Artillery Fungus: Identification and Prevention

When tiny, sticky, tar-like black spots appear on the siding of our home, on outdoor surfaces such windows, railings, and cars, or on the foliage of landscape plants, you might be inclined to think it’s insect droppings or some kind of sap. But the culprit could also very well be the mulch in your yard that is affected by artillery fungus, also known as shotgun or cannonball fungus. It forcefully ejects its mature spores high up in the air, leaving pesky stains. 

The good news is that while artillery fungus is a nuisance because it is unsightly, it is not harmful to humans or pets, it does not kill garden plants, nor does it cause any structural damage to the surfaces it lands on. 

To get rid of it and prevent it from coming back, it helps to understand what causes artillery fungus as well as its life cycle. 

What Is Artillery Fungus?

Artillery fungus (Sphaerobulus stellatus) is a fungus that develops on rotting wood in a moist environment. What makes it unique is the way it shoots its spores into the air after so much liquid has built up in its mature fruiting bodies that they burst open. Mature black spores, 1-2 mm in diameter, are propelled by internal force toward strong light about five hours later. They can land up to 20 feet away, preferably on light-colored surfaces. The spores have a sticky coating which makes them adhere to anything they land on.

If you are unsure if it’s artillery fungus, inspect the spots with a magnifying glass. The spores of artillery fungus are globe-shaped and when you scratch the outer dark brown coating open, you’ll see a whitish, finely granular, gummy-like center. 

What Causes Artillery Fungus?

The fungus is found mainly in wood chip mulch but also in decaying fallen trees and other rotting wood as well as in animal dung. It needs sunlight and moisture to grow. Artillery fungus mainly occurs in cool spring and fall weather in a temperature range between 50 and 70 degrees Fahrenheit. The fruiting bodies won’t develop in weather above 78 degrees. 

How to Remove Artillery Fungus

First, there is no fungicide to control the fungus; mechanical removal is your only option. Removing the black spots within two to three weeks after they appear is key for effective removal. The longer the stains remain on a surface, the more difficult it will be to clean the surface. 

Which removal method works best depends on the surface. Scraping followed by soap and hot water is the gentlest option. Using bleach water (1 cup bleach per 1 gallon water) is the next step up for non-porous surfaces, but make sure that the surface won’t get damaged by the bleach. Power-washing, the strongest option, should only be applied on suitable surfaces. On porous surfaces such as wood siding, you might not be able to remove more than the top layer of the spores and the stains may remain permanent. 

How to Prevent Artillery Fungus

Artillery fungus develops mostly in wood chip mulch. Choosing a different mulch is the best way to prevent it in the first place. Use bark mulch or cedar mulch instead, as they are resistant to artillery fungus. Redwood mulch and cypress mulch are not recommended for sustainability reasons as native forests have been depleted by timbering. 

Artillery fungus won’t grow in a dry environment. If you have a small yard, stirring up the mulch regularly to keep it dry is another option. 

If you have an ongoing problem with artillery fungus, it is best to remove all of the affected mulch and start from scratch with new, more resistant mulch. If you leave the old mulch with the artillery fungus in place and just cover it with a layer of new mulch, the fix is merely temporary, as the fruiting bodies will eventually reemerge. Dispose of all the contaminated mulch safely in the trash and don’t compost it or dump it in your yard, as the fungus could spread. 

Yet another alternative is to use a non-organic material to cover unplanted areas, such as stone or gravel.