The artillery fungus (Sphaerobolus stellatus) is a common saprophytic organism that frustrates homeowners, especially those using wood mulch. It is notorious for leaving dark, pinhead-sized spots resembling splattered tar on surrounding surfaces. Understanding the life cycle of this organism, particularly its behavior during cold months, is key to addressing the problem and determining if this nuisance fungus dies off in winter.
Understanding the Artillery Fungus Nuisance
The visible artillery fungus is a tiny, cup-shaped fruiting body, typically one to three millimeters in diameter. It disperses spores by forcibly ejecting a sticky, dark spore mass, called a peridiole, away from its host material. This explosive action, which can propel the spore masses up to 20 feet toward bright light sources like siding or vehicles, gives the fungus its name.
The ejected peridiole is covered in a highly adhesive substance that bonds securely to any surface it strikes. Once these dark spots dry, the strong adhesion makes them extremely difficult to remove without damaging the underlying paint or siding. This resilience means the spots are a long-lasting, persistent problem, not just a cosmetic issue.
Cold Tolerance and Winter Survival
The fungus does not die entirely when freezing temperatures arrive, which is a common misconception based on the disappearance of visible spots. Sphaerobolus stellatus is a perennial organism that employs a survival strategy to withstand the cold. The temporary fruiting bodies that eject spores naturally die off after reproduction.
The main body of the organism is a dense, thread-like network called mycelium, which remains alive deep within the mulch. This underground network enters a dormant state, tolerating freezing temperatures throughout the winter. The mycelium acts as the fungus’s permanent root system, ensuring the problem is perennial.
Furthermore, the dark, sticky peridioles that have already been launched are extremely hardy and can remain viable for several years after landing. This combination of a dormant mycelial network and long-lived spores ensures the fungus is ready to reappear as soon as the weather warms.
The Seasonal Cycle of Spore Production
The transition from winter dormancy to active spore production depends entirely on specific environmental conditions, primarily temperature and moisture. The artillery fungus is most active during cooler, damp periods, typically in early spring and again in the fall. Optimal sporulation occurs when temperatures are consistently maintained between 10°C and 20°C (50°F and 68°F).
The fungus relies on adequate moisture to develop the internal pressure necessary to launch its spores, thriving in consistently damp mulch beds. Spore activity ceases when conditions become unfavorable, such as during extreme summer heat or deep winter freezing. This dependence explains the distinct seasonal pattern in the appearance of the black spots.
Controlling the Fungus by Managing Its Habitat
Since the fungus survives the cold as dormant mycelium, control strategies must focus on eliminating or altering its habitat rather than chemical treatments, for which there are no effective options. The fungus thrives in moist, organic mulches, especially those derived from shredded hardwood.
The most definitive solution involves physically removing all infested mulch and disposing of it properly. Alternatively, homeowners can replace organic mulch with materials inhospitable to fungal growth, such as stone, gravel, or rubber mulch.
If organic material is still desired, blending fresh mushroom compost at a rate of at least 40% with existing mulch can suppress the fungus. Large pine bark nuggets are also a less favorable substrate, as they do not retain moisture and soften as readily as other wood mulches.