By Dr. Sharon M. Douglas
Department of Plant Pathology and Ecology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P. O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8601 Fax: (203) 974-8502
The presence of persistent, brown to black spots on cars, house siding, plants, and other surfaces may be an indication of the presence of the fungus Sphaerobolus stellatus. These brown spots, called peridioles or glebal masses, are approximately 2.0 mm (1/12 inch) in diameter and are the dispersal structures of this fungus. Sphaerobolus is a Basidiomycete and a member of the Nidulariales. It is a cosmopolitan saprophyte on wood and bark chips, dying and decaying wood, and dung, but is not considered a pathogen of herbaceous and woody plants, animals, or humans. The increased prevalence of this fungus during the past five years appears to be partially associated with the increased popularity and use of wood mulches in the landscape.
Sphaerobolus is commonly called the "artillery fungus" or "sphere thrower" since it forcibly ejects the peridiole for considerable distances. Peridioles can be projected vertically for more than 2 m (6 ft) and horizontally for over 6 m (20 ft). Some researchers have also reported that the discharge is accompanied by an audible sound! As with most fungi, growth is influenced by temperature, light, and moisture. When temperatures range between 10°C and 20°C (50°F and 68°F) and moisture levels are adequate, the fungus produces fruiting bodies. These structures usually form on the substrate (bark, dung, or decaying plant material) in autumn and spring and are quite small, approximately 2.5 mm (1/10 inch) in diameter. Because of their size, they are often very difficult to find in the mulch although areas of mulch supporting growth of the fungus may appear matted or gray and somewhat bleached in color. As the fruiting structures mature, they usually remain active or "shoot" for approximately 2-3 weeks. Adequate light and moisture are also necessary for ejection or discharge of the peridiole. Changes in osmotic pressure within the fruiting structures cause them to absorb water and become more turgid. This process creates enough force (1/10,000 horsepower) to propel the glebal masses or peridioles into the air. This ejection process is phototropic so the peridioles are projected toward the light. Peridioles are typically quite sticky and, since they are forcibly ejected, they readily adhere to objects upon which they are impacted. When the peridioles dry, they become very difficult to remove. Unfortunately, Sphaerobolus can be long-lived and peridioles have been found to still be viable for up to 12 years. Peridioles can also be naturally dispersed by wind or over great distances on plant debris, mulch, animal fur, and even animal dung.
STRATEGIES FOR CONTROL:
Prevention and avoidance are the major strategies for control of this fungus since chemicals are ineffective. This involves removing or disturbing the wood chips or bark mulch with a rake to disperse the fungus and to dislodge the fruiting bodies. This also helps to dry out the mulch by increasing air circulation and creating conditions which are less favorable for the growth of the fungus. Some success has been achieved by periodic overlaying with fresh mulch, thereby reducing the light which is important for peridiole discharge. However, when overlaying, it is important to avoid making the mulch layer too thick. It is also important to select mulches that contain at least 85% bark. Avoid mulches that contain a high proportion of wood since wood chips are better sources of carbon, an important food source for the fungus, than mulches that mainly contain bark. All types of wood mulches can potentially support the growth of this fungus and research is currently in progress to determine if one type of mulch is more frequently associated with this fungus than another. Another tactic to minimize this problem in the landscape is to use an alternative form of mulch such as black plastic, stone, pea gravel, or marble chips in areas directly adjacent to homes, cars, or other surfaces where the risk of damage is the greatest.
Sphaerobolus can occasionally be a problem in container-grown plants when bark or wood products are components of the potting media. It has been suggested that composting the bark or wood products prior to use may help to reduce the ability of the artillery fungus to colonize the wood or bark by promoting the growth of beneficial organisms that are antagonistic to it.
Peridioles should be removed from affected surfaces with a stiff water spray from a hose or by scrubbing with a wet cloth or stiff brush before they dry. Unfortunately, in most cases this is not practical and removal is usually attempted after the peridioles have dried. When this is the case, they are very difficult to remove and must be physically scrubbed and scraped from the affected surfaces. When the glebal masses are on glass surfaces, they can be easily removed by scraping with a razor blade. However, care must be exercised when removing the hardened masses from other surfaces since the removal process itself can often damage the substrate. Additionally, extensive staining can remain after the fungal masses are removed. These stains usually fade with time but can be unsightly. Pitting of the substrate has also occasionally been observed, especially on cars. Power washing (and double power washing with a rigorous scrubbing in between) has yielded mixed results and its success appears to be dependent upon the particular type and age of the siding. More effective results have been obtained with new vinyl siding whereas limited success has been reported for old vinyl, aluminum, and older painted wood siding.
Since the brown dots or peridioles of the fungus can remain viable for more than 10 years, they can serve as a means to spread the fungus. As a consequence, care should be taken when scraping them off affected surfaces. The peridioles won't grow on house siding or inert substrates such as concrete or paved walkways. However, if they fall into mulch or another suitable organic substrate, they can germinate and re-infest these substrates. A tarp or similar item should be placed under the area that is being scraped to catch the fungal structures as they drop in order to keep them from re-infesting the substrate.
The presence of persistent, small, brown spots on cars, house siding, plants, and other surfaces may indicate the presence of the fungus Sphaerobolus stellatus. This cosmopolitan saprophyte is also called the "artillery fungus" or "sphere thrower" since it forcibly ejects its brown spore masses for considerable distances. This fungus grows on wood and bark chips, dying and decaying wood, and dung. Although it is not pathogenic to plants or woody ornamentals, it is a nuisance to homeowners. This fact sheet discusses ways to minimize the growth and development of this problem fungus.