University of Wisconsin - Extension

Madison - Several months ago a homeowner from northern Wisconsin contacted me about a problem he was having with small black spots on the siding of his home. If the siding had been fiber board the spots might have been wax bleed, which occurs when moisture coming through the siding from the back forces some of the resins in the wood to the surface in the form of small dark dots. It turned out the siding was vinyl.

I asked if he thought it could be mold. He didn't think so; bleach and scrubbing didn't touch it. Then I asked if he had ever had vines growing on the siding. Once they dry, the feet that attach some vines to surfaces can be nearly impossible to remove. He assured me that there had never been any vines growing on the siding. Next, I asked if he had wood chips next to the house. In fact, he did. With this in mind we determined that his problem was the result of artillery fungus growing in the wood chips.

When this particular type of fungus fruits it produces a small cup containing a clump of spores. When conditions are right the fruit points toward a bright light source and shoots out its spores with sufficient force to hurl them up to 20 feet, hence the name artillery fungus. If the light source were the sun, the spores would fall short of the target and end up on the lawn or another surface where they would go unnoticed. However, the light source is often sunlight reflected off an automobile or siding on a house. In these cases the spores stick to the surface. When the spores land on a surface such as vinyl siding they attach themselves tightly to the surface leaving a tiny black spot. According to Don Davis, a plant pathologist at Pennsylvania State University, the spores are not likely to grow on the siding but they do remain viable for up to ten years.

While I have heard of only a few cases in the Midwest, artillery fungus is causing serious problems in the northeastern states where it has damaged the finishes on homes as well as cars. The growth of the problem is partly the result of the rapidly increasing demand for landscape mulch. This means that there is a lot more mulch in which the fungus may establish itself. Landscape mulch used to consist primarily of bark. With the increasing demand suppliers now are incorporating a higher percentage of wood. The wood breaks down more quickly than bark and seems to be more attractive to the fungus than bark, according to Davis. Damp decaying wood fibers are the preferred habitat for artillery fungus.

Davis and other sources with whom I have talked are not aware of any way of removing the spots without mechanically scrapping them off. In the case of vinyl this would probably damage the siding. I have suggested one solution, which as far as I know has not been tested. This is to mix some non-abrasive filler such as sanding dust with an enzyme laundry detergent and spread a paste of this over the affected surface. Cover it with plastic wrap to keep the paste damp for as long as possible. Then rinse off the surface. The theory behind this solution is to have the enzymes stay active and in contact with the spots for a while so that the paste has a chance to soften them. It is much like soaking a cooking pan to remove baked-on food debris. It is also the approach recommended for removing dried egg stains from the side of a home.

At present Davis is recommending that people who encounter the problem remove all the mulch, replace it with landscape stone and live with the spots.