The persistent, dark spots appearing on house siding and cars are caused by the tiny fungus known as artillery fungus, scientifically named Sphaerobolus stellatus. This organism is named for its unique, forceful spore dispersal mechanism, which acts much like a miniature cannon. The spots themselves are spore masses, about 1 to 2 millimeters in diameter, that stick tenaciously to any surface they hit, leaving behind characteristic, hard-to-remove black specks.
The Fungus and Its Primary Source
The artillery fungus is classified as a saprotrophic organism, meaning it obtains nutrients by feeding on dead or decaying organic matter. This natural process of decomposition is why the fungus is frequently associated with landscape materials. The primary food source for Sphaerobolus stellatus is well-rotted wood, which is most often encountered in the form of shredded wood chips or hardwood mulch used around planting beds. As the wood chips break down, the fungus colonizes the material and uses the carbon within the wood as its sustenance. Although all types of wood mulch can potentially support its growth, the fungus is known to especially favor finely shredded material because it retains moisture longer and provides ample surface area for colonization.
Environmental Conditions Required for Growth
Specific environmental conditions are needed for the fungus to grow and produce spore-launching bodies. Moisture is a requirement, as the fungus grows best in consistently damp or wet environments. Shaded areas, particularly the north side of a house, are often more affected because they retain moisture for longer periods and receive less direct sunlight.
Temperature also plays a significant role in triggering the fungus’s reproductive cycle. The fungus tends to produce its fruiting bodies during the cooler, moderate weather of spring and fall. Optimal temperatures for this growth are typically in the range of 10 to 20 degrees Celsius (50 to 68 degrees Fahrenheit). Production generally stops when the temperature exceeds 25 degrees Celsius (78 degrees Fahrenheit), limiting the problem during midsummer.
The Mechanism of Spore Dispersal
The tiny fruiting body, which is only about 2.0 to 2.5 millimeters in diameter, is the structure responsible for the black spots. Inside this structure, a single, dark, spherical mass of spores, called a peridiole or glebal mass, develops. Once mature, the fruiting body splits open into a star-like shape, revealing the peridiole resting in a liquid at the base of the cup.
The explosive launch is driven by a rapid change in osmotic pressure within the fruiting body’s inner lining. This pressure causes the lining to suddenly invert, much like a balloon being popped, which propels the sticky, dark spore mass into the air. This discharge mechanism is powerful, capable of launching the peridiole up to 6 meters (20 feet) away.
The fungus is phototropic, meaning its firing mechanism is aimed toward the brightest available light source. This characteristic explains why the spore masses are often found stuck to reflective surfaces like light-colored siding, windows, and cars. The peridiole is coated in a resilient, sticky substance that allows it to adhere strongly to any surface it strikes, making the resulting black spots extremely difficult to remove.
Summary of Causes
The formation of the persistent black spots is a direct consequence of the artillery fungus’s complete lifecycle. The initial cause is the organism’s colonization of decaying wood, typically found in residential mulch. This colonization is accelerated by high moisture and moderate temperatures, which are necessary for the fungus to develop its spore-producing structures. The visible nuisance occurs when the fungus forcibly launches its sticky, phototropic spore masses toward light sources, leaving dark specks on nearby property.