Artillery fungus (Sphaerobolus stellatus) is a nuisance organism commonly found in organic landscape mulch. It earns its name because it forcibly ejects tiny, sticky, black spore masses. These masses adhere firmly to nearby surfaces, creating tar-like spots on siding, cars, and windows. This leads to frustrating and difficult cleanup efforts for homeowners.
Identifying Artillery Fungus and Its Threat
The fungus is small, typically less than one-tenth of an inch in diameter, appearing as a tiny, cup-shaped fruiting body that is often yellowish or orange-brown. Inside is a single, dark, spherical spore mass, called a peridiole, which is forcibly launched upon maturity. This ejection mechanism is phototropic, meaning the fungus aims its spores toward bright surfaces, which is why light-colored siding and cars are frequently targeted.
The spores are propelled up to 20 feet horizontally and over 6 feet vertically, explaining their presence on the second stories of homes. The peridiole has a sticky coating that glues it immediately to any surface, making the spots exceptionally difficult to remove once dried. While the fungus poses no harm to plants, pets, or structural integrity, its main threat is severe cosmetic damage.
Strategies for Eliminating the Fungal Source
Chemical treatments, such as commercial fungicides, are generally ineffective against Sphaerobolus stellatus because they cannot penetrate the mulch deeply enough to destroy the widespread fungal network. Therefore, the most effective strategy for source control is environmental management and physical removal of the infested material. Reducing moisture is a primary step, as the fungus thrives in cool, damp conditions. Raking and turning the mulch regularly helps by aerating the material and allowing it to dry out, which discourages fungal growth.
If the problem is severe, the most drastic, yet most effective, solution is to completely remove all contaminated mulch. The infested material should be bagged and disposed of in a landfill, not composted, to prevent the spread of the spores. Covering small, isolated patches of the fungus with black plastic can also help by using solar heat to “cook” the fungus and dry out the underlying material. For those who wish to keep organic mulch, blending it with used mushroom compost at a concentration of about 40% has been shown to greatly suppress the fungus’s ability to produce spores.
Effective Methods for Removing Spore Stains
Removing the hard, tar-like spore masses requires a combination of softening, scraping, and chemical action. Quick action is necessary, as spots become much harder to remove after two to three weeks. For smooth, hard surfaces like glass, a sharp razor blade can often be used to gently scrape off the peridioles without causing damage. On vinyl siding, which is the most common target, a gentle approach is necessary to avoid permanent scratching or marring of the finish.
Begin by trying warm water and a mild detergent, scrubbing gently with a soft cloth or brush. For more stubborn spots on non-porous surfaces, a solution of one cup of chlorine bleach mixed with one gallon of water can be tested on an inconspicuous area. Some homeowners have reported success using mineral spirits, though these should be applied cautiously near paint or sealants. For car paint, use mild soap and warm water, followed by a light application of rubbing alcohol to a cloth for any remaining spots.
Long-Term Control and Recurrence Prevention
To prevent the artillery fungus from returning, the most reliable long-term solution is switching to a less hospitable ground cover. Since the fungus primarily colonizes shredded wood mulch, replacing this material is the best preventative step. Alternative ground covers that do not support fungal growth include inorganic materials like decorative stone, gravel, or shredded rubber mulch made from recycled tires. If an organic look is preferred, consider using large pine bark nuggets or pine straw, as the fungus is less likely to thrive in these materials. Maintaining a thin mulch layer, ideally less than three inches deep, also promotes quicker drying and better aeration. Regularly applying a new, thin layer of fresh mulch annually may also help to inhibit the fungus.