20170607A Kevco Building Services
What looks like tiny spots of black tar on white siding are actually artillery fungus spore masses. Photo: Kevco Building Services

Have you noticed tiny black bumps firmly glued to a white or pale outdoor surface, especially siding, but also a car, a fence, a window frame, a downspout or garden furniture? They may look like little balls of tar, but in fact they’re the spores (or rather masses of spores) of a very common fungus with a worldwide distribution, artillery fungus (Sphaerobolus stellatus or related species). Artillery fungus gets its name because it can shoot its balls of spores up to 20 feet (6 m). And the black spots stick firmly to the surface where they land on and are difficult, if not impossible, to remove.

The truth is that the same balls of spores also stick to dark surfaces and even to plant leaves, but generally go unnoticed there. It’s only on white or pale backgrounds that they are really visible.

At least they aren’t harmful to either humans or pets.

Nearly Invisible and Hard to Control

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The fruiting body opens into a star-shaped structure with a round ball of spores in the center. Photo: Eduardo A. Esquivel Rios, Wikimedia Commons

The fruiting body of the fungus in question is very tiny, in fact, scarcely visible, measuring only about 1/10 of an inch (3 mm) in diameter. Should you ever even see one, it would resemble a tiny cream or orange-brown star with a round ball in the center. It lives on dead wood and other decaying materials, including dead trees, stumps, fallen branches, animal feces and wood-based mulches, especially mulches that are in the process of decomposition. It’s generally more prolific in rather humid areas, especially in shady spots where the sun doesn’t reach the soil. In the case of siding, you usually see more spore masses on the north side of the building than the south side.

Usually, the spore masses mostly appear in the spring or fall, when the soil is moist and temperatures are relatively cool (50 to 70 °F/10 to 20 °C).

Stopping Its Spread

Artillery fungus is so common it’s hard to eliminate entirely, but to reduce damage, try removing any dead wood from the area and covering the soil completely with dense vegetation or fresh mulch. It may be necessary, in seriously affected areas, to replace the mulch annually, removing the old mulch each spring (it can go into the compost) and replacing it with fresh, less decomposed mulch.

How to Remove the Spots?

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Pressure washing at hot temperatures can help remove the spots. Photo: Steened, Wikimedia Commons

You can usually get the spots off by using a pressure washer heated to about 200 °F (93 °C), but you may need to rub the spots carefully with steel wool to get them off entirely. Fresh spores are easiest to remove; after 2 or 3 weeks, they attach more firmly.

Obviously, the easiest solution of all is to choose dark colored siding and garden furniture instead of white or pale shades. That way the spots will go unnoticed.20170607A Kevco Building Services

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

1 comment on “Black Spots on a White Wall

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