By Sandy Feather
Penn State Extension
Q. I am getting small dark spots on windows and the white siding of my house. The spots are about the size of the head of a pin and appear to be black or dark brown in color.
I have tried to remove these spots with mineral spirits without success. The only way I have found to remove them thus far is by scraping the windows with a razor blade. A friend of mine thinks they might be some type of spore from the trees. Is this possible? Also, if you have any idea how I might remove these spots, and possibly keep them from happening in the future, please advise.
A. The dark spots are indeed spores, but they do not come from your oak trees. They are the reproductive structures produced by artillery fungus (Sphaerobolus stellatus), one of several fungi common on wood mulches used around plantings in home and commercial landscapes. All of these fungi are involved in the natural decomposition of wood-based mulches. Artillery fungi are the only ones that cause such a problem. The unusually wet spring this year created very favorable conditions for artillery fungus to flourish. Unfortunately, there is no simple solution to preventing the fungus from colonizing mulch beds or to removing the persistent dark spots the spores leave behind.
The fruiting bodies of artillery fungi look like small, cream-colored or orange-brown cups that are about 1/10 of an inch in diameter. There is a single black “egg” in the cup, which is actually a sticky mass of spores. These fruiting bodies appear on the surface of the mulch, but are barely noticeable unless you are looking for them closely. Areas of mulch with artillery fungi may appear matted and lighter in color than mulch that has not been colonized. Bird’s nest fungus is also commonly found in mulch, but is much larger and more noticeable. It does not shoot a sticky spore mass as artillery fungus does.
Tar-like spores of artillery fungus next to a penny
Artillery fungus orients itself toward bright objects, such as light-colored siding, windows or shiny automobiles parked nearby. The fungi eject the sticky spore mass, which can be blown by the wind as high as the second story of a house. As you have discovered, the spore mass is difficult to remove without damaging the surface. It is likely to stain, even if you remove it successfully.
Although research is being done on this problem, there are no known controls for artillery fungus. There is nothing you can spray on mulch beds to prevent the growth of artillery fungus, nor can researchers state unequivocally that certain species of wood are absolutely resistant. Dr. Donald Davis of Penn State University has observed that wood based mulches are more likely to be colonized by artillery fungi than bark based mulches, possibly because of bark’s higher lignin content.
Research done at Penn State found that large pine bark nuggets were the most resistant to colonization by artillery fungus. It also found that artillery fungus was unable to produce spores on cocoa, cypress and licorice root mulch under the conditions used to conduct the experiments. It was also unable to colonize or produce spores on one hundred percent sewage or yard waste compost, or spent mushroom compost. Although dyed wood resisted colonization initially, the artillery fungus did successfully colonize it once it had begun to weather and break down.
At this point, the only surefire solution is to replace wood mulch with river rock or other stone-based mulches in areas that abut houses and parking areas. However, stone-based mulches do not provide the same benefits to plants that wood mulches offer, such as adding organic matter to the soil as they break down, and increasing soil microbial activity. There is evidence that mixing wood mulch with 40 percent spent mushroom substrate (also known as mushroom mulch or mushroom compost) helps suppress artillery fungus spore production.
Research at Penn State also looked at cleaning products to help remove the spores from aluminum and vinyl siding. While some were more helpful than others, one hundred percent removal required substantial effort and possible damage to siding. Bleech-White, Castrol Super Clean and Purple Muscle proved most effective on vinyl siding in her experiments. Foaming Wheel Cleaner and Planet Solutions were most effective on aluminum siding.
While there are no simple answers at present, research on this problem is continuing. Americans use millions of tons of mulch around landscape plantings annually. Despite the problem with artillery fungus, the benefit to the plants of properly applied organic mulches outweighs the negative. One of the reasons that artillery fungus is so frustrating is that it is part of the natural decomposition process that makes organic mulches so beneficial to plant health in the first place. Of course, no one minds when artillery fungus shoots its spores in the woods as it breaks a fallen tree down into nutrients that nurture germinating wildflower seeds and young saplings. It is just doing its job.