The small, black dots appearing suddenly on your car or light-colored house siding are the telltale sign of an infestation by the common but frustrating artillery fungus, Sphaerobolus stellatus. This organism is a wood-decaying fungus that thrives in moist, organic materials, making landscape mulch an ideal home. Its primary annoyance comes from its unique ability to forcibly launch its sticky, dark spore masses, known as peridioles, onto any nearby surface. These spores adhere with a tenacity that makes them incredibly difficult to remove, which is why homeowners seek practical solutions for both removal and long-term prevention.
Identifying Artillery Fungus and Its Persistence
The artillery fungus itself is a tiny, often overlooked organism, typically appearing as small, yellowish or orange-brown cups nestled within the mulch layer. These fruiting bodies are only about one to three millimeters in diameter, making them easy to miss. The real problem is the ejected spore mass, a tiny, dark brown or black sphere that adheres like a strong glue to surfaces.
These dark spots, about one to two millimeters across, are often mistaken for tar, insect droppings, or specks of dirt. The fungus earns its name because it can launch these spore sacs up to 20 feet horizontally and six feet vertically, generally aiming toward bright or reflective surfaces like white siding or car paint. The spores contain a strong adhesive that quickly bonds to any material it hits.
The fungus is saprophytic, meaning it feeds on decaying wood, and it favors the conditions found in many shredded hardwood and wood chip mulches. It is most active during the cool, wet weather of spring and fall when temperatures are typically between 50 and 70 degrees Fahrenheit. This preference for cool, moist environments explains why the fungus is frequently found on the north-facing side of a house, where shade and moisture linger.
Immediate Strategies for Treating Contaminated Mulch
When an infestation is active, the most effective solution is physical removal of the contaminated material. This labor-intensive process involves digging out the infected mulch, generally the top three to four inches, and disposing of it completely. Do not compost or dump the removed material elsewhere on the property, as this will simply spread the viable spores.
For areas where replacement is not immediately feasible, a technique called solarization can be attempted during hot weather. This involves covering the infected mulch bed with clear plastic sheeting for several weeks. The plastic traps solar heat, raising the temperature high enough to kill the fungus and its spores, though this method requires sustained heat to be successful.
Another practical option is to blend mushroom compost into the existing mulch bed. Studies show that mixing used mushroom compost at a ratio of about 40% by volume can significantly suppress the fungus’s ability to produce spores. This introduces beneficial microorganisms that compete with and inhibit the artillery fungus.
There are no effective, commercially available fungicides or chemical sprays for homeowners that can eradicate established artillery fungus. Chemical treatments on the mulch are generally ineffective because the fungus grows deep within the material, making it impossible for surface sprays to reach the entire organism. Homeowners should focus their efforts on physical removal, heat treatment, or preventative measures rather than chemical solutions.
Cleaning Spores from Adjacent Surfaces
Removing the dark spore spots from surfaces like vinyl siding, cars, and concrete requires a combination of cleaning agents and careful abrasion. Prompt action is best, as the longer the spores remain attached, the more difficult they become to remove. For vinyl siding and concrete, mild detergent and warm water, followed by gentle scrubbing with a soft brush, can remove fresh spots.
If the spots have cured, a plastic razor blade or a putty knife can be used to carefully scrape the black spore mass off the surface. This technique must be executed with caution to avoid scratching the underlying material. For car paint, specialized products like automotive cleaning clay bars or bug and tar removers are often necessary, as the spores penetrate the clear coat quickly.
For stubborn spots on a vehicle, a degreaser or even a light application of white vinegar can help break down the adhesive, but it is essential to test a small, inconspicuous area first. After applying any specialized cleaner, the area should be thoroughly rinsed with clean water to prevent any damage. Aggressive scraping or the use of harsh solvents like paint thinner must be avoided, as they will permanently damage the paint or siding.
Long-Term Prevention Through Mulch Management
The most reliable long-term strategy for preventing artillery fungus is to select mulch types less susceptible to infestation. The fungus thrives particularly well in shredded hardwood and wood chip mulches. Switching to large pine bark nuggets, cedar, or cypress mulch is a highly recommended preventative measure, as these materials are more resistant to fungal colonization.
For a non-organic solution, replacing wood mulch near structures with materials like gravel, stone, or shredded rubber eliminates the fungus’s food source entirely. If organic mulch is preferred, maintaining a shallow layer, ideally less than three inches, helps to promote drying and inhibits the moist conditions the fungus needs to grow.
Regularly turning or raking the mulch every few weeks is beneficial because it exposes the lower, moister layers to air and sunlight. This disruption prevents the establishment of the fungal network and promotes a drier environment, which significantly reduces spore production. Even with resistant mulch, applying a fresh, thin layer each year can help suppress fungal activity that may have developed in the older material underneath.