pressure settings on soft wood or vinyl. If the spots have been exposed to sun and heat for a long period, they become highly cured, making complete removal extremely difficult without causing noticeable surface abrasion.