Nurserymen and landscapers are occasionally contacted about small black spots on the leaves of ornamental plants, siding, and windows of a home. Measuring 1-2 mm in diameter, slightly raised, with a brown outer coating which darkens with age, these spots are off-white, finely granular, and gummy when scraped open. Efforts to remove the spots usually prove futile. Often these spots are identified as scale insects or insect excrement.
These black spots have nothing to do with insects, but instead are mature spores expelled from fruiting bodies of the fungus Sphaerobolus stellatus Tode, commonly called artillery fungus. The spores are generally shot toward the strongest source of light, usually glass or light colored walls. They can launch themselves as far as 36 feet. The spores then use their sticky coating to adhere themselves to whatever surface they land on. A favorite host for the fungus are decomposed bark and wood chips found in foundation mulch. The fungus thrive in bright light, little shade, and sufficient moisture. Fruiting bodies are produced when temperatures range from 10-20 degrees Celsius; therefore the problem is limited to spring and fall.
The best way to control the fungal spots is to change the type of mulch used in the affected area. Each year add new mulch to completely cover the old mulch. Remember that scraping the masses from the window or siding may reinfest the area, as the masses may be viable for 11 years. The best options for removing the spots from walls and windows is with acetone or trisodium phosphate.