A recent tree sample was sent to the Plant Clinic with small, pinhead sized, black dots on the foliage about the size of scale insects. The real source of concern was the appearance of these same dots on cars parked nearby. When the sample was examined with a microscope, it became clear that this was a fungal problem. The fungus is not housed on the trees and shrubs as you might expect but on mulch in the landscape. We sometimes receive similar reports of spots appearing on houses.
The small dots are the spores, called peridioles, of a fungus, probably Sphaerobolus. We call this an artillery, or shotgun, fungus because it can shoot its spore caps as much as 20 feet. The fungus is probably living in the mulch. It is not harming the mulch but prefers to live in moist environments with a source of organic matter.
How do the spores shoot so far? The peridioles are located on top of cup-shaped cells that accumulate water. When enough liquid is accumulated, the cupped cells invert, causing the cells to burst and shoot the peridioles as far as 20 feet. The peridioles are very sticky and readily adhere to surfaces.
What can you do to get rid of the spore masses? We are not aware of a product that removes these spore masses without harming siding, car finish, or other painted surfaces. You can prevent more problems by raking the mulch and trying to get it to dry more quickly. If the area is not too large, remove the mulch and use something without an organic source, such as pea gravel, or stone. Artillery fungi are more common on wood mulch (versus bark mulch). Composting the wood mulch before use may help reduce colonization by artillery fungi. If wood products are used as mulch, the addition of a fresh mulch layer to cover old mulch each year may lessen the problem. Use of bark products may also lessen the fungal spread.
For pictures of the artillery fungus and peridioles, use a Google search on the Internet for “artillery fungus.” There are several good sources with photos.