The following questions were sent to the P&PDL diagnosticians here at Purdue University:
Question 1: There are tiny black specks that look like fly dung all over my siding! When I look up close they are not fly specks, but they look like little bumps. I can scrape them off, but a red-brown stain is left behind. What are they and how can I get rid of them or prevent them?
Question 2: Many people in my area have this but don't know what to do about it! Apparently there is something in some of the mulches that releases tiny black dots in the air. They land on the house, windows and decks. If you try and remove them - they leave a brown ring behind that can't be washed off. Any ideas on this? Help in clearing it up, cleaning it off, and suggestions for any mulch that wouldn't cause this?
Answer: The spots on houses are a type of spore (peridiole) produced by a group of fungi called "shotgun" or "artillery" fungi. Sphaerobolus is the genus name of one of these fungi, and is most likely the specific culprit. These fungi colonize dung (manure) or other organic matter, such as wood mulch. Sphaerobolus-like fungi have a unique mechanism for dispersing these peridioles (spores are to fungi what seeds are to flowering plants). Very small fruiting bodies of this fungus are in the organic matter. The dark brown peridiole sits on top of specialized cells which accumulate water and cell contents. When enough liquid is accumulated, the cells burst open, propelling the peridiole as high as a two story building and can spot siding, downspouts, windows, cars, etc.
The contents of the "exploding" cells are sticky, and enable the peridioles to adhere to surfaces such as plants or structures. The fungus is sensitive to light, and will direct the peridioles towards a light source, including reflected light off of white buildings or vehicles. The sticky substance is designed to withstand the rigors of animal digestive systems (some species complete their life cycle by returning to dung), so it is indeed difficult to remove the peridioles from surfaces.
The use of gravel mulch, stone, pea gravel, black plastic, etc. next to buildings instead of using wood chips will reduce the problem, although this solution may not fit in with the overall garden design. Attempting to remove the spots from surfaces is generally futile. To remove these spots from buildings, try soaking the area with soap and water. Gentle scraping may be needed to loosen the peridioles, but care must be taken to avoid damage to the surface or the paint. Bleach water may also help to remove these spots, however, be sure to do a small test area first to avoid damage to the surface or the paint.
Answers from Dr. Donald D. Davis, Penn State